CHAPTER 2 ARE WE MADE OF STARDUST?contemporarychemistry.com/ReviewerChapters/Ch2_BigBangBtns.pdf · Chapter 2 Is the Earth Made of Stardust? Page 2-1 CHAPTER 2 ARE WE MADE OF STARDUST?

Chapter 2 Is the Earth Made of Stardust? Page 2-1

CHAPTER 2

ARE WE MADE OF STARDUST?

If we can believe the most current theories of our origins, the answer to the above question is “yes.” However, scientists and non-scientists are still asking questions such as: What is the origin of matter? What is the origin of the Earth? Did life evolve or was it created? Science currently offers no final answers to these questions. However, hypotheses are being proposed as a result of the combined studies of astronomy, geology, biology, physics, and chemistry. Recent observations from the Hubble and other sophisticated telescopes and scientific equipment have provided dramatic new pictures and other critical scientific data. Scientists are using this evidence to speculate about how our universe, our Earth, and humans originated. Many clues are available and critical to understanding our origins and those of our surroundings.

Questions Answered in This Chapter:

1. What are the current theories of the formation of the universe and its galaxies?

2. What is the evidence for these theories?

3. What is thought to be the origin of the solar system and the Earth?

4. What major changes have occurred on the Earth throughout its history?

5. What special conditions have led to our current situation on the Earth?

“Why Worry About This Stuff?”

Sometimes scientists are asked, why bother with the Big Bang and related subjects? What has this got to do with what’s happening today? Some people are interested in genealogy and others in history of their country, region, or neighborhood. The history of our universe and its structure is only now beginning to be understood. We are in a very active period of the discovery of that history. As well as the latest twists and turns regarding the Big Bang model, you will be hearing about and reading about bubble universes, mirror worlds, branes, cosmic threats to the Earth from comets, and

Why worry about this subject?

http://contemporarychemistry.com/MultimediaModule/chapter_2.swf?frameTarget=5


other disturbing concepts. Science fiction has many fans, but the developments and suggestions made in the scientific studies of the origin and history of the universe are rapidly approaching the level of the astounding. It could be argued – and the authors of this book do– that knowledge of the origins of our universe is a very important part of our cultural history. The Big Bang model and related topics reveal our common human condition in a vast, unknown, and mysterious universe. Primitive peoples studied and wondered about the stars. Have we in the modern world taken these same stars for granted? Because we are apparently composed of matter from the remnants of exploding stars, ought we not take at least a little time to learn more about our atomic ancestors and their history? If there is a desire to look at the “big picture,” there is none larger than this one!

Scientific Observations Used to Justify Cosmological Models

We begin our discussion of the complex and fascinating area known as cosmology, the science of the origin and structure of the universe, with experimental observations that are used to justify cosmological models. Cosmology is in a period of rapid change, with an abundance of newly acquired experimental data and a number of new and controversial cosmological models. It is important to examine first the types of scientific data that are used by those who propose these models of the origin and structure of the universe. Stars, such as our Sun, are organized in galaxies. Our own galaxy is a flattened disk of several hundred billion stars called the Milky Way. It takes light about 100,000 years to transit the width of our galaxy. It is estimated that there are on the order of hundreds of billions of galaxies in our observable universe. Our ability to travel to nearby planets in the solar system is highly limited. Our ability to travel to nearby stars is, with current rocketry, nearly impossible, and not even contemplated for travel to nearby galaxies. So how can we possibly obtain structural information about the universe?

We are limited to collecting and studying the particles and electromagnetic radiation (light of all wavelengths, including radio waves, infrared, and X-rays) that stream into Earth’s atmosphere from outer space. The fast moving, high-energy particles are called cosmic rays. The study of incoming electromagnetic radiation provides the most valuable data for detailed models of the solar system, our own galaxy, the Milky Way, and the rest of the galaxies making up the rest of the universe. Let’s examine the way in which the information extracted from widely different parts of the electromagnetic spectrum can be used to illuminate our knowledge of our universe.

Experimental observations

used in models

Figure 2-1 An estimate of what the large-scale structure of the universe looks like, with billions of galaxies forming a web-like structure with vast regions of empty space.


The Electromagnetic Spectrum

Most of the visible light from outer space enters the atmosphere without interference. Some radiations entering the Earth’s atmosphere are scattered or absorbed by atmospheric molecules. These radiations from the Sun and other parts of the universe are part of the electromagnetic spectrum (Fig. 2-2). The electromagnetic spectrum consists of a wide variety of radiations that have the following features in common: they all travel with the speed of light, they all have both wave-like and particle-like properties, and they all have well-defined wavelengths (see below) and energies. Each of these wave-like, energy-containing packets of energy is called a photon regardless of its location in the electromagnetic spectrum.

The wavelength is somewhat like the distance between the crests of a series of incoming waves at the seashore. The distance between the peaks of the succeeding waves is called the wavelength. The shorter the distance between waves, the more energy is delivered to a bather in the surf during a given time. The shorter the wavelength of the electromagnetic radiation, the higher is the radiation’s energy. This means that ultraviolet radiation is much more energetic than infrared radiation. The frequency is defined as the number of wave peaks passing a fixed observation point in a certain time period. This corresponds to an observer on a pier that extends into the ocean counting the number of waves passing the observer in a fixed time. The flux of photons is a measure of the intensity of the source of illumination, and is the number of photons passing through a square centimeter area in a given time period.

Fig. 2-2 The electromagnetic spectrum, showing the relationship among wavelength and frequency. Hz is the units of frequency in hertz (number of cycles per second). (From Astronomy, M. Zelik, Wiley and Sons, 1994, p.92)

Electro-magnetic Spectrum



Analysis of Starlight

When pure white light passes through a prism or a raindrop, or is reflected off a solid etched with very close, evenly spaced straight lines (a diffraction grating), the light splits into its different component wavelengths, giving a continuum spectrum (see top figure on left). When the spectrum of light emitted from a hot flame containing a metal vapor such as sodium (Na) is analyzed, it gives a distinct characteristic pattern of colored lines (middle figure on left). However, when light from stars is similarly analyzed, spectra with very sharp dark lines (bottom spectrum on left) are found that are nearly identical in their position on the spectrum with different elements found in laboratory flame studies of various elements. These line spectra have been studied carefully and are known with great precision with modern instruments called spectrometers,

which record electromagnetic radiation spectra of all types. These dark lines arise because light coming from the star is completely absorbed by metal or other atoms in the star’s atmosphere in only certain very narrow regions of the starlight’s spectrum. Thus, the dark line spectrum can be used to identify the elements contained in the atmosphere surrounding a star. If the element is found in the star’s atmosphere, it is most likely also contained within the star itself, thus giving a clue as to the star’s chemical composition.

Spectral Analysis


When spectral lines present in visible starlight from some weaker stars are

analyzed, all of the spectral lines are shifted toward the longer wavelength infrared than the locations of the same spectral lines in found for the same elements on the Earth. This so-called “red shift” is correlated with the distance of the star from the Earth in much the same manner as sound is affected by the Doppler effect. That is, if an object that emits a sound at a certain frequency when at rest moves rapidly toward an observer, that stationary observer will hear a higher frequency. If the object moves past the observer, away from the observer, the opposite effect is heard. That is, as the object moves away from the observer, the frequency is lower that when it is moving toward the observer. When trains blowing a horn pass by another train, there is a change in the pitch of the horn, which is because of the Doppler effect.

By using the red

shift analysis as a scientific tool (consult figure on left), a three dimensional analysis of the positions of stars and galaxies throughout the universe has been conducted. This analysis has shown that galaxies are not uniformly distributed, but instead are grouped in clusters and ribbons of thousands of galaxies, and these

clusters are present in long strings and webs of clusters (Fig. 2-1). This leaves vast amounts of space without any galaxies. This arrangement has been described by astronomers as a “foam-like” distribution or as a “great wall” of galaxies.

We pause now and ask the question, why are there sharp lines in these

starlight spectra? The answer involves a very important principle of physics and chemistry.

Line Spectra and the Bohr Atom

Recall the animation on ionization in Chapter 1 [link]. When you drag the electron away from the proton of the H atom, the red bar representing the energy of the electron-proton pair rises in an interesting non-smooth, “jerky” manner [ed. note: if this is not so, we’re working on it!] until it reaches the ionization limit, and the electron is freed from the proton positive charge. When you return the electron to the proton, discrete photons are ejected from the reassembled H atom. When you analyze the actual spectrum of the light from a large number of highly excited H atoms, you observe a collection of very sharp lines. What is the source of these lines?

Red Shift



Analysis of these lines inspired Niels Bohr in 1913 to propose what is called

the Bohr model of the atom. Previously, based on his experiments with scattering of alpha particles from thin metal films, Rutherford proposed that the atom consisted of a small positively charged central core with an equal number of negative charges more widely distributed around that core. Bohr suggested that the H atom electron was restricted to certain circular orbits around the proton nucleus. His theory accounted for the general pattern of the H atom spectral lines and was accepted almost immediately.

Bohr explained the spectral lines using the following model: The hydrogen electron is only able to occupy certain circular orbits around the proton. That is, the energy of the H atom was quantized. Each orbit represents a fixed electron energy. If an electron moves from a higher energy orbit to a lower energy orbit, it must emit a photon that has exactly the difference in energy of the two orbits (the higher energy minus the lower energy). Because of the orbit energy restrictions, the emitted photon energies were restricted to a certain set that gives rise to line spectra. All H atoms have the same orbit energy levels. Therefore, they all give exactly the same spectral lines when large numbers are excited to higher energy levels and then drop to lower energy levels.

The absorption spectrum of H atoms was also quantized. An H atom electron

could only be promoted from a lower energy level A to a higher energy level B if the photon about to be absorbed had an energy corresponding exactly to the difference between energy level A and energy level B. This could account for sharp absorption line spectra from the stars. Only certain narrow energy regions of the electromagnetic spectrum could provide the correct energy photons to excite atomic electrons of certain atoms. Thus, atoms in the atmosphere of stars could be identified by their spectra. However, there were problems with Bohr’s proposed model.

The Bohr model failed to agree with more refined H atom spectral data.

Elliptical electron orbits were proposed, but again failed to agree with even more refined experimental data. Finally, in 1926, the quantum mechanical model of Schrödinger was published. It predicted exactly the H atom line spectrum. However, this new model of the atom was not so easy to understand. It assumed that the electron had both wave and particle characteristics. Another proposal, the Heisenberg uncertainty principle, considered the role of the critical role of the measurement tool in disturbing the measured atomic-level particles. A consequence of this principle was that it did not allow any prediction of any path for an electron within an atom. Instead, it only allowed the Schrödinger model to predict the probability of finding the electron within a certain region of space at a given time. This model of the atom has survived to the present time, despite its non-intuitive nature. The message of quantum theory is that when scientists start to explore the nature of very small particles, do not expect the same properties as in the macroscopic world, i.e., that of the world of bowling and tennis balls. However, the crowning success of this model was its ability to quantitatively predict or explain experimental data.

The Bohr Atom

Quantum Mechanical

Model



The above quantum theory is important for not only understanding the very small electron and nucleus, but also the very large universe. What kind of information can we gather from the electromagnetic spectrum that will allow us to comprehend the nature and structure of the universe? Photons from the electromagnetic spectrum, along with cosmic rays, are the only tools we have on this universe without physically leaving the Earth’s surface. Let’s now see how these photons are put to use.

Raw Data for the Model of the Universe

Who would have thought that radio static could give us one of the most important clues to the nature of our universe? Scientists have discovered cosmic microwave background (CMB) radiation bombarding the Earth from all directions. Parts of this radiation are responsible for the hiss in a radio when the radio is not tuned to a particular station. This radiation is remarkably uniform and is almost equal in magnitude and quality regardless of its direction. This radiation has a characteristic “temperature” that can be determined from the distribution of microwave frequencies contributing to this cosmic radiation. You’ve perhaps heard the phrases “red hot” and “white hot.” This cosmic radiation is at the very cold end of the electromagnetic spectrum, in the microwave region. From the shape of the spectrum, scientists can measure the temperature of the emitter of the radiation in the same way a steelmaker can measure the temperature of molten steel within a blast furnace by measuring its color. This CMB temperature corresponds to that of an object at 2.7 degrees Celsius above absolute zero – exceedingly cold! When the radiation was very carefully measured and mapped across the entire universe, as viewed from Earth, in 1990 by a specially designed satellite (COBE), there were found to be very small variations in the temperature of the radiation that reveal patterns that have been carefully analyzed by astronomers. Two later satellites made much more detailed measurements of the cosmic background microwave radiation field and very small variations were found and analyzed. These variations in this CMB radiation have been interpreted as radiation that is indicative of one of the earliest events in the origin of the universe. More about this later.

As their telescopes become more sophisticated and powerful, astronomers

have been intensely studying light coming from more distant stars and galaxies. As they do so, they are reaching farther back in time, as indicated by red shifts in spectral lines. This is because at the distances calculated from the red shifts in the spectrum of the light coming from the most distant stars, it takes billions of years for this light to reach Earth. Thus, the light that scientists are observing was initially emitted billions of years ago.

Astronomers discovered that most galaxies are expanding away from each

other at a rapid rate, even though the stars within these galaxies are not expanding away from each other. Indeed, some stars within a galaxy are being drawn together. Furthermore, it has been recently reported that as the distance between the galaxies increases, this rate of expansion also accelerates. This leads to the conclusion that the universe appears to be expanding at an ever-increasing rate, with no hints of slowing down.

Cosmic microwave background

radiation


Astronomers have extrapolated this galaxy expansion backwards in time and asked what happens and when? Do all of the galaxies coalesce in a common location? Yes, about 14 billion years ago they coalesce into a very small region. Thus, the above observations and calculations have formed the basis for the Big Bang model as the origin of our current universe.

An Overview of the Big Bang Model

In order to explain the above the astronomical observations in a comprehensive model, it is necessary to break up the model into a number of periods:

(1) A very short inflation segment in which space expanded in a fraction of a second from an exceedingly small volume to huge, unknown dimensions; (2) Following a period of seconds with nothing but energy yielding fundamental particles that make up nuclei, there was a sudden formation of a plasma consisting of neutrons, protons, electrons and dark matter (see below) tightly coupled with a very high temperature radiation (photon) field; (3) An expansion of this plasma during which nuclear reactions formed helium nuclei from protons and during which the plasma cooled; (4) A further expansion period in which the plasma temperature was too cool to support nuclear reactions; (5) Further cooling to the point at which electrons were finally able to combine with protons and helium ions; (6) Formation of H and He atoms in a relatively short period of time with the release everywhere in the universe of the trapped plasma radiation in the form of photons that are observed currently as cosmic microwave background (CMB) radiation, also everywhere in the universe; (7) Gravitational attraction of H and He atoms in regions of slightly higher than average density of dark matter to ultimately form stars and then galaxies; (8) Further expansion of space between galaxies at an ever-increasing rate, while gravitational attraction between matter and dark matter within galaxies prevents the expansion of space between stars.

The Inflation and Expansion Periods

We live in a period of great change in cosmology, the science of the origin and development of the universe. At the time of this writing, the starting point of the universe and the nature of a proposed huge expansion period are still not universally accepted by all cosmologists. Many of the details of this initiation process for the current universe are based on theoretical calculations. Future satellite data will probably yield experimental insights into the very early events in this process. Rather than describe the details of the controversy, we merely state that there appears to be a consensus that within the first few seconds following the birth of the current universe, there was a sudden expansion.

Steps in Big Bang

Models

The Inflation Period



According to the Big Bang model, 13.8* (*footnote: a recent figure was listed as 13.75 ± 0.11, where the ± 0.11is the estimated experimental error in this figure, meaning that this figure could be either 0.11 billion years greater or less than 13.75 billion years) billion years ago, all of the matter-energy in the universe was initially contained in an exceedingly small volume, within a volume of approximately 10–33cm3 (cubic centimeters). Suddenly, in ~10–35 second, this matter-energy entity expanded exponentially to huge dimensions for an unknown reason in a process designated as inflation. That is, huge volumes of space were created within much less than the blink of an eye. Matter did not explode, as the name Big Bang implies – instead space was suddenly created! No space existed outside that which was created during the Big Bang. It is postulated that during this very short inflationary period there was no matter, just energy in the form of a type of field with some very special properties. These concepts may sound rather strange, but it is a critical part of most, but not all, of the proposed cosmological models.

The Beginnings of Chemistry

According to current cosmological theories, chemistry didn’t begin until the end of the inflation period. This was because the high-energy field that was rapidly expanding weakened to the point where it became unstable. At this point, the energy of this field was transformed initially into the fundamental particles from which nuclei are composed and then into an extremely hot, expanding plasma consisting of unstable neutrons tightly coupled with high temperature radiation. According to the Big Bang model, the temperature of this expanding plasma cooled in less than a minute after the inflation period from an initial value of tens of billions of degrees to about one billion degrees, the temperature inside one of today’s hottest stars. At this time, theorists postulate, the universe consisted of a uniform plasma containing photons coupled with three kinds of particles: neutral neutrons, positively charged protons, and negatively charged electrons (Fig. 2-2). These particles were continually colliding with each other and were strongly influenced by the trapped and coupled radiation that maintained the very high temperatures of the plasma. However, space was still expanding between these particles in the plasma at a much slower rate than during the inflation period.

Neutrons are stable inside a stable nucleus. However, free neutrons (n) outside the nucleus are known to decompose into protons (H+) and electrons (e–) within a matter of minutes. The information contained in the previous sentence is represented in equation form below. Stated in words, the equation designated (2-1) reads “An isolated neutron spontaneously decomposes, yielding in its place a positively charged

Immediately following

the inflation period

neutron

electron

proton

Figure 2-2 Representation of the plasma during the Big Bang as a random array of neutrons, protons and electrons colliding with each other at very high speeds. Not shown is the trapped radiation that keeps the temperature very high.

neutron

electron

proton


proton and a negatively charged electron”*(*footnote: another product of this reaction is a neutrino, a neutral particle with near zero mass that rarely reacts with matter and is of little consequence in cosmological chemistry.) : n → H+ + e– (2-1) The reverse of reaction (2-1), the recombination of protons (H+) and electrons (e–) to form a neutron (n), does not occur, so equation (2-1) is irreversible. Instead, the recombination of a proton and an electron forms a neutral hydrogen atom (H) according to equation (2-2), provided the temperature is low enough to keep the resulting hydrogen atom stable. H+ + e– → H + photons (2-2) As equation (2-2) indicates, energy from the recombination of a proton and an electron is given off in the form of several photons as the H atom electron cascades down in energy while forming a stable (“ground state” – lowest energy) H atom. The energy given off in the photons is equal to the energy that it takes to ionize the H atom. However, the estimated temperatures of the initial Big Bang plasma were too high for any H atoms to survive and would therefore immediately be ionized as soon as they formed (2-3). H + plasma energy → H+ + e– (2-3)

Thus, according to the Big Bang model, during the first several minutes

following the Big Bang, the expanding hot plasma consisted of decreasing numbers of neutrons and increasing numbers of protons and electrons. However, during the first several minutes of cooling, it is proposed that protons and neutrons underwent fusion reactions, a collision-induced fusing together of plasma neutrons and protons. This nuclear reaction is represented in equation form as: n + H+→ D+ , where D+ represents a bare deuterium nucleus composed of a positively charged proton and a neutron. D can also be represented symbolically as H-2, since deuterium is an isotope of hydrogen (H) and the -2 represents a mass number for deuterium of 2.

Soon after formation, the D nucleus

took part in a series of nuclear reactions that ultimately resulted in the formation of a helium nucleus (He-4) consisting of two neutrons and two protons (Fig. 2-3). Because the helium nucleus is significantly more stable than its constituent neutrons and protons, the formation of this helium isotope in the fusion reaction releases very large amounts of energy when formed. The

Proton - hydrogen nucleus

Helium-4 nucleus (2 neutrons and 2 protons)

Figure 2-3 Composition of the two most abundant nuclei present in the expanding Big Bang plasma, the proton (H-1) and the helium-4 (He-4) nucleus

Chemical reactions

immediately following

the Big Bang

Formation of very

stable He nucleus





gamma rays that are given off in this reaction are absorbed in the plasma, maintaining its high temperature. Gamma rays are photons at the highest energy in the electromagnetic spectrum.

Astronomical measurements estimate that today 25% of the observable mass in the universe is He-4, in excellent agreement with the predictions of the Big Bang model. These measurements, which established the ratio of H atoms to He atoms in the universe, are considered one of the key scientific observations supporting the Big Bang model.

The Big Bang plasma cools as it expands

According to the Big Bang model, the temperature of the expanding plasma rapidly decreased with time. The plasma cooled, in part, because the expansion of space was stretching the plasma radiation, lowering its energy.

If you try to escape from the Earth by jumping up away from the ground, your mass is strongly attracted by gravitational force of the huge mass of the Earth (Fig.2-4). Thus, your upward speed slows rapidly to zero and you quickly fall back to Earth. If you are light enough or are propelled vigorously enough, as in the case of space vehicles, you can escape the gravitational pull of the Earth. The gravitational attractions among particles in the Big Bang plasma are much smaller because their masses are so much smaller than you or the Earth, and their speed is also much larger than yours as you jump up away from the Earth. Nevertheless, the speed of particles in the hot plasma during the Big Bang expansion was reduced because of this gravitational attraction. However, scientists find that this

attraction was insufficient to explain the observed results so that hypotheses about other forms of matter called dark matter (see below) have to be introduced into models to explain experimental observations. After about 380,000 years of expansion and cooling, nuclear reactions were no longer possible between positively charged nuclei because there was no longer sufficient collision energy to overcome the large repulsive forces between colliding positively charged nuclei and neutrons had long since disappeared. Neutral atom

The plasma cools

Figure 2-4 Illustrations of the effects and force of gravity: (1) a ball thrown up stops and then returns with a relatively small force; (2) a person who jumps up away from the earth returns quickly to earth with a larger force; (3) a one ton weight returns to the earth with a tremendous gravitational force of attraction.



formation reactions eliminated the plasma and at the same time liberated plasma-trapped radiation. During this transition from plasma to atoms, the temperature of the plasma is estimated to have been about 3000 degrees above absolute zero (3000 K).

The Formation of Neutral Hydrogen and Helium Atoms

The chemical reactions that destroyed the plasma were the recombination of electrons with protons and He-4 nuclei to form neutral H and He atoms. Negatively charged electrons were originally formed from the early decomposition of neutrons and were part of the cooling plasma. These electrons were attracted to the positive charges of the hydrogen and helium nuclei, forming neutral hydrogen and helium atoms as shown in Fig. 2-5 and chemical equations (2-2) and (2-4). H+ (hydrogen nucleus/proton) + e– → H (hydrogen atom) + photons (2-2) He 2+ (helium nucleus) + 2 e– → He (helium atom) + photons (2-4) Equations (2-2) and (2-4) are chemical rather than nuclear reactions because in chemical reactions the nuclei do not change their chemical identity. Hydrogen remains hydrogen in equation (2-2) and helium remains helium in equation (2-4).

Formation of

atoms

Exercise 2-1 Neutralization of an ion Write the chemical equation describing the reaction of electrons with the lithium ion (Li+3) to form a neutral lithium atom (Z = 3 for lithium).

Figure 2-5 Representations of the recombination of protons and helium nuclei with plasma electrons to form H and He atoms.

He-4 nucleus electrons He-4 atom atomatomatom




H and He Atom Formation and the Release of Cosmic Background Radiation

When H and He atoms formed, the electrically charged plasma vanished, causing the release of all the photons trapped in the plasma. Before the formation of atoms, the radiation was intimately coupled with the charges of the protons and electrons. After the formation of neutral atoms, this trapped radiation energy was released into space in a different form, namely free, uncoupled photons that traveled with the speed of light (Fig. 2-6). Thus, in a relatively short cosmological time period approximately 400,000 years following the inflation period of the Big Bang, the nature of the expanding and cooling universe changed from a uniform, expanding hot plasma to an expanding, cooling group of atoms and uncoupled, free photons traveling through space in all directions, originating from all parts of the expanding universe. Thus the source of this cosmic background photon radiation was – and still is – the entire expanding universe as it existed over 13 billion years ago. From the point of view of a person on Earth, this released radiation, still present in the universe coming to the observer over a period of tens of billions of light years away, comes equally from all directions. This relatively sudden release of the plasma radiation is one of the key predictions of the Big Bang model because it can be experimentally analyzed and compared with theoretical predictions. This predicted radiation has been discovered as a special kind of “static.” Discovered accidentally as unwanted background static in a special type of radio receiver, the radiation, called cosmic microwave background (CMB), has no one single source, as predicted, and its detailed characteristics are almost exactly those predicted by the Big Bang model (Fig. 2-7). This uniform radiation is considered perhaps the most important confirmation of the Big Bang model. This CMB radiation is also exceedingly valuable in helping determine the spatial distribution of matter about 400,000 years following the Big Bang, allowing scientists to predict from the structure of this radiation the detailed manner in which galaxies and stars were formed. Scientists have acquired very high-resolution microwave data from satellites and have used these data to test various Big Bang models.

Release of trapped CMB

radiation when H and

He atoms formed

Figure 2-6 (a) Trapped radiation with a temperature of about 3000 degrees Celsius coupled with electrons and protons in a plasma state; (b) H and He atoms and free photons traveling at the speed of light that are observed as cosmic background radiation today.


Let’s again recall why the hydrogen and helium nuclei were being separated in the period following inflation of the universe: because space was being created between them! The previously trapped radiation was also present in this expanding space. These released photons were initially high-energy photons whose average energy reflected that of plasma at 3000K (2727 ºC). Photons have wave properties, including the property of wavelength, a characteristic that is determined by its energy, in this case by its temperature. High photon energy corresponds to short distances between the crests of the waves and lower photon energies correspond to larger distances between wave crests. When a photon travels through expanding space, the distance between wave crests increases because of this expansion, that is, the photon is stretched, and the energy of the photon decreases. In the 13.8 billion years of expansion, the universe has expanded by a factor of approximately 1000, increasing the distance between wave crests drastically and making them now detectable in the microwave region of the electromagnetic spectrum (Chapter 3). Thus, the cosmic microwave background radiation, initially having a temperature of 3000 K, 13.8 billion years later has a temperature around 3 K. This temperature is almost exactly that predicted (2.7 K) from the Big Bang expansion model of the universe. This is another of the significant successes of the Big Bang model.

The analysis and interpretation of recent, very high resolution CMB radiation

data and new theoretical investigations has given rise to a number of variations of the Big Bang Model discussed in the next advanced section. Beware, the material in this section may seem bizarre at times, but is serious cosmological science! For example, dark matter and dark energy have been postulated based on theory alone, and scientists are desperately searching for experimental proof for these entities. In the absence of experimental evidence, however, some scientists are claiming that they should be changing their theories since there is no experiment (yet!) that has demonstrated the direct observation of either dark matter or dark energy. Multiple universes have been proposed. Ours is a “Goldilocks” universe in which everything is “just right.” In some of these universes, time may run backwards. Others may be mirror universes, ones in which exactly the same universe and inhabitants as ours have different outcomes. Other strange concepts that have been proposed in science fiction such as wormholes through spacetime are beginning to have their counterparts proposed in serious cosmological theory. There is one intriguing new scientific theory

Figure 2-7 360 degree map projection of the cosmic microwave background (CMB) radiation given off around 400,000 years after the initiation of the Big Bang observed by the COBE satellite. The different colors are very highly magnified, infinitesimal differences in the radiation field temperature.

Analysis of

CMB radiation


called Biocentrism that proposes that time does not exist independently of the life that observes it. After all, say the proponents of this theory, time does not play a role in the natural laws of physics. Proponents of this theory suggest that life creates the universe as well as time. That is, the universe exists because we do!

As you ponder these unusual concepts, you should always remember that

science is based on experiment. It is up to theory to explain all experimental results. In rare instances, theory precedes and predicts subsequent experimental results. Einstein’s theories are good examples of this. However, all scientific endeavors ultimately depend on reproducible experimental facts. Many of the above concepts are without experimental backup and should be treated as “interesting” but not verified proposals. ––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––– ---------Advanced material (reached only by clicking button marked “Advanced Material”) --------

Modified Big Bang Models and Related Concepts

A warning: Some of what is written below in this and other sections of this chapter may either be out of date by the time you read this or may be a disputed model. Some of what you read may sound like science fiction. The author assures you that this material is his best effort to summarize the scientific literature comprehensible to him at the time of writing. However, he cannot guarantee it will be taught to your children or grandchildren. This scientific area is very fast moving in its output of new and controversial ideas, some of which may vanish with new observations that prove a particular model to be incorrect. You may not want to read too much of this section, indeed this chapter, at one sitting. The head begins to spin – at least it did for the author when he was initially researching this subject.

We are now in a period of rapid advances in the development of cosmological models for several reasons. First, vast improvements in astronomers’ equipment, with much higher resolution satellite telescopes and more sophisticated collectors of non-visible radiation, have allowed deeper penetration into outer space with higher resolution. This is especially true for detectors of cosmic microwave background radiation. Second, new advances in theoretical physics and their application to cosmology have introduced new, fascinating, and often hard-to-believe theories of the origin and structure of the universe.

The development of the so-called “string theory,” proposed by some physicists as the “theory of everything,” has helped to spawn some of the most controversial cosmological models. In string theory, fundamental particles are conceived as being exceedingly tiny excited vibrating strings. This satisfies the concept that these particles have both wave and particle properties. Prior to this theory, we were assured that time began with the initiation of the Big Bang. There was no way of knowing what preceded the Big Bang. String theorists are now suggesting that there need not be a beginning or an end of time. One cosmological string theory suggests that the initiation period of the Big Bang was a transition from

Fast-paced developments in Big Bang

Models


a contraction of one universe to a very rapid inflation period that initiated the expansion of another new universe. However, there are many problems with string theory that are being investigated in an effort to overcome these problems. Stay tuned.

Cold Dark Matter

Cosmologists have postulated a more refined model of the universe to account for all of the features of the data (Fig. 2-8) from the Wilkinson Microwave Anisotropy Probe (WMAP), which was able to collect cosmic microwave background radiation at very high-resolution. Cold dark matter is proposed to be matter that does not emit or

absorb radiation and does not readily react with observable matter or with photons. It is matter that is not protons, neutrons, or electrons, but is instead postulated to be composed of a fundamentally different kind of matter. Because of this strange property of not interacting with radiation or observable matter, it also cannot couple with plasma photons the way protons, neutrons, and electrons do. Dark matter is estimated to be six times as abundant as observable matter and is suggested to be the controlling factor in determining the structure of our universe. It is proposed that gravitational forces attract observable matter to dark matter “haloes.” Dark matter is thought to have been cold at the beginning of the hot plasma Big Bang expansion, which must mean that dark matter does not suffer collisions with nuclei or electrons. If there were collisions, the very high energy of these plasma particles would be transferred to the dark matter, making it hot dark matter. However, dark matter is thought to attract other dark matter as well as observable matter through gravitational forces. Thus, the proposed dark matter haloes attract observable matter. These observable matter-containing haloes then condense to form larger units to the point that they are large enough to form observable galaxies.

Dark Matter

Figure 2-8 Cosmic microwave background radiation projection from data obtained from the Wilkinson Microwave Anisotropy Project satellite. Notice the much higher degree of precision and finer detail than that of the data taken from the COBE satellite (Fig 2-7)


Dark Energy

Dark energy is one of the most controversial entities in cosmology. Scientists propose that vast amounts of dark energy exist in the universe and that this repulsive energy is responsible for the currently observed accelerating expansion of the universe. They postulate there is a struggle between dark matter and dark energy [shades of “May the Force be with you…”!]. When dark matter “wins,” there is gravitational collapse. When dark energy “wins,” there is an accelerating expansion brought about by accelerated creation and expansion of space. Einstein was forced to add a negative gravitation factor as a “cosmological constant” [in more recent parlance, a “fudge factor’] to his general relativity theory. However, later, when Hubble discovered that the universe was expanding, Einstein decided that there was no need for this constant and claimed that adding this constant was his greatest scientific blunder. Today, cosmologists are attributing this needed correction to dark energy. Some think that dark energy is a characteristic of a vacuum in space, a sort of vacuum energy. Some think dark energy is responsible for the early inflation period. To quote one science writer, “dark energy will likely be the darkest mystery in a very dark universe.” A cosmologist frankly states, “In the self-proclaimed age of precision cosmology, we know the amount of each component [dark matter and dark energy], but in the spirit of ‘honest cosmology,’ we also have to admit we also do not know precisely what either of them is. But we are not helpless.”

According to recent theories, dark matter and dark energy are very important

quantities in the universe! Most have heard of Einstein’s equation, E = mc2, stating the equivalence between mass (m) and energy (E) and suggesting an all-encompassing concept mass-energy, where c is the speed of light. Approximately a quarter of the total mass-energy of the universe is estimated to be present as dark matter. Approximately two thirds of the total mass-energy of the universe is dark energy, the least understood quantity of any in the universe.

Neither dark matter nor dark energy has the capacity to absorb or emit light. A

suggested analogy is that of flying at night over a dark territory with pinpoints and small regions of light piercing the passing jet-black landscape below. Dark matter and energy are the portions of the landscape below that you can’t see. You only know that something must be there, but you’re not allowed to fly over the territory during the day. From the density of light, you propose that you are over a city, a village, or farmland. Some intelligent decisions can be made in this situation, but suppose you have never seen the land during daylight before? Moreover, suppose that you’ve never seen land before and the land below is that of a planet that is in perpetual darkness!

Dark Energy


The Battle Between Dark Matter and Dark Energy

It is proposed that dark matter won over dark energy in the period of the early part of the expansion of the universe and caused a slowdown in the expansion. Then there was apparently a period when the dark energy and dark matter had an equal effect. Currently, dark energy has apparently taken over and the expansion is accelerating, with no sign of abating. At this point, scientists speculate that there can be two extreme outcomes, one involving the domination of dark energy to the point of tearing all matter apart, the other involving a weakening of dark energy with the passage of time, leading to a collapsing universe. An alternative model for the accelerating expansion comes from the string-theory camp proposing that gravity is somehow leaking from our universe, allowing accelerated expansion of the universe. Cosmologists are studying the temperature of the cosmic background microwave radiation looking for variations in this temperature on the order of a millionth of a degree for data in an attempt to find data to justify various theories.

One important piece of evidence obtained from the analysis of the cosmic

background microwave radiation from the WMAP project is that the universe is flat. Not the Earth – the universe! What does this mean? According to cosmologists, there are three possible shapes for our space-time universe: (1) a warped saddle that will expand so rapidly that there is no time to form galaxies; (2) spacetime is flat, i.e., the universe is exactly between the point of eternal expansion and gravitational collapse; (3) the universe is a

curved ball and will quickly collapse in on itself. Even an exceedingly small amount of curvature would prevent the universe from being flat. Cosmologists have indicated that the very high precision of the WMAP data allows them to conclude that the universe is definitely very flat. A further conclusion from this analysis is that the universe was made flat by the inflation process.

An analysis of the gravitational lensing

data (where stars bend and focus light)

Which wins? dark

energy or

dark matter?

Saddle Shape

Flat Universe

Curved Ball

Figure 2-9 Sketch of a proposed dark matter halo within which a galaxy was formed.


mentioned earlier and the WMAP data have prompted cosmologists to propose that dark matter was not completely homogeneous during the period in which the homogeneous Big Bang plasma was expanding. They propose that the dark matter formed large haloes, within which the first stars, and then the first galaxies were formed (Fig. 2-9).

Multiverse?

One intriguing prediction of some string theorists is that ours is just one out of many possible universes (one part of the “multiverse”) that can intersect with each other. Some say that, if a proposal that the universe is infinite is correct, there must be mirror worlds that have exactly the same arrangement of atoms and molecules as ours. To make matters even more disturbing is that the theoretical foundation of atomic matter, quantum mechanics, predicts that particles can be in two distant places at once and that they become localized in one of these two places only when measured. One interpretation of this idea is that the two particles actually exist in parallel universes. As a recent commentator on cosmology put it, “the borderline between physics and philosophy has shifted dramatically in the last century.”

For more details on this and other fascinating ideas and other difficult-to-

contemplate models, consult the articles in the reading list at the end of this chapter. We will also keep you posted on our web site on these and other fascinating topics of cosmology that are no longer merely science fiction but are seriously debated by cosmologists. Let’s now move on to the proposed formation of stars and galaxies based on the above ideas.

Formation and Expansion of Galaxies

Models regarding the formation of stars and galaxies are still in their own formative stages. There have been some apparent successes at modeling the larger structure of the observable universe (Fig. 2-1 is an example). If dark matter exists, there is agreement that its distribution after the initial inflation period and in particular at the time of release of the cosmic background microwave radiation is critical.

The dark ages

After the formation of H and He atoms and the release of the cosmic background radiation, the further expansion of space caused the cooling of these very hot atoms. During this period, there was localized gravitational attraction that ultimately caused the formation of stars. However, other than the cosmic background radiation, there was a vanishing of visible light, because the background radiation was being stretched by the expansion of space causing visible radiation to shift to lower frequencies and longer wavelengths, causing visible radiation to shift to infrared and then to microwave frequencies. This would have left the universe dark until the formation of first stars. Hence the name “dark ages.”

Multiple universes?


The role of dark matter

Scientists are in agreement that, if the cosmic microwave background radiation coming from the vanishing plasma had been completely uniform in the expanding Big Bang universe, the universe today would be nothing more than expanding hydrogen and helium atoms and perfectly homogeneous cosmic background radiation. There would be no galaxies, no Sun, and no Earth. Satellites and other instruments have made careful and highly sensitive measurements of the cosmic microwave background radiation collected from all parts of space. These measurements have revealed that the density of this radiation, and therefore the density of H and He atoms, was almost, but not completely uniform throughout the expanding universe. This minute variation in the density of atoms observed about 400,000 years following the Big Bang makes life, the Earth, the Sun, and the observable universe possible today! A possible reason for these small variations in density comes from an analysis of the structure of the cosmic microwave background radiation from the COBE and WMAP satellites. Some scientists conclude that there were quantum perturbations that resulted in acoustical waves during the inflation period. Proposals are made that these acoustical waves could have caused regions of higher and of lower dark matter densities in the plasma. These were very small deviations, no more than one part in 100,000 (the equivalent of variations between 100,000 to 100,001) in the relative dark matter densities of the plasma (“primordial freckles”). The acoustical waves are similar to those coming from musical instruments and all of these characteristics are found in the analysis of the data from the WMAP microwave radiation. The detailed features of these wave fingerprints are exactly those predicted by quantum theory, so their observance in the WMAP data is treated as justification for the inflation hypothesis of the Big Bang. In the regions with the slightly higher dark matter densities (Fig. 2-10), gravitational forces are thought to have pulled atoms toward each other rather than allowing them to expand away from each another despite the continuing expansion of space elsewhere. The higher density regions had more gravitational pull and robbed matter from the regions of lower density and through this accelerating gravitational pull caused what is known as a gravitational collapse. It is claimed that dark matter “wins” over dark energy so that space is not created in these more dense regions and

Figure 2-10 Shown is a cluster of stars in the middle of a sea of red stars. The blue background is the estimated region of dark matter that may have been responsible for the formation of the cluster in the center of the picture.

Galaxy formation


therefore there is no expansion within these regions. During the gravitational collapse of slightly higher density regions, large amounts of energy were released from gravitational collapse releasing heat energy (thermal energy) and causing temperatures within the collapsed, high atom density regions to rise to many millions of degrees. These high temperatures and densities were sufficient to initiate nuclear reactions within the collapsing matter. These nuclear fusion reactions released tremendous amounts of energy, thereby forming stars that gave off visible light. Astronomers now believe that, within a billion years or less after the Big Bang, galaxies were being formed. Galaxies are large volumes of space containing up to billions of stars, huge amounts of dust, hydrogen and helium atoms, including black holes, assemblies of matter so dense that light cannot escape from its huge gravitational pull. In the center of most galaxies is a super-massive black hole. As increasingly sophisticated telescopes probe further back into early times following the Big Bang, there are some surprising results. For example, fully formed complex galaxies are found at times that are too early to conform to proposed models of galaxy formation. These models suggest that stars are formed first, then stars are gravitationally attracted to other stars, etc., until a galaxy is formed, then galaxies interact to form larger galaxies. Such findings are causing theorists to go back to their computers to try to alter their models to account for these surprising findings.

Expanding Galaxies

The Expansion of Space

There is solid evidence that most galaxies are expanding away from each other. There are exceptions such as the nearby Andromeda Galaxy, which is on a collision course with the Milky Way galaxy. However, most distant galaxies are not only expanding away from each other, but the farther they get from each other, the faster they move away. According to Einstein, space is being created between these galaxies, the galaxies are not physically moving.

How do we understand this

expansion of space? There are a number of models that have been used to illustrate the creation of space and the resulting expansion. One is a very large cake containing raisins spaced

Expansion of

space

Figure 2-11 Analogy for the expansion of space: raisins, representing galaxies, in a cake being baked and each raisin expanding away from every other raisin; the expanding dough represents the space being created. The raisin galaxies do not change shape or size.




randomly throughout the cake. Consider the possibility of being able to be a very small observer inside this cake sitting on one raisin, able to see the raisins through the transparent dough and without being able to see the top, sides or bottom of this very large cake (Fig. 2-11). Now, allow the cake to rise or expand in all directions. The raisins do not change their shape or size, but the dough expands at a rapid rate. Each raisin is expanding away from every other raisin. In this example, the expanding dough is an analogy for space being created, causing expansion. The raisins in this case represent galaxies. Since the cook was a little sloppy, some of the raisins were clumped together and stayed that way because the dough could not get between them.

Perhaps it is easier to comprehend a two-dimensional analogy, illustrated in Fig. 2-12. Take a rubber sheet and attach marbles to it. Now stretch the sheet in both the x and the y direction. The marbles will separate from each other, but they do not change size. The marbles here represent galaxies, NOT stars. There is no creation of space inside the galaxies and as a result stars within the galaxy do not move away from each other. Gravity and dark matter apparently hold stars within the galaxy. Each marble (galaxy) is separating from every other marble. The stretched rubber sheet is a representation of expanding space. To an outside observer, the marbles appear to be moving away from each other. From the point of view of an observer sitting on a particular marble, it is not moving, but all other marbles are moving away from it. In a similar manner, in the raisin cake analogy, all other raisins (galaxies) are moving away from each other in a three dimensional array of raisins.

The creation of space, according to this model, is still ongoing today, but at a slower pace than during the extremely short inflation period, and is responsible for the continued expansion of the universe. Thus, the term “Big Bang” is misleading. In an explosion, there is a center of the explosion. In this view, the expansion is caused by space being created between all galaxies, except those that have a strong gravitational attraction for each other because they are formed relatively close to each other. According to this model, matter was not flung out into pre-existing space, because there was no space! Instead, vast amounts of space are being created to drive the expansion of the

Fig. 2-12 A rubber sheet with a marked grid has four marbles fixed to it. It is stretched equally along both its width and breadth. The sheet (used as an analogy for expanding space) expands, carrying along the marbles, but the marbles (used as an analogy for galaxies) remain the same size. Each marble moves away from every other marble, just as galaxies do when space expands.

Expansion of

space


universe. According to this idea, the Big Bang still continues today – and yet we hear nothing!

The question naturally arises how big is the universe today? The answer is problematical because different suggestions apparently propose different sizes. One such proposal suggests a diameter of 1 followed by a trillion zeros centimeters, a number that is almost inconceivable.[ This number is not 1 trillion, which equals 1 followed by 12 zeros.] Others go all the way and have the universe expanding to infinity, with many associated philosophical problems as well as problems of comprehension. Ideas such as infinitely large bubble universes inside which other infinitely large bubble universes are created boggle the mind! Nevertheless, all models propose that our observable universe is a fraction of the total observable universe(s). Even though large numbers of atoms were attracted to each other to form galaxies, each galaxy is expanding away from every other galaxy at high speeds, except in certain closely spaced galaxy clusters. Measurements have indicated that the farther the galaxies are from each other, the faster is this expansion rate. It must be stressed that galaxies are not whizzing through space. Rather, space is expanding between the galaxies. We see the results of this in the radiation coming from stars. Earlier, we discussed the shifts in the very sharp spectral lines in the photons of light coming from distant stars. The energy and other characteristics of these photons change during their journey because these photons traverse through very large regions of space. In effect, the photons are stretched during their journey because space is expanding significantly. By measuring the amount of the red shift in starlight, scientists can measure the distance of the light-emitting star from the Earth. Because light travels through air at 3 x 1010 cm/sec, it takes 1 year for light to travel from a star that is 6 x 1012 (6,000,000,000,000 – six trillion) miles from Earth.

The NASA Hubble Space Telescope and other telescopes on Earth have explored regions of the sky that have red-shifted light that has traveled well over 10 billion years to reach us. The Hubble Deep Field experiment exposed one very small part of the sky for a long period to collect enough light to make visible the galaxies shown in the photograph to the left. Thus, this light pattern shown on the left indicates what galaxies looked like during the early part of the expansion of the universe and provide us with clues to the early history of galaxies similar to our own Milky Way galaxy. It is for this reason that astronomers are interested in

Far-away galaxies observed


observing the most distant galaxies, because the light from these galaxies may provide information regarding processes leading to and responsible for the formation of the first as well as subsequent galaxies and stars. Most experimental data that can be obtained from stars and galaxies are the photons streaming into the Earth from them. There are now a wide variety of photon energies that are being studied and that form the basis of any new cosmological models for the formation of the fine structure of the universe, the galaxies and stars. The highest energy photons are X-rays, which come mainly from the gases that are about to be sucked into the exceedingly strong gravitational pull of black holes. Ultraviolet radiation comes mainly from nests of newborn stars. Optical (visible) light originates mainly in ordinary stars. Infrared light comes from warm regions or cocoons of stellar nurseries and dust clouds. Each of these radiations will be stretched and its energy lowered according to the length of time it takes to reach the Earth from its source. Thus a UV photon emitted from a newborn star may appear on Earth as a visible photon because the space through which it is traveling is continuously expanding. By studying photons from a wide spectrum collected from one particular region of space, cosmologists, and astronomers are able to propose and support new models of galaxy or star formation.

The mystery of cosmic rays

Cosmic rays are very high-energy nuclei that bombard the Earth’s atmosphere. These particles move so fast that when they lose this energy slowing down in the atmosphere, they create a shower of secondary particles. It was generally thought that these cosmic rays were protons, but recent studies have shown some of these are nuclei as heavy as Fe (iron). This result was unexpected because heavy nuclei were thought to be much more fragile and would not survive the long journey from their source. Equipment is being constructed to study the particle shower pattern and use it as an astronomical tool to identify the source of the ultra high-energy particles.

Sources relatively close to Earth are prime suspects, especially active galactic nuclei, which are massive black holes that form in the center of many galaxies, including our Milky Way. These black holes literally consume streams of gas like wisps of smoke captured by an exhaust fan. Only nothing can escape, not even light, hence the name black hole. There is a region, designated the event horizon, beyond which nothing can escape being consumed. As the black hole feeds, the core of the galaxy lights up with brilliance of many million suns. The light comes from the excited atoms and molecules outside the event horizon. The black hole shoots out two opposing jets of particles at nearly the speed of light, along with shock waves that heat the surrounding gas causing a “bubble” around the black hole. These particles are the suspected source of cosmic rays that bombard the Earth. The other possible sources are from supernovae (see section below).


----------------------------------------End of advanced material-------------------------------- –––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––

Formation of Elements Other Than Hydrogen and Helium

The manner in which elements other than hydrogen and helium were created is of considerable interest because data on the distribution of elements in the universe have been used in testing the Big Bang model as well as models of the formation of galaxies, solar systems and planets. When combined with the theoretical predictions of the large amount of helium formation during the Big Bang, proposed models are tested with information regarding the detailed distribution of the chemical elements in the different building blocks of the universe. Stars that are much more massive than our Sun consume their hydrogen (proton) fuel in nuclear fusion reactions faster than in smaller stars. When large stars

are formed, there is more gravitational energy released, causing higher temperatures in the center of the newly formed star than in smaller stars. Therefore there are more frequent, highly energetic nuclear collisions in larger stars and stable helium nuclei (He-4) are formed more quickly than in smaller stars. In massive “red giant” stars (see illustration to the left), the hydrogen fuel is used up relatively rapidly, on the order of millions of years, and new types of

nuclear fusion reactions occur, including the “burning” of He-4 nuclei. The sequence of reactions leading to the production of He in stars is somewhat different from that responsible for forming He immediately following the inflation period. During this early plasma expansion period, there were neutrons, but the fuel in stars is only protons. The neutrons present in the expanding plasma have all decomposed. As protons are consumed in helium-forming nuclear reactions, the outward radiation pressure diminishes because of a shortage of the proton fuel and the helium nuclei collapse inward. This collapse converts more gravitational energy into heat energy, and temperatures reach even higher levels than before, increasing the number and intensity of collisions among nuclei. At these higher temperatures, on the order of hundreds of millions of degrees, helium nuclei, with their 2+ charges, collide with each other multiple times with sufficient energy to promote nuclear reactions that produce heavier nuclei such as carbon (C-12), nitrogen (14), and oxygen (O-16). To produce nuclei heavier than carbon or oxygen, the temperatures must be even higher in massive stars to activate the required types of fusion processes. Such processes take place in regions closer to the interior of the larger red giant stars. The closer to the higher temperature interior of the star, the larger are the nuclei being formed through fusion processes. In the largest stars, many different types of nuclear fuel are employed to form heavier elements, up to and including a critical element, iron. Fusion processes can only produce elements with an atomic number up to about 30; energy is given off by

Formation of the

elements heavier than

helium

Burning of helium nuclei

produces elements

with higher atomic number

Hydrogen fuel in star runs out:

what next?


the formation of all of these nuclei. Energy must be added to make nuclei with atomic numbers greater than 30, because nuclei of iron (Z = 26) and elements with atomic numbers close to iron are among the most stable nuclei known. However, there are no nuclear charge barriers to neutrons, so neutrons can enter many nuclei and form new elements with higher atomic numbers following decomposition of the resulting neutron rich nucleus. A limited number of neutrons are available in the interiors of stars, so there are a few neutron-induced elements formed by a slow process. However, luckily for us, there is an abundance of neutrons during supernova events.

Synthesis of elements in supernova

When the innermost core of a large star is converted into iron nuclei, the star has nearly reached the end of its life. Massive stars can have violent deaths, during which their core suddenly collapses inward, forming an unstable, super-dense, neutron-rich region. This collapsed core erupts, causing a catastrophic explosion that

tears the star apart in what is called a supernova (Fig. 2-13). During supernovae, exceedingly large numbers and very high densities of free neutrons are created. In a period of seconds, a large number of neutron-rich nuclei are created. As soon as they are formed, these highly unstable radioactive nuclei decompose by ejecting fast electrons (beta

decay). The products of this decay are lower energy, more stable nuclei, giving rise to an element with one higher atomic number. These new product nuclei absorb more neutrons to form new neutron-rich highly radioactive elements, etc. The neutron density is so large that, within a very short period, atomic numbers of product nuclei can build up very quickly. Very heavy elements with a large number of protons can be formed during the relatively short supernova explosion. For example, elements such as uranium (U), with an atomic number of 92, are created in supernovae. Astronomers know that these processes are occurring because they are able to observe light emitted from exploding supernovae that can be identified through line spectra as

Supernova- the result of star collapse

Formation of heavy elements

in supernova

Figure 2-13 Cycling of material causing the buildup of high atomic number elements. This figure shows the stages leading up to the supernova. “Recycling” in the above figure represents the “stardust” that is ultimately condensed in further stars that may, themselves, become supernova. A neutron star is a small, extremely high-density star composed mainly of very tightly packed neutrons. (NASA)



coming from specific known heavy elements. The character of the sharp lines in the light from these supernovae corresponds to that obtained from experiments performed on Earth with light emitted by these same elements at high temperatures. Supernovae explosions of very large stars fling material containing elements with high atomic number that we designate as “stardust” out into the universe (“Recycling” in Fig. 2-13). It is from this material that “second or third generation” stars such as our Sun are formed. It is proposed that through the birth and death of about a hundred million of these red giant stars, about one percent of the observable universe’s hydrogen and helium has been converted into the elements heavier than H and He that we observe in the universe today. Thus, scientists suggest that all of the known chemical elements were formed from one of four events: (1) hydrogen and helium formation immediately following the Big Bang; (2) the burning of protons, helium, and heavier nuclei in the interior of stars; (3) neutron capture deep with the core of relatively stable stars; and (4) neutron capture during supernovae explosions.

Formation of the Solar System

What is the fate of stardust ejected into space from supernovae? According to cosmologists, gravitational force causes the formation of very large regions of dust and gas clouds. One very small part of these regions contained the precursor to our solar system within the Milky Way galaxy, estimated to have formed thirteen billion years ago. The Sun and its planets originated as a result of the condensation of this stardust. We now explore the events thought to be responsible for the formation of the solar system and the Earth’s solid layers, its oceans, and its atmosphere.

Formation of the Solar System

There is a high probability that our Sun started in a cluster of stars, but that all the other stars ultimately were scattered, possibly by the effects of a massive star near the center of an enormous gas cloud. Scientists propose that our solar system formed from a nearly flat, spinning disc of supernova debris mixed with H and He atoms. Thus, the planets that formed from the aggregation of this dust have similar circular orbits around the Sun, as indicated in the sketch to the right. The Sun is composed of 99% hydrogen and helium. The remaining 1% is composed of elements with atomic numbers larger than that of helium. The existence of these heavier elements in the Sun identifies it as a second or third generation star, because some of the heaviest elements could only have been formed in a supernova. Cosmologists calculate that the Sun formed 4.6 billion years ago. The Sun was probably the first object formed in our solar system. The Earth and the other terrestrial planets (Earth, Mercury, Venus and Mars) were then probably formed

Gravitational attraction of

stardust particles to form new

stars



from aggregated solid material contained in the disk in the vicinity of the Sun.

The nine original planets (now 8) in our solar system can be divided into the small, solid terrestrial planets, the giant Jovian planets (Jupiter, Saturn, Uranus and Neptune), and the very small Pluto, whose orbit is slightly different from all the other planets and whose classification as a planet has been successfully challenged. The Jovian planets are all large balls of very hot gas, primarily protons, and are farther away from the Sun than the terrestrial planets. Each of the planets has been found to be unique. The reasons for these differences are not currently understood. There have been no explanations thus far for the unique character of the Earth that has provided the conditions for life that thus far have not been found elsewhere in the solar system or in the universe. An early, violent reshuffling of early bodies may explain such characteristics such as the pockmarks on our Moon and other strange characteristics of our solar system. The intense radiation emanating from the young Sun, called the solar “wind,” probably blew away from the nearby terrestrial planets most of the lighter gaseous elements (e.g. helium and hydrogen). Before the Earth’s magnetic field formed, there was no protection from the solar wind. Primarily solid material remained to form the Earth and the other terrestrial planets closest to the Sun. This separation of gaseous and solid materials is thought to be responsible for the different compositions of the planets and the Sun. The four dominant elements on Earth are: oxygen (O), magnesium (Mg), silicon (Si), and iron (Fe), all of which were probably present in condensing solid matter during the formation of the Earth as magnesium oxide, silicon dioxide, and iron oxides. These four elements make up 85% of the mass of the terrestrial planets.

The Search for Extra-Solar Planets

A very active area of astronomy at present is the search for rocky exoplanets, planets that orbit stars other than our Sun, that have conditions that are either Earth-like or


might be capable of supporting life of some kind. Over 400 exoplanets have been identified as of this writing. Masses and distances from their stars as well as elements in their atmosphere have been measured, but very few candidates have been found with Earth-like conditions. However, given that it is not easy to identify these planets, their current abundance is a good indication that there are probably millions, if not billions of exoplanets in the rest of the universe. As data from new space-based observatories dedicated to the search for exoplanets are analyzed, the pace of extra planet discovery is about to explode. Models predict that even those rocky planets that are larger than Earth may have atmospheres and climates favorable to living systems.

Water is a critical molecule that would need to be present in liquid form for life as we know it to exist. However, scientists can envision other forms of life without the presence of oxygen similar to that found in Earth’s own deep oceans. Planets have been observed with methane atmospheres that could support living systems. Molecular precursors for living systems on Earth have been found in deep space, so that the raw materials for living systems are available elsewhere in the universe. It is increasingly probable that living systems will be discovered in locations other than the Earth. There are some hints of life on Mars and Saturn’s moon Titan, but these have not been confirmed.

We are, of course, most interested in those in nearby star systems, so that if intelligent life does exist, there might be a possibility of communicating with them. For years, there has been an intensive search for such communications, but no credible contact has been made.

Leftovers in Orbit

Not all of the matter that was spinning around the Sun condensed into planets. Asteroids and comets are two different classifications of this solar refuse that orbit around the Sun. Asteroids are any of the numerous small celestial bodies composed of rock and metal that move around the Sun (mainly between the orbits of Mars and Jupiter). A comet is a relatively small extraterrestrial body consisting of a frozen, often icy mass that travels around the Sun in a highly elliptical orbit. There is great interest in determining the composition of comets because their compositions probably represent that of the primordial material that formed the solar system.

Every day hundreds of millions of small pebble size bits of asteroids and comets enter and harmlessly burn up in the Earth’s atmosphere, sometimes observed as “shooting stars.” However, occasionally asteroids of a size that do not burn up completely in the atmosphere directly impact the Earth’s surface. In 1908 a rocky asteroid about 60 meters in diameter is thought to have impacted the Tunguska region of Siberia and flattened 2,000 square kilometers of forest with a force of around 10 megatons of TNT. If an asteroid 100 meters in diameter were to land in the ocean, it would cause a tsunami of monumental proportions, killing millions of people in coastal cities. It is thought that an asteroid 10km (6 miles) wide was responsible for a direct hit on Mexico’s Yucatan peninsula that caused the extinction of the dinosaurs

Exoplanets

Asteroids and

meteorites


and many other forms of life around the Earth. Efforts are being made to identify and track asteroids that might be candidates for future collisions with Earth. It is the consensus that the use of a nuclear weapon to demolish such an asteroid would lead to unpredictable consequences, and some planning efforts are focused on identifying and steering asteroids gently out of an Earth-impact trajectory. Some question whether these efforts are insufficient, given their potential consequences.

The Kuiper belt is a doughnut shaped belt of thousands of icy chunks beyond

Neptune in a circular belt in roughly the same region as Pluto. Scientists believe that the material in this belt is debris from the original disk that formed out solar system that wasn’t used to form the Sun or any of the planets. Therefore, its composition may represent that of this original disk. The density of objects in this belt is so low that the likelihood of a collision between any two of them is very low. The materials in the Kuiper belt and the disturbed orbits of many of the objects in this belt may help to unravel the history of the formation of our solar system.

Formation of the Earth

The Earth formed about 4.5 billion years ago, according to scientific dating techniques. There are two theories concerning its origin, one micrometer size grain-by-grain, chunk-by-chunk, relatively slow aggregation, with kilometer size and larger solid objects in space being attracted by gravity until the primitive Earth was formed. The gravitational collapse of larger particles during the Earth’s formation created immense amounts of heat energy and very high temperatures. This heat energy probably caused the formation of pockets of gas and molten rock, which escaped to the surface from deep in the Earth, giving rise to a large amount of volcanic activity

in the primitive Earth (Fig. 2-14). In addition, there were undoubtedly many severe impacts by comets and meteorites on the Earth’s surface, causing deep craters and cracks that allowed hot gases such as water vapor to escape from deep inside the

Formation of

the Earth

Figure 2-14 The structure of the differentiated Earth, illustrating the proposed processes that contributed to the evolution of the Earth’s early atmosphere. (R. P. Turco, Earth Under Siege, Oxford Press 1997, p.87)




Earth. At this stage, the Earth must have been an exceedingly inhospitable planet for life. One theory suggests that asteroid strikes were numerous during its early history and that nine major strikes between 3.8 and 2.5 billion years ago might have helped trigger the formation of the Earth’s continents. There is evidence that iron and nickel in this newly aggregated hot planet liquefied because of the intense heat and their high density flowed to the core of the Earth (Fig. 2-7). These metals formed an outer liquid core surrounding a solid inner core, both composed predominantly of iron with small amounts of nickel. Convection currents of the outer liquid metal core are thought to produce the Earth’s magnetic field. Early in its history, this high density, outer molten core forced lower-density, higher-melting solid silicate materials, composed of silicon (Si), oxygen (O), iron (Fe), aluminum (Al), and magnesium (Mg), up away from the liquid core toward the surface of the Earth. Because of the large number of meteor and comet impacts as well as volcanic activity during its early history, gaseous matter was probably forced out of the Earth's hot interior to the surface to become the Earth's atmosphere, a process known as outgassing. However, some scientists believe that water supplied by these same impacting meteorites may have been at least part of the source of our oceans and Earth’s other water resources.

Water, carbon dioxide, and nitrogen (N2) are thought to be among the dominant gases that constituted Earth’s early atmosphere.

Changes in the Earth’s Crust

Following the Earth’s formation, the overall composition of the Earth and its atmosphere has remained essentially constant, but the surface of the Earth has changed markedly and continuously since its formation. Three main forces bring about this change: weathering, release of heat from the Earth’s interior, and processes associated with the evolution of living species. The heat has come from both the original heat of gravitational compaction and the heat generated from radioactive decay within the Earth. Weathering is the result of water and wind interacting with rocks and sediments, dispersing materials over the landmasses and into the sea. New life forms probably evolved in some region of the Earth that contained water. These and succeeding life forms have also contributed to the transformation of the Earth’s surface. The release of oxygen (O2) by these life forms caused transformations in the crust of the Earth that ultimately allowed a buildup of O2, which allowed more advanced life forms to emerge that further changed the Earth’s surface. The release of heat from the hot, molten interior of the Earth is somewhat like pouring steaming hot water into a thick, cold ceramic coffee mug. The outside surface of the coffee mug is initially cold, but it gradually becomes hot and is cooled by the air surrounding the hot surface of the mug. The flow of heat in both the mug and in the Earth is from the interior to the exterior surface. A fundamental principle of

Changes in the Earth

following its

formation


science is that heat always flows from hot to cold regions, never the other way. The heat flow in the coffee mug does nothing to alter the structure of the ceramic mug, that is, of course, if it does not crack the mug. However, the flow of heat from the hot interior of the Earth to its cool surface has a major dynamic effect on the Earth’s structure.

The two outer sections of the Earth are designated as its lithosphere, which consists of the mantle and crust (Fig. 2-15). The crust at the Earth’s surface is very thin (6-70 km) in comparison with its radius (6378 km at the equator). The part of the crust under the oceans is thinner than that beneath the continents. Even though it is solid, the upper portion of the mantle is pliable and can flow very slowly when it is warm or under pressure. Heat is generated in the interior of the Earth by the radioactive decay of certain potassium (K), uranium (U), and thorium (Th) isotopes. To help release this and other heat stored in the interior of the Earth, the mantle moves slowly in the cyclic manner called convection. This mantle movement induces a slow movement of the Earth’s surface crust. In this manner, a significant part of the crust has been formed and reformed over the Earth's history. The surface of the continental crust, the only part of the Earth’s crust that is not recycled, is

continually being modified by weathering processes, volcanic activity, and crustal movements. The convection process described above causes continual movements of the inner mantle and outer crustal portions of the Earth, called continental plates. The movement of these continental plates, called continental drift is induced by molten lava seeping into cracks between the plates at the ridge, forcing the plates apart. Continental plate movements and the resulting changes in the Earth’s crust explain the current shape of the continents and the regional nature of volcanic activity. The Earth’s magnetic field, caused by the internal rotation of the molten metal core, was formed before 3.4 billion years ago. This timing was critically important in deflecting highly energetic particles from the Sun that could strip away the Earth’s oxygen-containing atmosphere, evaporate water, and snuff out life on its surface. Microfossils more than 3 billion years old demonstrate that precursors to human life date back to these critical times, so the development of this magnetic field literally saved life on Earth.

Structure of

the Earth

Fig 2-15 Symbolic model of the Earth showing the inner cores of the Earth and the effects of the slow mantle circulation on the lithosphere plates that move apart because of the mantle circulation, filling in new crust at the ridge and recycling lithosphere below the trenches.


During early times, there were apparently not enough greenhouse gases (chapter 6) to keep the oceans from freezing. The Sun was weaker than it is now, so there must have been something that absorbed a large fraction of the Sun’s incoming photons. Scientists propose that the ocean was darker than it is today and there were fewer clouds, making the Earth’s surface less reflective, thus making the oceans warmer. However, these ideas may not be sufficient to explain why the early Earth didn’t freeze.

Composition of the Earth's early atmosphere

The exact composition of the Earth’s early atmosphere is unknown, but there is experimental evidence that indicates that before 2.4 billion years ago, oxygen was a minor constituent of the Earth’s atmosphere, less than 1%. During the extensive volcanic activity of the first million years of the Earth's existence, the majority of the gases that formed the earliest atmosphere probably were expelled from the interior of the Earth. This expulsion is thought to have continued over the next four billion years to form the rest of the atmosphere. Along with water (H2O) vapor, the early atmospheric gases are thought to have been carbon dioxide (CO2) and nitrogen (N2). Trace gases, gases present in very small concentrations in comparison with other gases, probably included methane (CH4), ammonia (NH3), sulfur dioxide (SO2), and hydrochloric acid (HCl). In most respects, the Earth’s early atmospheric composition was probably comparable to that currently found in the atmospheres of Mars and Venus. Water vapor was probably abundant in the Earth’s early atmosphere. There are persistent claims that much of the Earth’s current water supply was delivered to Earth by asteroids and helped to form the oceans. As the Earth's temperature cooled, water vapor must have condensed into liquid water, creating the Earth’s hydrological cycle, the Sun-driven cycling of water through lakes, rivers, oceans, underground movement (aquifers), and evaporation into the atmosphere (Chapter 3). This cycle helped set in motion the weathering of rocks, which probably removed large amounts of atmospheric carbon dioxide by forming carbonate minerals. The oceans took up vast quantities of carbon dioxide. Reduction in the atmospheric carbon dioxide concentration prevented the atmosphere from trapping excessive quantities of solar heat energy. This heat trapping, which is the basis of the greenhouse effect (Chapter 6), kept temperatures on Earth low enough so that water could remain in the liquid

The Earth’s early

atmosphere

Cooled, outgassed

water and the

hydrologic cycle


state, which is critical to sustaining life. Had the carbon dioxide concentration remained what it was in the earliest days of the Earth's existence, the planet may have become very hot and lifeless, similar to that of Venus. However, about a billion years after the Earth's formation, organisms, whose origins are unknown, apparently multiplied in the Earth’s ocean to large enough quantities that they consumed a very high percentage of the atmospheric carbon dioxide by incorporating the carbon atoms it into their structures. As these organisms near the surface of the ocean died, a significant portion of their remains (containing the carbon from the absorbed carbon dioxide) dropped to the sea bottom, delaying the release of carbon dioxide back into the atmosphere. At the same time, living organisms formed oxygen (O2) as a byproduct of sunlight-induced synthesis of

biological molecules.

Slow evolution of oxygen in the atmosphere

Oxygen reacts readily with many of the elements that are thought to have been present on the surface of the early Earth. Very small amounts of oxygen may have present at this time. However, as oxygen was produced early in the Earth’s history by primitive forms of aquatic life, it probably reacted with any of those elements that were exposed on the Earth’s surface until these reactive elements were depleted. Only then

did the oxygen concentration start to build up to current atmospheric levels, according to current theories. This change in oxygen concentration is thought to have taken place between two and three billion years ago. Primitive life forms could and still do exist without oxygen. However, animal life, with its complex cellular structure, was able to emerge only after the atmosphere had sufficient oxygen some three to four billion years after the Earth's formation. About 2.4 billion years ago, possibly even much earlier, in what is designated the Great Oxidation Event, oxygen arose on Earth with the inception of photosynthesis, the process by which green plants and some other organisms convert photons from the Sun, carbon dioxide (CO2), and water (H2O) into foods and other plant compounds. The other crucially important product of this photochemical reaction is molecular oxygen (O2). The identity of the first photosynthetic organisms is unknown, but may have been some form of bacteria. However, a recent proposal suggests an earlier source preceding bacteria. The Great Oxidation Event marked the beginnings of a drastic change in the Earth’s surface, setting the stage for oxidative weathering of the continents, successive changes in ocean chemistry, and the eventual rise in multicellular life.

Once sufficiently high oxygen (O2) concentrations were established in the atmosphere, an ozone (O3) layer (Chapter 3) developed in the upper atmosphere. This

Primitive forms of life responsible for oxygen evolution

Oxygen production caused the creation of the ozone

layer


layer filtered out life-threatening ultraviolet radiation from the Sun. Formation of the ozone layer allowed life to emerge from deep waters, which had previously acted as a filter to keep damaging radiation from reaching and destroying the earliest forms of life. This ability of life to emerge from the sea onto land has been proposed as a major step in the evolution of higher forms of life. At this time, the atmospheric oxygen concentration rose to its current level. The various chemicals on the Earth's surface, previously in an oxygen-limited environment, now found themselves in an environment with abundant oxygen. About 500 million years ago, the percentage oxygen rose to about 20% and, except for a peak of about 30% around 300 million years ago arising from lush growth and burial of plants responsible for our currently used fossil fuels, has stayed remarkably steady at around 20%. Iron and other elements were chemically transformed by reaction with oxygen. These chemical reactions formed a barrier to the further penetration of oxygen deeper into the soil. Because there is a nearly impenetrable crustal barrier between the oxygen-containing atmosphere and the chemicals that readily react with oxygen below the barrier, the oxygen in the atmosphere remains at a level high enough to sustain life. In the absence of this crustal barrier, nearly all of the atmospheric oxygen would be consumed in chemical reactions, and there would probably be insufficient free oxygen to support current life forms.

Molecules Formed in Space

Scientists propose that massive first generation stars exploded when they began consuming new fuels such as helium and carbon nuclei in their nuclear reactions. As a result, carbon (C), nitrogen (N), and oxygen (O) nuclei were ejected into space, joining with the abundant H and He nuclei already present from the Big Bang. The likelihood of these atoms colliding in space with each other and forming chemical compounds is exceedingly small. These atoms probably collided with and adsorbed to

fine grains of space dust composed of carbon (C) and silicon (Si). Therefore as millions of years passed, the probability of C, H, N and O atoms being on the same particle increased. Even at the very low temperatures of these interstellar dust cloud particles (~ – 440 ºF), chemical reactions among these adsorbed atoms are possible. Thus, as these dust particles were attracted to each other and condensed into larger planets or stars, with increasing temperatures, more chemical reactions could occur and the increasing temperatures

Molecules formed in or near stars

Fig. 2-16 Birthplace of stars in the Eagle Nebulae. Gravitational attraction of the stardust in the dust clouds is creating new stars. This dust is the result of previous supernovae explosions.


could cause the resulting chemical compounds to evaporate or desorb from the solid material and escape into space. Light from stars and dust clouds has been extensively analyzed in a search for the existence of molecules in outer space. Modern infrared and radio telescopes detect such organic molecules in deep space. In this case, an organic molecule is designated as one that contains primarily carbon (C), hydrogen (H), and possibly oxygen (O) and nitrogen (N) atoms chemically bonded together (Chapter 3). Early in this research, indications were found of the formation in space of simple molecules such as CO, carbon monoxide, an inorganic molecule, since it has no hydrogen. However, well over a hundred chemical compounds have been discovered through this type of research. Recent research on comets and interstellar dust has identified rather complex molecular solids. Some of these solids are silicon-containing solids, which might be expected because of the extensive amount of silicon in the Earth. More surprising, there have been solids containing compounds with extensive networks of carbon atoms that also contain smaller amounts nitrogen and oxygen. Because of experiments carried out on Earth under outer space conditions, scientists propose that highly complex compounds can be formed in these solids under these conditions, including amino acids and precursors to RNA and DNA (Chapter xx). Further, they propose that the compounds in these grains of dust survive the severe conditions of interstellar space. These speculations now have experimental support.

Recent analyses of the contents of well-documented meteorites have revealed that these visitors to Earth from outer space contain millions of organic compounds. These compounds include many those that also can be created in outer space simulations on Earth. Among these and of special interest are critical molecular precursors to proteins and to DNA and RNA, the molecules that are key to life itself (Chapter xx).

Highly energetic ultraviolet (UV) radiation and cosmic rays are abundant in

outer space and can fragment any molecules present in space or that are adsorbed to very cold dust grains. The resulting molecular fragments are highly reactive chemically but may not react because of the very cold temperatures. As the dust particles containing these fragments coalesce into larger fragments and as they become warmer from a nearby supernova, these fragments can move around on the particles and combine into larger chemical compounds or react with molecules to form larger fragments. In this manner, a large variety of molecules can be formed. More complex reaction products can survive in dust clouds because they shield these products from UV. When dust clouds condense to form stars, some of the solid material that does not condense into the star can survive with complex compounds in asteroid belts or meteoroids.

Organic chemical

compounds in space


Life in the Universe and On Earth

Scientists believe that if there were life on Mars, it could have made its way to Earth by “piggybacking” on meteorites ejected during an impact of a large meteor on Mars. The same is true of meteorites ejected from the Earth landing on Mars. These results have led scientists to speculate that life can arise spontaneously under many different circumstances in many different parts of the universe, given the appropriate conditions. There is a continuing effort to communicate with other intelligent life forms in space because of this supposition, thus far without success. As the search for exoplanets becomes more sophisticated, scientists are now focusing on the exploration of the atmospheres of these planets. For example, the existence of oxygen molecules (O2) in significant quantities would probably indicate photosynthetic processes taking place on the exoplanet.

Moderate temperatures on the Earth’s surface or the oceans made it possible for life to possibly evolve and survive. The distance from the Sun, the intensity of the sunlight, the ability of Earth to reflect light and re-emit absorbed light energy, and the amounts of greenhouse gases (Chapter 6) in the Earth's atmosphere all combine to make these conditions possible. The greenhouse gases helped trap energy from the Sun in the form of heat energy, thus increasing the temperature of the atmosphere. It is useful to compare Earth with Venus. The two planets are comparable in size and in bulk composition, but differ significantly in their habitability. The ground temperature of Venus is 430˚C (806˚F) because most of the carbon on that planet is present in its atmosphere as carbon dioxide (CO2), a greenhouse gas that efficiently traps heat energy coming from the sun. In contrast, most of the carbon on Earth became locked up in other, non-gaseous forms. Today carbon dioxide is a trace atmospheric gas with a relatively low concentration, even though it is rising to potentially dangerous levels (Chapter 6). Most important for life, water has been available in liquid form throughout most of the history of the Earth, despite the relatively narrow temperature range in which water is a liquid (between 0˚ and 100˚C). Water would be of little use to support life as we know it as ice or steam. The amount of water available also is of importance. During its evolution, the Earth has been able to retain just the right amount of water for life to occur on land and in the sea. If the Earth had too much water, there would be little land surface. If it had too little water, there would be vast deserts. Over the last several billion years, the total amount of liquid water apparently has been constant. The above considerations assume the early existence of life on the Earth. The earliest fossils of living organisms found on Earth are apparently 2.1 billion years old, just after the Great Oxidation Event at 2.4 billion years ago. There are now a number of theories about possible chemical processes leading to the creation of the first living cell. Molecules that are needed to build the key molecules of life such as DNA, RNA, and proteins (see Chapter xx) can be synthesized in a number of different experiments. In some of these experiments, using only ingredients and conditions thought to be present in the early Earth, scientists have prepared key nucleic acids

Moderate Earth

temperatures because of greenhouse

gases

Liquid water makes life possible


(Chapter xx), which are the chemical keys to self-replication. In other experiments, gaseous molecules that may have been present in the early Earth’s atmosphere were exposed to ultraviolet light, electrical discharges, or gamma radiation. There seems to be a natural tendency to form the chemical building blocks of critical molecules that are precursors to DNA and proteins. In some experiments, molecules synthesized from a crude mixture of atmospheric gases, formed an oily substance which self-assembled into a membrane-like film when added to water. There are large numbers of organisms living deep within the Earth and under the sea floor, where there is no oxygen and very little liquid water. Drilling deep under the seabed as well as deep mines has produced living bacteria. One shock has been the estimates that the amount of life below the surface of the land and oceans is greater than that at and above the surface. Such ideas have caused scientists to reevaluate their ideas about natural cycles, especially in the sea. Life has been found in unlikely places such as steaming pools in Yellowstone Park. These organisms, called extremophiles, demonstrate that living organisms can tolerate extreme temperatures, pressures, and even radiation environments. During the history of the Earth, there have been many periods where catastrophic changes took place, causing extinction of many different species. The reasons for these mass extinctions are still being debated. Among the proposed causes are meteor impact, volcanic activity, and major climate changes. Many hypotheses have been proposed for the extinction of the dinosaurs, but none have been proved. Less discussed is the near extinction of humans. Although the global population of homo sapiens is approaching 7 billion, they were apparently threatened with extinction on the African continent around 150,000 years ago. Nevertheless, they survived very cold, dry conditions of that era on the southern coast of Africa, where seafood and plants were plentiful. Paleoanthropologists estimate that there were on the order of just hundreds of homo sapiens left at this critical point in human history. (More on human migration patterns based on DNA analysis in chapter xx)

Humans did survive, but they still ask the question, how did it all start? No experiments or theoretical studies have come close to bridging the gap between molecular raw materials that compose living substances and the deliberate synthesis of a simple living cell containing the highly complex chemical processes responsible for life. There is intense study in this area, with no significant breakthroughs at present. Some say it is impossible. Others say that synthesis of living cells is just a matter of time.

The role of ecosystems in the evolution of life

Some scientists propose that life has emerged and evolved over many billions of years and that there were periods of slow and rapid evolution. Other scientists believe there was a gradual evolution of living species. In either case, the survival of newly evolved species depended on how these species fit into their ecosystem. Those species that survive fit into and indeed help define their ecosystem. Today, humans find themselves living in natural ecosystems that are not fully understood. This is


especially true when one or more parts of their ecosystem are altered and the ecosystem as a whole responds in an unexpected manner. Humans are altering

worldwide ecosystems with unpredictable consequences, as we will try to elaborate in the following chapters.

In Conclusion

According to the modern theories of stellar evolution, all of the chemical building blocks of all living species, including

ourselves and our environment, were created: (a) during the early moments of the Big Bang; (b) in the interior of massive stars; and (c) in the momentous supernovae explosions that spewed matter out into the universe. The elements that formed from these three processes ultimately condensed through gravitational attraction to form galaxies, our solar system, and planet Earth. If these theories are correct, our environment and we are, indeed, composed of "stardust!" However, the manner by which we, living human beings, came to be who we are today is a far more complex story.

Summary

1. What are the current theories of the formation of the universe and its galaxies?

The majority of cosmologists favor the inflation/Big Bang model as an explanation of the origin of the universe some 13.75 billion years ago. The following are proposed as the major steps in the process:

(1) A sudden expansion or inflation of an incredibly small, super-dense

region containing all the energy currently in the universe. This expansion came about because of the creation of space and did not involve expansion out into already existing space.

(2) After the inflation period and while further expansion progressed, the

universe cooled rapidly from multi-billion degree temperatures through a sequence of events that included:

(a) formation of free neutrons that decomposed into electrons and

protons, forming an exceedingly high temperature plasma; formation

Summary


of dark matter distributed throughout the universe; (b) formation of helium nuclei through nuclear reactions during the first few minutes in the super-hot plasma; (c) cooling during the expansion due to expansion of space and

formation of regions of slightly increased density of dark matter; cessation of nuclear reactions.

(d) formation of neutral hydrogen and helium atoms about 380,00 years

ago with the release of plasma-trapped radiation from all parts of the universe at about the same time, radiation that is still observable today;

(e) gravitational collapse of slightly increased density regions that

ultimately led to the formation of galaxies and billions of stars within each galaxy; the role of inhomogeneous regions of dark matter in this process is suggested; haloes of dark matter attracted visible matter, which condensed into massive stars

(f) fusion reactions within stars forming helium and elements heavier than helium such as carbon, nitrogen, oxygen, and on up to iron in the heaviest stars; (g) gravitational collapse of more massive stars to form supernovae with the resulting creation of heavier elements and ejection of these solid materials into space as “stardust;” (h) aggregation of this stardust with other interstellar matter into new

stars and planets, including the solar system and the Earth. (i) continuing expansion of the distant galaxies from each other at an

accelerating rate; dark energy is suggested to play an important role in this repulsive acceleration.

2. What is the evidence for these theories? (a) cosmic background microwave radiation observed everywhere in space

with no single point of origin; characteristics of this radiation are exactly those predicted; the structure of this radiation has been a goldmine for cosmologists and has, among other things, led them to conclude that dark energy and dark matter are needed to explain these results

(b) the abundance of the various elements in the universe, especially the

relatively large amounts of helium found distributed throughout the universe;


(c) analysis of light from supernovae indicating the formation of heavy elements. (d) astronomical observations using many different techniques and analyses

of the radiation emanating from starts and galaxies. 3. What is thought to be the origin of the solar system and the Earth? Debris (stardust) from supernovae along with hydrogen and helium is thought

to have aggregated in a very small region of the Milky Way into a whirling disk of dust and gas. The Sun probably formed first in the center of this disk, followed by the Earth and the other planets. The Earth probably formed by a process of accretion that generated large amounts of heat. During an early period in the evolution of the solar system, intense solar winds swept away light gaseous material from the region around the Earth, leaving heavier elements that formed a rock-crusted planet. Temperatures were high enough to melt iron and nickel, which formed a liquid core and forced lighter solid metal oxides to the surface.

4. What major changes have occurred on the Earth throughout its history? Following the Earth’s initial aggregation into a solid planet, the following series of events are postulated: a. The hot Earth melted elemental iron and nickel, creating a dense central

core. b. A semi-solid mantle formed from less dense matter covering the core. c. The least dense solid outer crust of the Earth continuously changed during

its history, driven by heat released from the inner regions and by weathering.

d. Water (H2O) and other gases such as nitrogen (N2) and carbon dioxide

(CO2) were expelled from the interior of the Earth to form an atmosphere. Water may have also entered the atmosphere on meteorites and comets.

e. As the Earth cooled, water vapor condensed to form oceans, lakes, and

rivers, initiating the hydrological cycle driven by radiation from the Sun. f. Primitive life forms were created by unknown processes from unknown

sources approximately 2 billion years ago or earlier. g. Photosynthetic life in the seas consumed CO2 and generated O2. Most of

this CO2 was deposited either in the ocean or on the ocean bottom, removing vast quantities from the atmosphere, thus keeping the


temperature of the Earth relatively low in comparison with other planets close to the Sun.

h. O2 built up slowly in the atmosphere after first reacting with crustal material

to form different minerals, and the ozone layer was formed. i. Complex life forms on Earth evolved due, in part, to the presence of

oxygen and liquid water. 5. What special conditions have led to our current situation on the Earth? a. Conditions on Earth have allowed water to remain in the liquid form

continuously for many billions of years. Without liquid water, life as we know it would not have been possible.

b. Without a crustal barrier on the surface of the Earth, oxygen in the

atmosphere would have been used up, and complex life on land could not have evolved. Oxygen allowed the formation of the ozone layer, which protected living species from harmful ultraviolet radiation from the Sun.

c. The complexity and diversity of life is thought to be dependent on the final

increases of the oxygen concentration in the atmosphere.

Review Questions

1. Was it possible for anyone to hear the "bang" from the Big Bang? 2. As the early universe expanded, it cooled. What difference did this make in

the average speed of the particles present in the expanding plasma? What role is dark matter supposed to have made in this process?

3. What processes can occur in a hot plasma containing electrons, protons, and

neutrons as the mixture cools from billions to millions of degrees? 4. Why is the universe composed mainly of hydrogen and helium? 5. What is the reason for a supernova? 6. What is the chemical result of a supernova?

7. What are the reasons for galaxy and star formation? What role does dark

matter play in these processes? 8. Why are both our Sun and other stars composed of a relatively small

percentage of elements heavier than helium?


9. Why is it thought that there is so little gaseous helium and hydrogen present in

the Earth’s atmosphere despite its abundance in the rest of the universe? 10. Why is it thought that the Sun’s planets have orbits that all lie in the same

plane? 11. In what manner do you think the Earth was formed? 12. In what ways do the terrestrial planets differ from the Jovian planets? 13. What is thought to have been the source of the Earth’s: (a) original atmosphere shortly after the Earth’s formation? ; (b) water for its hydrological cycle?; (c) oxygen in the current Earth atmosphere? 14. Why does the current atmosphere of the Earth have very little of some of the

gases such as carbon dioxide that are thought to have been present in its atmosphere shortly after the formation of the Earth?

15. Why have complex forms of life emerged only after the buildup of significant

amounts of oxygen in the atmosphere? 16. What kind of a planet would Earth have had if carbon dioxide had not been

removed from the early atmosphere? 17. Why is it so surprising that water has been mainly in the liquid form for most

of the Earth’s history? What significance has this made to the maintenance of life?

18. Why is the composition of the Earth’s crust different from that of its interior? 19. Why is the core of the Earth thought to be iron and nickel? Why not some

other metals? 20. What is the source of heat energy in the Earth's mantle? 21. By what mechanism is heat in the Earth’s mantle released to the Earth’s

surface? 22. Life has continued to evolve over long periods of time in the Earth’s history.

What is the connection between this fact and the dependence of the state of water on temperature?


23. Why is it thought that life and the atmosphere have coevolved? 24. What would happen to the atmospheric oxygen concentration if there were no

barrier to oxygen between the interior of the Earth and the atmosphere?

Discussion Questions

25. Consider two different collisions that occur between a proton and deuteron. One produces a nuclear reaction with new products and the other produces nothing but the original collision partners moving in different directions. What might be the difference between the conditions during these two collisions?

26. A heavy object is dropped from a tower and impacts with a "thud" on the

ground. Immediately after the object lands, the temperature of the earth beneath the object is measured and is found to have increased following the impact. Why did this happen?

27. List two “models” of the universe you have heard or read about. Which of

these can be characterized as scientific? Justify your reasoning in each case. 28. Do you personally believe in the Big Bang model? Why? Why not? 29. Do you support the expenditure of federal funds for projects that: (a) explore

the solar system? (b) explore other galaxies? (c) search for intelligent life on other planets? Defend your answers.

30. Why does it take so much energy to lift space vehicles off the Earth’s surface

during rocket launches? Why do these vehicles need heat shields during reentry?

31. Why are Hubble Space Telescope astronomers focusing so much attention on

the most distant stars and galaxies?

Projects

32. (Semester project) Trace the steps that take place during the transformation of a proton, created during the Big Bang, from its creation in the Big Bang to its final stages as a carbon nucleus in a muscle of a student doing aerobic exercise. You'll have to use your imagination for many of these steps. Creativity is encouraged within the bounds of the chemistry known to you. You will probably have to do some additional research beyond this book. Keep a continuing log for this problem as you learn more during each chapter.


33. Construct a timeline containing each of the events described in this chapter. 34. For each of the following elements, pick a final fate for the element in today’s

world and an initial source of its creation and construct a rough outline of a plausible history of that element from its creation to its current status. The elements are: He, C, O, N, Fe, Au, U.

35. Search the World Wide Web, using a Web search engine such as Google,

Yahoo, Dogpile, etc., to acquire additional material on the following topics: Big Bang, supernova, astronomy, stars, comets, creation of the universe, etc. Be sure to visit the home pages of NASA. Look especially for materials that either conflict with your text or expand the information contained in it. Call your instructor's attention to any sites with conflicting information by listing their URLs (Uniform Resource Locators) with a summary of that information. Consult your library on the same topics and compare the quality of information.

Readings

Cowen, R., Mapping the Earliest Moments of Time, Science News, April 11,2009, pp16-20. Earliest moments during the Big Bang explored by a new sattelite. Hand, E., The Test of Inflation, Nature, April 16, 2009, pp 820-824. A more technical review of the early moments of the Big Bang. Turner, M., Origin of the Universe, Scientific American, September, 2009, pp36-43. Cosmologists are closing in on processes that created the universe Cowen, R., Beyond Galileo’s Universe, Science News, May 23, 2009, pp 22-28.The puzzles of dark matter and its effects on astronomy and cosmology. Jenkins, A., and Perez, G., Looking for Life in the Multiverse, Scientific American, January, 2010, pp 42-49. Universes with different physical laws might be inhabitable. Plait, P., Invisible Planetoids, Discover Magazine, July/August, 2010, pp34-37. Search for planets that might harbor life. Sasseliv, D. D., and Valencia, D., Planets We Could Call Home, Scientific American, August, 2010, pp38-45. Theoretical thoughts about Earth-like exoplanets. Ricardo, A., and Szostak, J. W., Life on Earth, Scientific American, September, 2009, pp 5461. Clues about how the first living organisms arose from inanimate matter. Grant, A., Cosmic Blueprint of Life, Discover Magazine, November, 2010, pp 39-44. How life may have been imported from space.


Ehrenberg, R., The Final Chemistry Frontier, Science News, January 30, 2010, pp 26-29. Molecules found in space – what are they, and how they might be made? Quill, E., Can You Hear Me Now?, Science News, April 24, 2010, pp 22-25. Searching for signals from outer space. Bradley, A. S., Expanding the Limits of Life, Scientific American, December, 2009, pp62-67. How life may have evolved in undersea hot vents. Lanzsa, R., and Berman, B., The Biocentric Universe, Discover Magazine, May, 2009, pp 53-55. The revolutionary idea that we create the universe by observing it.

Documents

CHAPTER 2 ARE WE MADE OF STARDUST?contemporarychemistry.com/ReviewerChapters/Ch2_BigBangBtns.pdf · Chapter 2 Is the Earth Made of Stardust? Page 2-1 CHAPTER 2 ARE WE MADE OF STARDUST?