Science 3210 001 : Introduction to Astronomy

Lecture 13 : Cosmology, Interstellar Travel, and the Search for Life in the Universe

Robert Fisher

Items

Nathan Hearn guest lecture. Lunch at Frontera Fresco and the Macy’s food court was amazing.

Adler Planetarium field trip next week on May 4th. $16/person -- to be collected today. Waiver forms to be signed today.

Final projects due May 11th, along with a short (5-10 minute) presentation that day. Important : If for whatever reason you are unable to attend on May 11th, you MUST get your final project to me before then.

Return midterms and homeworks.

Final Project

Your final project is to construct a creative interpretation a scientific theme we encountered during the class. You will present your work in a five minute presentation in front of the entire class on May 11.

The project must have both a scientific component and a creative one.

For instance, a Jackson Pollock-lookalike painting would fly, but ONLY if you said that it was your interpretation of the big bang cosmological model AND you could also demonstrate mastery of the basic astrophysics of the big bang while presenting your work.

Be prepared to be grilled!

Ideas : Mount your camera on a tripod and shoot star trails. Create a “harmony of the worlds” soundtrack for the Upsilon Andromeda system. Paint the night sky as viewed from an observer about to fall behind the horizon of a

black hole. Write a short science fiction story about the discovery of intelligent life in the universe.

Major Astronomical News Item of the Week -- Discovery of Gliese 581c

Gliese 581c is the smallest extrasolar planet discovered to date, at about 5 times the mass of the Earth.

It is believed based on models (not yet demonstrated by direct observation) that this could very well be a massive Earthlike rocky planet, unlike the gaseous giants discovered to date.

What is more, even though the planet orbits Gliese 581 at a very close distance (.07 AU), the star is a red dwarf of about a third the mass of our sun, and so is far dimmer -- about 1/100 the luminosity.

Because the intensity of light falls off with the square of the distance, it turns out that the effective intensity at Gliese 581 is roughly the same as that of the sun on the Earth -- it is possible that it Gliese 581c has a surface temperature which can support liquid water.

Two Weeks Ago

Black Holes, White Holes, Wormholes

Galaxies Distances in the universe Types of galaxies

Ellipticals Spirals Irregulars

Last Week

Cosmology Why is the night sky dark? Newtonian Cosmology Einstein, Hubble and the Expansion of the Universe

Today

Cosmology Cosmic Microwave Background Clusters of Galaxies, Superclusters Quasars

Interstellar Travel

The Search for Life in the Universe

Cosmic Microwave Background

During the post-WWII era, physics and astronomy grew rapidly, fueled by discoveries made during WWII.

In 1965, Robert Wilson and Arno Penzias were conducting observations of the Milky Way galaxy using a radio telescope at Bell Laboratories.

When conducting their observations, they needed to separate out the signal (the microwave emission of the galaxy) from the noise generated by other sources.

After taking their measured signal and subtracting out all known sources of noise (the sky and the ground, the galaxy, and the telescope itself), they were still left with a background signal.

This background signal persisted regardless of where they looked on the sky.

Blackbody Radiation : A Review

Each and every body in the universe radiates a characteristic thermal spectrum which depends only upon its temperature.

As a consequence, for instance, the interior of a kiln heated to a white-hot uniform temperature, each surface glows with the same characteristic spectrum, regardless of its composition and color in reflected light. One sees only a uniform glow, and individual objects are indistinguishable.

Cosmic Microwave Background

Starting with a total signal measuring several hundreds of degrees Kelvin (degrees on the Centigrade scale above absolute zero), and after subtracting out all known signals, Penzias and Wilson were left with a residual background signal corresponding to blackbody radiation at the tiny temperature of 3 degrees Kelvin.

This signal persisted at the same level regardless of where the telescope were pointed on the sky.

Many people would disregard the tiny discrepancy as an unexplained curiosity.

Because Penzias and Wilson took extraordinary care in isolating the residual background, they concluded that they must be observing a real feature of the universe -- the cosmic microwave background.

Cosmological Redshift

Wait… three degrees above absolute zero is an incredibly cold temperature; why is it that we believe the universe began in a hot big bang??

The answer to this question has to do with the fact that in an expanding universe, all distances are expanding in time.

The hot plasma in the early universe emitted short-wavelength, very hot photons.

As the universe expanded, so too do all other distances within the universe, including individual wavelengths of light.

Cosmological Redshift

As the universe expanded, so too do all other distances within the universe, including individual wavelengths of light.

Hot Big Bang Model of the Universe

The detection of the cosmic microwave background by Penzias and Wilson led to the development of the hot big bang model of the universe.

In this model, the universe began from an incredibly dense hot plasma state, and rapidly expanded.

As the universe expanded, it cooled down. At the point that it had cooled down sufficiently (to about 4000 K) to allow individual protons and electrons to combine to form hydrogen atoms, it became transparent.

The microwave background we see today are the same photons that left the early universe at the point of recombination, and have been streaming freely since.

The cosmic microwave background demonstrates that at the point of recombination, the universe was in an extremely homogeneous state.

Surface of Last Scattering

When we see the microwave background, we are actually seeing the surface of last scattering of the intense radiation field of the early universe.

Cosmic Microwave Background Anisotropies

The cosmic microwave background is incredibly uniform -- to within one part in one hundred thousand. If the microwave background were the blades of grass in a football field, then the blades would all be identical to within one centimeter.

The uniformity of the cosmic microwave background is due to the fact that the early universe was itself nearly perfectly homogeneous and smooth.

Yet today, we have evidence of structure on all scales in the cosmos -- from planets to stars to galaxies and even bigger.

In the framework of the hot big bang model of the universe, these structures must have originated from small fluctuations in the early universe, and grown under the influence of gravity.

Detecting the Cosmic Microwave Background Anisotropies -- The “Fingerprint of God”

For nearly thirty years following Penzias and Wilson’s discovery, scientists probed the cosmic microwave background with a series of experiments of ever-increasing precision, in an effort to detect the anisotropies in the cosmic microwave background.

The issue came to a crux in the early 1990s, when scientists had not yet detected any of the intrinsic anisotropies in the microwave background.

Scientists studying galaxy formation knew that the anisotropies had to be there, and be large enough that they could grow into galaxies in the current age of the universe.

However, if the level of anisotropy were very small, then no theory of structure formation could have been able to account for the existence of galaxies in the universe, and the hot big bang model itself would be in serious jeopardy.

COBE And George Smoot

In 1992, George Smoot led a team of scientists to design and fly a microwave detector onboard a satellite which became known as the Cosmic microwave Background Explorer (COBE).

Smoot and his colleagues gathered their data and spent many months carefully analyzing it.

Finally, after a long wait, Smoot and his colleagues announced the discovery of the intrinsic anisotropies in the cosmic microwave background, from nearly 14 billion years ago, and from which all structures in the universe emerged.

For this work, Smoot was awarded the Nobel Prize in Physics in 2006.

George Smoot

COBE Results

The COBE science team’s discovery of the anisotropy in the microwave background revolutionized cosmology. For the first time, scientists had a glimpse of structures of the early universe, when it was less than 500,000 years old.

To some, this breakthrough was just the tip of the iceberg. They asked, where did these fluctuations come from in the first place?

COBE All-Sky Map of Temperature Anisotropies

Inflationary Cosmology

The discovery of anisotropies in the microwave background came along at a serendipitous time -- during the 1980s, theorists studying the very early universe -- at times 10-32 s ! -- were making major progress.

At these very early epochs, the universe was incredibly hot and dense, and if one goes back far enough in time, the energies achieved exceed those of any particle accelerator on the Earth.

The consequences of the high energy state of the very early universe are profound -- specifically, one cannot understand the largest scales in the cosmos without an understanding of the smallest scales as well.

Inflationary Cosmology

During the early 1980s, Alan Guth was attempting to understand several surprising properties of the early universe -- including,

Why is the microwave background so uniform?

Why are strange particles predicted to exist by some cosmological theories absent or at least very rare in nature?

Why does the curvature of the universe appear to correspond to a “flat” cosmologiy?

Guth’s proposal was radical. He suggested that instead of the expansion rate expected from the regular Einstein equations, the universe expanded enormously faster in the universe -- exponentially.

Inflationary Cosmology

Guth’s model was based on a heady mix of general relativistic cosmology and particle physics.

The mechanism producing the inflation can best be understood by the analogy of a supercooled container of water being held beneath its freezing point.

Similarly, in an inflationary cosmology, certain particles may find themselves in a “false vacuum” as the universe expands and cools.

During the period of time that the particles remain in the false vacuum, the universe is endowed with an enormous amount of energy that causes the size of the universe to expand exponentially with time.

In this picture, the anisotropies which develop are initially nothing more than quantum fluctuations within a tiny region of spacetime.

Wow… Space is so full of … Space

Once it begins to sink in, one realizes the inflationary cosmology picture is an amazingly startling one.

The entire observable universe began with a tremendously small region of space and then blew up by a huge amount in a very short span of time.

Every structure which we observe in the universe today… superclusters and clusters of galaxies, stars, planets… originated from quantum fluctuations in the very early universe.

Schematic Diagram of History of Universe

Inflationary Cosmology

One of the most startling realizations of inflationary cosmology is that the entire observable universe is only a tiny fraction of what is really out there.

As the universe expands, more and more of the rest of the universe comes into our observable horizon. However,in this picture, vast regions of space are entirely unknowable until very distant points in the far future.

But Is Inflation Correct?

The map of the anisotropies on the cosmic microwave background can be used to constrain theories of the very early universe, including inflation.

The key idea here is that very distant regions on the microwave background were not in causal contact at the surface of last scattering. Hence, whatever anisotropies exist on these large scales, they must be leftover from much earlier epochs still.

It turns out that COBE-measured anisotropies on large scales are consistent with those predicted by inflation.

This provides a tantalizing hint that there may actually be something to the inflationary cosmology and the radical ideas it advances.

Large-Scale Structure of the Universe

Zel’dovich Pancakes and Filaments

The structures seen in the large-scale structure surveys such as SDSS and in the cosmological simulations were originally predicted to exist by Yakov Zel’dovich (1914 - 1987).

Zel’dovich was a monumental figure in 20th century physics. He did much of his early work on fundamental contributions to chemical combustion and detonation. During and after WWII, he played a leading role in the development of Soviet nuclear weapons.

In 1965, at the age of 49, he entered the field of astrophysics, and subsequently made so many fundamental contributions that Stephen Hawking once said, “…before I met you here, I believed you to be a collective author.”

Zel’dovich Pancakes and Filaments

In the early 1970s, long before the advent of extensive large-scale surveys or large computer simulations, Yakov Zel’dovich and colleagues worked out much of the basic physics of structure formation in the early universe using little more than pure thought.

In order to understand the development of large-scale structure in the early universe, they considered what would happen if one perturbed an initially uniform density region.

Overdense RegionUniform Background

Zel’dovich Pancakes and Filaments

If the perturbed overdense region is nonspherical, it will collapse the fastest along the shortest axis, which causes the region to become even more distorted.

From this line of thinking, it is clear the general outcome of a gravitational collapse will be a pancake-like structure.

Note that this mechanism does not rely upon rotation.

Overdense Region Overdense Region

Zel’dovich Pancakes and Filaments

Once a pancake is formed, a similar process begins to occur within it.

The thin pancake become unstable along its shortest dimension, which results in the production of filamentary structures.

The outcome of this process should be a network of highly complex, interwoven filaments and pancakes containing most of the mass in the universe, interspersed with enormous voids.

Overdense Region Overdense Region

Clusters of Galaxies

Zel’dovich realized that when one combined numerous perturbations on the background, the resulting pattern would resemble a complex spiderwork of filaments.

In this picture, the intersection of filaments provide seeds for the growth of clusters of galaxies.

At these intersections, the density is significantly higher than the background density or even the density in individual filaments.

As a consequence, the regions of intersection collapse the fastest, and produce some of the first galaxies in the universe.

Superclusters of Galaxies

In addition to individual clusters, clusters of galaxies themselves aggregate into even larger-scale bound structures known as superclusters.

The superclusters are bound together by the complex network of filaments in large-scale structure.

Over time, these superclusters will tend to swallow one another up, along with mass contained in the filaments.

The resulting picture of structure formation is fasctinating, and suggests that we are looking at the universe at unique epoch, just as structure formation has really begun in earnest, but before it has fully quenched itself out.

Large Scale Structure

As large-scale structure surveys of the nearby universe were completed in the 1980s, structures similar to those predicted by Zel’dovich and others began to emerge : Clusters of Galaxies Superclusters of galaxies Walls, Sheets, and Filaments Voids

Cfa Survey of Galaxies

LCDM Animation

Zoom-in of Millenium Simulation

Flythrough of Large-Scale Structure of the Millenium Simulation

Interstellar Space Travel

Interstellar Space Travel

Travel through interstellar space has become a staple in science fiction -- often by fictional devices with little or no basis in real science.

Is interstellar travel scientifically possible?

The Slow Boat to… Aldebaran

Even with today’s technology, it is possible to travel to the nearest stars.

Pioneer 10 was the first device created by humanity to leave the system, after its encounter with Jupiter in 1973.

It is so distant, and its radioactive power sources have become so weak, that it has not been heard from since 2002 when it was 80 AU away from the sun.

It is nearly 90 AU away today.

Pioneer 10 Record Plaque Cover

As humanity’s first envoy outside of the solar system, Pioneer 10 carried with it a record with sounds of Earth, along with a cover etched with a kind of scientific Rosetta stone explaining who we are, and when and where the spacecraft was launched.

The Slow Boat to… Aldebaran

Pioneer 10 is headed towards the red giant star Aldebaran in the constellation of Taurus.

At its current speed, it will take nearly 2 million years to reach Aldebaran.

By aiming towards a nearer system (for instance, Alpha Centauri, our nearest neighbor at about 4 light years distance), it would be possible to cut the travel time to about 100,000 years.

An interstellar voyage lasting such a long time would require a enormous spacecraft ark capable of sustaining itself over many generations.

Interstellar Arks

An interstellar ark would be an enormous spaceship (possibly hollowed out of a smaller asteroid) with everything needed to sustain a society for many generations on its voyage.

While technically feasible even today, it seems difficult to imagine a small society could be sociologically stable for such a long time when few societies on Earth have been stable for hundreds of years.

Long a staple of science fiction, many authors describe inhabitants in “suspended animation” and a spaceship run by robots.

What is more probelmatic is the likelihood that if humanity survives, the ark will likely be long superceded by the time of its arrival.

Can’t Wait for Another Lifetime? We Have Faster Rocket Technologies Just for You

Many other concepts for faster rocket technologies exist, using varying degrees of existing and far-fetched science.

Perhaps the most unusual example using an existing technology is the Orion spacecraft concept, designed by General Atomics in the late 1950s, and led by the physicist Freeman Dyson.

By detonating a succession of hydrogen bombs and absorbing part of the blastwave energy through a pusher plate, in principle it can achieve about 10 percent of the speed of light.

Greener Energy to the Stars

Another spaceship concept design using existing technology is the light sail.

The sail reflects light over a wide area (many square kilometers) using a lightweight material.

The reflection of individual photons

produces a thrust, by Newton’s third law.

The simplest source of light is the sun itself, though this can only be used in the inner solar system.

More advanced designs rely upon banks of lasers to provide the impulse -- up to 20 percent the speed of light.

Cosmos 1 Sail

Relativistic Spaceflight

The ultimate limit for any spacecraft is the speed of light itself.

Approaching this limit requires vast amounts of energy, beyond any known technology -- even matter/antimatter engines.

If it is indeed possible to achieve relativistic spaceflight, the possibilities are amazing.

As we learned, according to Einstein’s theory of relativity, the time of a moving observer is slowed down relative to a stationary observer.

Interstellar rocket travel is an example of a famous paradox in relativity theory.

Twin Paradox

One of the famous apparent paradoxes of special relativity theory is the twin paradox.

Imagine we have two twins, Albert and Heindrik. Heindrik remains on Earth, while Albert sets off on a relativistic spaceflight to Alpha Centauri at a speed close to that of light, and returns.

In the middle of his flight, according to Albert, Heindrik’s clock is running slowly relative to his own.

According to Heindrik, Albert’s clock is running slowly relative to his own.

Yet when Albert returns to Earth, surely he is either older or younger than Heindrik.

Twin Paradox

Which twin is older on return to Earth? Albert or Heindrik? Why?

Relativistic Spaceflight

If relativistic spaceflight is indeed possible, then it is possible to travel to very distant parts of the galaxy with a single lifetime.

Relativistic spaceflight results in a strange kind of time travel into the future.

For instance, special relativity predicts that if one can move at 99.9999% the speed of light, then (by one’s own clock) one will be able to traverse the 50,000 LY radius of the Milky Way in just 70 years.

However, the clocks on Earth back home will advance 50,000 years in the same time -- highly relativistic spaceflight is in a sense an enormous leap into the future.

Life in the Cosmos

It has taken about 14 billion years in the time since the Big Bang for the universe to produce conditions suitable for life and intelligence on Earth.

When viewed on a broad scope, it appears the rate of “progress” of the universe is accelerating rapidly.

Question : What is Life?

Definition of Life

It makes sense to first address the question of what life is, specifically.

A reasonable (though not necessarily exhaustive) definition of life is that A) A lifeform can react to its environment. B) It can grow by taking in nourishment from its surroundings. C) It can reproduce. D) It can pass on its genetic structure to its heirs.

Urey-Miller Experiment

In a classic experiment conducted at the University of Chicago in the 1940s. Urey and Miller demonstrated that beginning with very simple chemicals (which were chosen to approximate the unevolved state of the early Earth atmosphere), and adding some energy in the form of a spark, highly complex organic chemicals fundamental to life are rapidly produced.

Urey-Miller Experiment

Analysis of the chemical products revealed the presence of amino acids, which are the building blocks of DNA.

However, not the original experiment, and not any repetitions of the experiment have ever been able to demonstrate the manufacture of DNA in this process.

It remains a major scientific mystery how DNA was itself produced, and how single-celled beings came to exist.

Planetary Habitable Zones

The range of distances from a central star over which a planet can support liquid water is known as the stellar habitable zone.

The more massive the central star, the wider the habitable zone, and the further out it becomes.

Gliese 581c is in the habitable zone of its parent star.

The Fermi Paradox -- Where are They??

If planets are common, it seems quite plausible that life should arise on many worlds.

This poses an immediate paradox -- if life (and quite possibly intelligent life) is common throughout the universe, why is it that we have no evidence for extraterrestrial civilizations?

There are many clever proposed solutions to this paradox, including We are too meager for aliens to contact. The aliens aren’t talkative. They are far from us, and haven’t yet received our radio signals. They are here… among us…

Gathering at the ol’ water hole

The most natural way to communicate in interstellar space that we currently know of is through radio wave transmissions.

There is a preferred range of frequencies in which the background from astrophysical sources and atmospheric absorption is minimal, and which some astronomers believe may be a natural place for interstellar civilizations to communicate.

Who Speaks for Earth?

Next Week : Field Trip!