Recreating the Universe 10,000,000 Times a Second · field that studies the entire cosmos, across billions of light years and the 10–15 billion years since the creation of the universe,

I want to know how God created this world. I am notinterested in this or that phenomenon, in the spectrum ofthis or that element. I want to know His thoughts; the restare details.

— Albert Einstein

There are many marvelous books that are simply brimming with dis-cussions of the newest ideas and discoveries pertaining to the cosmos.This is not one of those books. This book is fundamentally about par-ticle physics, yet the two fields are inextricably linked. Cosmology, thefield that studies the entire cosmos, across billions of light years andthe 10–15 billion years since the creation of the universe, stands handin hand with particle physics, which is concerned with the behavior ofunstable particles with the most fleeting of lifetimes, many of whichhave not been generally present in the universe since the first instantsfollowing the Big Bang.

Given that these fields are seemingly so dissimilar, how is it thatthe study of particle physics can reveal so much about the birth and

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the ultimate fate of the universe? First, one must recall that in the tinyfractions of a second after the Big Bang, the universe was unimagin-ably hot. When matter (e.g. particles) is so hot, it is moving extremelyquickly; that is to say, the matter (the particles) has (have) a lot ofenergy. And the study of highly energetic subatomic particles isexactly the topic that elementary particle physicists pursue. In thehuge leviathan experiments with which you are now quite familiar,physicists collide particles together millions of times a second, rou-tinely recreating the conditions of the early universe. Cosmologyis fundamentally an observational science—in that we can onlylook out and see the universe—but we can’t really do experiments(after all, creating and destroying universes is pretty exhaustingwork … conventional wisdom is that each one takes a week). We havebut one universe and we learn about it by staring at it with ever moresophisticated instruments, trying to winnow out its secrets. In con-trast, in particle physics we do experiments. We can change the energyof the particles. We collide baryons, mesons and leptons. We havecontrol over the experimental conditions and directly observe thebehavior of our experiments. Cosmologists can only infer the initialconditions of the universe by observation literally billions of yearsafter the fact. Particle physics experiments can directly observe thebehavior of matter under the conditions of the primordial inferno,thus the knowledge obtained from particle physics experiments isdirectly applicable to the study of cosmology.

In addition to the creation of the universe, cosmologists use theknown laws of physics to describe the behavior of heavenly bodies. Ingeneral, they are very successful, yet they do occasionally experiencefailure. The rotation rates of the outer arms of galaxies are much toorapid to be explained by the matter that we can see (stars, planets, gas,etc.) So either the laws of gravity that we use to describe the worldare wrong, or there are new phenomena to be discovered. We will dis-cuss why cosmologists postulate the so-called “Dark Matter” (i.e.matter that makes its presence known solely through its gravitationaleffects and is somehow not observable in the traditional meaning of

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the word). Particle physicists potentially have something to say aboutthis as well. How is it that particle physics can contribute to the dis-cussion of the rotation of galaxies? This is because it is possible thatwe may discover massive particles that interact, not through thestrong or electromagnetic force, but through only the weak force andperhaps not even that. Recall that after the primordial Big Bang wascomplete (a whole second after it began), the laws of physics and thepopulations of subatomic particles were frozen. As discussed inChapter 7, by that time, there were essentially no antimatter particlesand for every matter quark or lepton, there were about one billion(109) neutrinos and photons. If each neutrino had a small mass, thiswould contribute to the mass of the universe and perhaps explain themystery. The discovery of neutrino oscillations, also discussed inChapter 7, shows that neutrinos do have a mass and so perhaps theconundrum is solved. We’ll talk more about this soon, but we believethat neutrinos cannot solve the galactic rotation problem by them-selves. So again, we turn to particle physics, this time for more specu-lative theories. For instance, if supersymmetry turns out to be true,then there exists a lightest supersymmetric particle (or LSP). As welearned in Chapter 8, the LSP is thought to be massive, stable anddoes not interact with matter via any of the known forces except, con-veniently, gravity. So the discovery of supersymmetry could directlycontribute to studies of the large structures of the universe … galaxies,galaxy clusters and even larger structures.

In a single chapter, we cannot possibly describe all of the excitingdevelopments and avenues of research followed by modern cosmolo-gists. There are entire books, many listed in the bibliography, whichdo just that. Instead, we will follow the arrow of time backwards, dis-cussing the various observations that are relevant to particle physics,pushing through the observation of the universe to the experimentsperformed in particle physics laboratories, past even that field’s fron-tier and on to some of the ideas discussed in the previous chapter. Bythe end, I hope to have convinced you that the study of the very smalland the highly energetic will supplement much of the beautiful vistas

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seen by the Hubble Telescope and other equally impressive astro-nomical observational instruments.

While in order to fully understand the universe you need tounderstand the particles and forces described in earlier chapters, tounderstand the universe in its cosmological or astronomical sense, itis gravity that reigns supreme. Even though in the particle physicsrealm gravity is the mysterious weak cousin of the better understoodother forces, in the realm of the heavens, gravity’s infinite range andsolely attractive nature gives it the edge it needs to be the dominantforce. The strong and weak force, both much larger than gravity atthe size of the proton or smaller, disappear entirely when two parti-cles are separated by as small a range as the size of an atom. Even theelectromagnetic force, with its own infinite range, has both attractiveand repulsive aspects. Averaged over the large number of subatomicparticles that comprise a star, planet or asteroid, the attractive andrepulsive contributions cancel out, yielding no net electromagneticforce at all. So gravity finally gets the attention that our senses suggestthat it should.

For centuries, Newton’s universal law of gravity was used todescribe the motion of the heavens. It was only unseated in 1916 bythe ideas of another great man, Albert Einstein. Einstein postulatedhis law of general relativity, which described gravity as a warping ofspace itself. Regardless of the theory used, we must focus on the factthat gravity is an attractive force. An attractive force makes objectstend to come closer together. Thus, after a long time, one wouldexpect the various bits of matter that comprise the universe (i.e. thegalaxies) would have all come together in a single lump. Given that weobserve this not to be true, if we know the mindset of the astronomersof the early 1920s (during which time this debate raged), we cancome to only one conclusion. While there certainly was discussionon the issue, the prevailing opinion was that the universe was nei-ther expanding nor contracting, rather it was in a “steady state.”Accordingly, Einstein modified his equations to include what he calleda “cosmological constant.”




The Shape of the Universe

The cosmological constant was designed with a single purpose … tocounteract gravity’s pull and keep the universe in the static, unchang-ing state that was the consensus view at the time. Basically, the cos-mological constant was Einstein’s name for a hypothetical energy fieldthat had a repulsive character. Because of its repulsive nature, itspreads out across the universe, filling it completely. (If you thinkabout it, if every object repels every other object, the only way theycan have the maximum distance between each other (in a universe offinite size) is to spread uniformly across the cosmos.) Essentially, thecosmological constant can be thought of as a uniform field, consist-ing of energy that is “self-repulsive.” In a steady state universe, thestrength of the repulsive cosmological constant is carefully tuned tocounteract the tendency of gravity to collapse the universe, a pointillustrated in Figure 9.1.

In 1929, Edwin Hubble presented initial evidence, followed byan improved result in 1931, which suggested that the universe wasnot static, but rather was expanding very rapidly. After much debate,an explanation emerged. In a cataclysmic explosion, termed the BigBang, the universe was created at a single point and at a single time.



Figure 9.1 Gravity is an attractive force in the universe. The cosmologicalconstant provides an outwards pressure. In Einstein’s early vision of the uni-verse, the two forces were balanced, providing a static and non-changinguniverse.


Starting from a single spot in a place that can’t even properly be calledspace, the matter that constitutes the universe was flung by the BigBang outwards at great velocities. In an explosion of a house, like youmight see in a war movie, the roof is blown off and ejected upwardsvery rapidly. As the explosive fireball expands, it cools off and it nolonger forces the roof upwards. Eventually, the effects due to theforce of gravity become dominant and the bits of the roof fall back tothe ground. Similarly, the effect of the Big Bang is to fling the matterthat makes up the beautiful stars and galaxies you see under a clearmidnight sky across the universe. (In fact, the reality is more compli-cated, as the expansion of matter actually creates the universe as itgoes. In addition, strictly speaking the Big Bang is still ongoing, as theuniverse continues to expand … essentially we are in the later stages ofthe explosion. We’ll gloss over these points right now and instead usethe word “Big Bang” in a sloppy way that signifies the original explo-sion only.) Since the Big Bang is long over, one expects that the grav-itational force between the constituents of the universe would causethe initial expansion to slow down and possibly even stop and crashthe matter of the universe back together, like the bits of the roofcrashing back to Earth. The fact that the universe was not staticcaused Einstein to remove from his equations the cosmological con-stant, calling it “the greatest blunder in his life.” Ironically, nearly80 years later the cosmological constant is making a comeback. Moreon this later.

As astronomers understood the phenomenon of the Big Bang andthe slowing effect of the universe’s self gravity, naturally a questionarose. What happens to the matter in the universe after the initialexplosion? Does the universe expand forever, slowing while it goes?Does it expand and eventually stop? Does the force of gravity cause itto eventually contract, making the matter of the universe racetogether in a “Big Crunch?” How can we resolve these questions?

Before we talk about these questions within the context of the fateof the universe itself, let’s discuss a somewhat simpler example.Suppose you have a giant slingshot and you want to launch an object




into a specific orbit around the Earth. This is a very high-tech sling-shot and can launch your object at any speed you want. As you chooseyour launch speed, you realize that three things can happen. Launchyour object too slowly and it will crash back to Earth. Launch theobject with too much energy and you’ll fling it off into the darkdepths of space. However, at a single particular velocity, which we callthe “critical velocity,” we are able to attain the desired orbit. Onevelocity among all possibilities is special.

In determining the fate of the expansion of the universe, whetherit will expand forever or not, the critical parameter is the density ofmatter in the universe. Too much matter and the universe will even-tually collapse, not enough and it will expand forever, never stopping.If the amount of matter is “just right,” the universe will expand for-ever, moving ever slower until the expansion eventually stops in theinfinite future. The whole thing has a very “Goldilocks” quality toit … too much, too little or just right.

We call the “magic” amount of mass needed to just stop theexpansion of the universe in the far future the “critical density.”Density in this context has the usual meaning, so one takes the ratioof the mass (or equivalently energy) of the universe to its volume. Inorder to easily communicate about this whole question, cosmologistshave defined a quantity called � (omega), which is simply the ratio ofthe mass density of our universe (denoted �) to the critical mass den-sity of the universe (denoted �c). Mathematically, we say � � �/�c. Ifthe mass density of our universe is equal to the critical density, then� � 1. If our density is greater, then omega is greater than 1 (� � 1),while obviously too low a mass density will make omega less than one(� � 1). Thus, the determination of � will reveal the ultimate fate ofthe universe.

While our discussion thus far has been relatively intuitive, whenthe whole question is cast in Einstein’s theoretical framework, the dis-cussion becomes a bit murkier. Since many of the accounts you willread in newspapers and other sources are explained in Einstein’s lan-guage, we’ll talk a little about it here. Recall that Einstein’s theory of




general relativity cast gravity in a geometrical framework, describinggravity as a curvature of space itself. It should not surprise you thatthe question of the critical mass of the universe has a geometric ana-log. Since we are discussing the mass that permeates the universe, thismass gives the universe its shape. The concept of curved space is apretty tricky one, requiring that one understands the distortion of ourfamiliar three-dimensional space. As usual, intuition (and artistic tal-ent) can fail us in this endeavor, so let’s instead talk in two dimen-sions. If � � 1, we can say that on average, the universe is “flat” likea plane. If the mass density of the universe is too high (� � 1), theuniverse has a spherical shape, while if the density is too low (� � 1),the universe has a “saddle” or “hyperbolic” shape.

In Figure 9.2, we see the three shapes that space can take. Thinkof two ants walking along two perpendicular lines in the grids of eachtype of space. In all cases each ant moves at a constant “local” speed.Local speed means how fast he is moving with respect to the placethat his feet are touching. The counterintuitive thing one must real-ize is that due to the curvature of space, the ants in the three differ-ently shaped spaces will separate at different speeds. Fundamentally,it is this aspect of space that will govern the fate of the universe.

Since you, gentle reader, have made it this far in this book, youare a curious person, with a deep-seated interest in the structure ofthe universe. I expect that you are becoming impatient. I can imaginewhat’s going on in your mind. The burning question must be “Well?What is it? Is space curved or flat?” Cosmologists have finally been



Figure 9.2 Three types of space, flat, spherical and hyperbolic, or “saddle-shaped.” Until recently the exact type of space that makes up the universewas not known. Recent work suggests that our universe is flat.


able to make the relevant measurements and they find … a drum rollplease … space is flat. We know this because of subtle variations in theradio waves emitted by space itself. This measurement is somewhatbeyond the scope of this book, although it is described in some of thesuggested reading. We will revisit the radio waves from space in a lit-tle bit, albeit not at quite so technical a level as would be needed tofully convince you of the flatness of space. You’ll have to trust me.

The Dark Side of the Universe

With the knowledge that space is flat (and � � 1), we know what themass (technically energy) density of the universe must be … it must beequal to the critical density discussed earlier. As a crosscheck,astronomers can look out at the universe and catalog the matter thatthey observe. They do this by looking out at the cosmos and cata-loging stars. From what is known of stellar evolution, they can convertthe brightness and color of each star they observe into a stellar mass.The visible mass of galaxies can be determined by similar studiesand through the application of statistical techniques. What they find isthat the amount of luminous matter in the universe is only about 0.5%of that needed to make space flat. So where is the missing matter?

This question is not a new one. Astronomers have long realizedthat the combination of the observed distribution of luminous matterand Einstein’s law of gravity could not explain the rotation rates ofgalaxies. The rates at which a star orbits the center of an extendedobject like a galaxy is determined by two things. The first is the amountof matter (other stars and gas and such) contained within the sphericalvolume circumscribed by a star’s orbit. The second parameter is thedistance the star is from the galaxy’s center. In a galaxy such as ours,with a large central bulge and long graceful and relatively sparse arms,these two effects compete. For stars at a radius greater than the extentof the central bulge, it’s the size of the orbit that dominates.

When astronomers measured the speed of stars at various orbitalradii in our own Milky Way galaxy (and other nearby galaxies), they




found that the galaxies rotate differently than Einstein’s theory wouldpredict. The Milky Way rotates in a complex way, but essentially oneexpects the stars in the spiral arms to revolve more slowly as the radiusof the orbit increases (much in the same way that Pluto moves muchmore slowly than Mercury). However, as shown in Figure 9.3, whatone finds instead is that the rotational velocity of stars in the arms isindependent of radius.

The favored (although not unique) explanation for this discrep-ancy is the idea that perhaps there exists matter throughout the galaxythat is not luminous. Luminous, in this context, means giving offelectromagnetic energy. An object that we can detect, whether itemits visible light, infrared, ultraviolet, microwaves, radio, x-rays orother electromagnetic energy, is luminous.

Such a hypothesis is fairly arresting, if not exactly new. In the mid1930s, Caltech astronomer Fritz Zwicky proposed non-luminous ordark matter to explain the motion of galaxies within galactic clusters.However, if dark matter exists, what is its nature and how could we



Figure 9.3 The rotation rates of galaxies are quite different than predictedfrom conventional gravitational theory and the observed distribution ofmatter in a galaxy. In contrast to predictions, in which the outer stars ofthe galaxy are expected to revolve more slowly, the revolution speed of thegalaxy appears to be independent of radius. Observation of this fact has ledto the idea of dark matter.


find it? Many options have been proposed which we introduce inincreasing degrees of exoticness. Hydrogen gas within the galaxy, butnot tied up in a star, can be excluded as it emits radio waves and istherefore luminous. The next most plausible explanation is the so-called “brown dwarfs.” Brown dwarfs are essentially stars too small toignite and burn. Somewhat larger than our own Jupiter, they can’tquite make up their mind whether they are large planets or small, failedstars. There’s nothing that forbids such objects from forming, indeedrecent attempts to find planets around nearby stars have revealedobjects that would qualify as being small brown dwarfs. However, sincethey are so small on the stellar scale, in order to make up the invisiblemass that seems to permeate our galaxy, there needs to be a lot of them.

So how would you find invisible brown dwarfs? Essentially, yousee them by the shadow they create. If brown dwarfs are so ubiqui-tous, you should be able to look at a distant star and eventually abrown dwarf would wander across of the line of sight between youand the relevant star, and you would see a dimming of the star’s light.Space is large and stars are small, so any individual star is unlikely tobe eclipsed in any reasonable amount of time, consequentlyastronomers simultaneously observe many stars. The usual approachis to look towards the center of our galaxy, which has the greatestconcentration of stars and see if any of these are ever eclipsed. Longstudies have seen very few such events, conclusively proving that apreponderance of brown dwarfs is not the explanation for dark mat-ter, although the amount of matter tied up in brown dwarfs andrelated objects exceeds the mass tied up in luminous matter.

Another possible astronomical explanation of the dark matter ques-tion is black holes. We can rule out black holes as an explanation fairlyeasily. While black holes are, by definition, black (i.e. non-luminous),they play havoc with the matter that surrounds them. As matterencounters a black hole, it accelerates inwards. Accelerating matterusually radiates electromagnetic energy. Thus while the black holes areinvisible, the lack of disturbances in the interstellar medium rules outthe existence of so many black holes.




You may have heard of a super massive black hole at the center ofour galaxy. While a consensus seems to have arisen that a black holewith the mass of millions of times greater than our Sun probably gov-erns the galaxy’s overall rotational dynamics, to explain the uniformrotational speed observed in the galactic arms requires a spherical andextended distribution of dark matter. Thus the central black hole,interesting though it may be, does not provide the explanation.

Collectively, these relatively mundane candidates for dark matterare called MACHO’s (for MAssive Compact Halo Objects). Thename stems from the fact that these objects have significant mass, arecompact (like brown dwarfs, rather than gas clouds) and make theirpresence felt most strongly in the galactic halo (i.e. periphery) of thegalaxy. All of the matter mentioned thus far is called baryonic, as it ismade of common baryons (protons and neutrons). As the leptonicelectrons contribute little to an atom’s mass, their presence is ignoredin the name.

Leaving the traditional explanations for dark matter, we now turnto our particle physics knowledge for options. If the Big Bang idea istrue, neutrinos were produced copiously in the primordial inferno.One can calculate the number of neutrinos that should be present inthe universe. It turns out that for every stable baryon (i.e. protons orneutrons) in the universe, there should be about 109 (one billion)neutrinos. While we don’t know the mass of neutrinos, Chapter 7suggests that we have enough information to make a reasonable guessas to their mass. If one combines the current best guess of the massof the various flavors of neutrinos with the number of neutrinosinferred from the Big Bang model, one finds that the neutrinos canaccount for only about 1–4% of the mass needed to make the universeflat and only about 10% of that needed to explain the observed rota-tion rate of galaxies.

So, of the particles and objects that we know exist, we have onlyabout 5% of the matter necessary to make space flat and about 15%needed to solve the galactic rotation problem. So now what? We needto find enough matter, first to explain the rotation of galaxies (which




would be about 5 times the total potentially visible matter, i.e. allbaryonic matter, even the dark stuff as it would emit light if it wereheated enough) and then another source of mass to explain the flat-ness of the universe itself.

Let’s first start with the matter needed to explain the rotation ofgalaxies. We need to find matter which is not affected by the electro-magnetic force (or we could see it) or the strong force (or we couldsee it interact with ordinary matter). This form of matter may feel theweak force and by definition, it must feel the force due to gravity.While we have no real experimental evidence as to what sort of mat-ter would make up this dark matter, we have found in Chapter 8 ahypothetical particle that might fit the bill. When we were discussingsupersymmetry, we talked about the lightest supersymmetric particleor LSP. Because the LSP is the lightest of its brethren, there are nolighter supersymmetric particles into which it could decay. In addi-tion, because supersymmetry is “conserved,” these particles cannotdecay into ordinary (and luminous) matter and therefore are stable.Further, since we have not detected the particle yet, if it exists, it mustbe electrically neutral, impervious to the strong force and massive.The LSP, while wholly theoretical, would prove to be an attractivecandidate for the dark matter that governs galactic rotation.

Of course, since the LSP may not exist, there have been other par-ticles proposed that might also prove to be the culprit. All of theseparticles are exotic, completely theoretical and quite possibly non-existent. However, the upcoming generation of particle physics exper-iments will be looking for heavy stable particles. Cosmologists willkeep a close eye on these experiments, in the event that they providecosmologically relevant information.

However, all the matter discussed thus far; baryonic luminous and“normal” dark matter (baryonic brown dwarfs and such) and non-baryonic “exotic” dark matter (LSPs or equivalent), while necessaryto successfully describe galactic rotation measurements, can onlyaccount for about 30% of the matter needed to make space flat (orequivalently � � 1). So now what? Cosmologists have now proposed




another idea …dark energy. The dark energy can take several forms,one provided by Einstein’s resurgent cosmological constant andanother related idea called quintessence. The important point is thatthe dark energy provides a repulsive force. The cosmological constantis sort of a vacuum energy … something analogous to the Higgs fieldof Chapter 5. Basically, the vacuum itself is permeated by an energyfield of a repulsive nature. Quintessence is somewhat more analogousto the discredited idea of the aether, the non-existent material whichphysicists once thought was needed to allow light to propagate.Quintessence, if true, would also be an energy field that permeates theuniverse. It can be disturbed and interact with itself. It is this aspectthat distinguishes it from the cosmological constant, whichis…well…constant. These two ideas make similar predictions and willrequire fairly precise measurements to say which of the two is correct,if either. If this all sounds rather fuzzy, this is because it is. This isresearch in progress. In research, confusion is good. It means thatsomething doesn’t hang together and you’re about to learn somethingnew. New equipment, like the Dark Energy Survey in Chile, plus aplanned orbital mission that might fly in the future will add crucial newdata to our understanding. This is an exciting time in cosmology.

The whole idea of what constitutes the matter of the universe is arather complicated one. The matter that makes up the beautiful andsparkling night sky is only responsible for 0.5–1.0% of the energy ofthe universe. The luminous matter is a very thin icing on a very darkcake. Table 9.1 shows the contribution of the various components tothe makeup of the universe.

A skeptical reader might find this whole discussion to be suspect,as it seems very complicated. It very well might be that all of these dif-ferent types of matter and energy are needed to fully describe the uni-verse. On the other hand, it may be that there is a much simplerexplanation … one we have not yet formulated. The possible solutionsproposed here are not unique. For instance about 20 years ago,Mordehai Milgrom at the Weizmann Institute in Israel proposed asolution to the galactic rotation problem that did not invoke dark




matter. He proposed a rather small modification to the laws ofphysics. His proposal would have no effect except in situations inwhich the acceleration was very small, as is the case in the outer armsof the galaxy. So, which of the two explanations is right? I don’t know.Nobody does, although studies of the Bullet Cluster published in2006, are strongly supportive of the dark matter hypothesis. Indeedsome researchers have claimed this evidence to be definitive, althoughmodifications of Milgrom’s original hypothesis were quickly pub-lished, keeping the debate alive for at least a little while. These debatesare what make the whole question so much fun. Luckily, experiments



Table 9.1 Various components of the universe.

Likely Source of Percentage of Material Composition Information the Mass of

the Universe

Visible Luminous

baryonic matter, stars Telescopes, etc. �0.5%gas, etc.

Hydrogen and Normal, but helium

Dark dark matter abundances in �5%baryonic (brown dwarfs, the universe,planets, etc.) “eclipse”

experiments

Non-baryonic matter (no

protons and Rotation speeds

neutrons) LSPs of galaxies, Exotic dark and other motion of �25%matter unusual matter galaxies within

to be discovered galactic

by particle clusters

physicists

Cosmological Observed Dark energy constant, flatness of �70%quintessence, spaceetc.


are now possible which may resolve the whole question. A full dis-cussion of these ideas is outside the scope of this book, but the inter-ested reader can peruse the suggested reading where these aspects ofcosmological research are discussed in greater detail.

While the question of exactly what constitutes the universe is a burn-ing one, there is another interesting question. Intricately interwovenwith the question of what makes up the universe is the story of its birthand evolution. In this, there is one clearly favored explanation.

The Big Bang cosmology was originally suggested in 1922 byAleksandr Friedmann at the University of Petrograd and developedindependently about 5 years later by Georges Henri LeMaître, aCatholic priest turned astronomer. LeMaître later said that he had anadvantage over Einstein, as his priestly training made him look favor-ably upon the idea that the universe had a distinct beginning.LeMaître called his progenitor of the universe the “primeval atom.”Among other evidence, this cosmology was designed to explain theobserved expansion of the universe, first discovered by Edwin Hubblein 1929. The term “Big Bang” was not offered by the proponents ofthe theory, but was intended to be a denigrating term, first suggestedby a key opponent. Fred Hoyle was an architect of a competingtheory, the so-called Steady State hypothesis (initially so beloved ofEinstein). The Steady State theory postulated that the universe wasin a … well … steady state, that is to say that matter was being createdand consumed in equal quantities and thus on average nothing waschanging. Hoyle, in a criticism of the competition, was unimpressedwith the need for a unique event and thus offered the disparagingterm “Big Bang” as a way to show how silly the theory was. Much tohis chagrin, proponents loved the term and the Big Bang cosmologywas named.

The Big Bang cosmology is the only one that clearly agrees withthe observational evidence, as we will discuss in the following pages.Competing scientific theories and all ancient myths, including biblicalones, have been discredited. This is not to say that the Big Bang cos-mology is without its mysteries. Details of what the universe looked




like earlier, and how it got to be so smooth and homogeneous, arestill topics of research and debate. Unfortunately, the popular presssometimes uses that debate to report sensational stories, “The BigBang is Dead” being my particular favorite. Adherents of competingtheories use these reports to try to convince others that the scientificcommunity is in a much greater turmoil than it is. Biblical literalistsinsist that at best, the Big Bang cosmology be taught as a theory, onpar with, but less true than their own Genesis-based ideas. Such anapproach is nonsense. While the Big Bang cosmology is not withoutits own internal debates, no reputable scientist can dispute the evi-dence that the universe was once much smaller and hotter and that itis now expanding at great speed. The evidence for this is simply over-whelming. Theology based counterarguments must now join thesame debate as scientists; Why are the laws of physics what they are?While an unlikely explanation, a deist answer to this question remainstenable.

When one considers how one might ascertain the nature of theuniverse immediately after the Big Bang, one is struck by the magni-tude of the task. The Big Bang occurred between 10–15 billion yearsago at an unknown point probably many billions of light years away(and quite possibly in a now-inaccessible dimension). Given that theprimordial explosion was such a long time ago, it is difficult to inferany details. One might as well take air pressure measurements todayand infer from them the details of that first nuclear detonation at theTrinity site in New Mexico.

Where Are the Galaxies?

As hard as the task may seem, astronomers actually have had animpressive success rate. Edwin Hubble found that other galaxies tendto be moving away from us. Even more interesting was the observa-tion that the greater the distance to the galaxy, the faster it was mov-ing away. Subsequent studies have verified Hubble’s initial result andgreatly improved the accuracy of his measurement. Scientists can use




exquisitely precise telescopes, the Hubble Space Telescope being themost famous of them, and measure the speed of a galaxy. By knowingthe relationship between speed and distance, they can determine thegalaxy’s distance. While the precise number assigned to the distancestill has some experimental and theoretical uncertainties, at presentwe can see galaxies over 10 billion light years away.

A light year is the distance that light, that fleetest of messengers,can travel in a year. Light travels at 186,000 miles per second. In ayear, it can travel 6 � 1012 (six trillion) miles and, in 10 billion years travel a whopping 6 � 1022 miles. These distances, while impres-sive, are not the most useful fact. The important thing to rememberis that as fast as light travels, the size of space is incomparably greater.The Earth orbits the Sun at a distance of 93 million miles. It takeslight a little over 8 minutes to travel from the Sun to our eyes. So thelight you see from the Sun shows us not the Sun as it is right now, butrather as it was 8 minutes ago. The nearest star, Proxima Centauri, is4.3 light years away. If you were to see it go nova the night you readthis book (a highly unlikely prospect), any hypothetical people livingthere would be already dead for 4.3 years, as that’s the amount of timeit takes for the news to get here. The most important consequenceof this observation is the following. The farther away an object isfrom Earth, the longer it takes for light to get here. When it does gethere, you see the object not as it appears now, but as it appeared inthe past. If you rigged three cameras on Earth to simultaneouslyrecord the Sun, Proxima Centauri and the nearest “real” galaxy toour own (M31, also called Andromeda), you’d be taking pictures ofobjects 8 minutes, 4.3 years and 2.2 million years in the past.

Once we realize this fact, it becomes obvious how to study theevolution of the universe. Take your most powerful telescopes andtrain them outwards, looking at ever more distant objects. The fartheraway you look, the farther back in time you see. If you’re interestedin how galaxies have changed over the years, look at our nearby galax-ies and study their properties. To see a galaxy 2.2 million years ago(a cosmological blink of an eye), you merely need to look at our




neighboring galaxy in Andromeda. As you look at galaxies at an ever-increasing distance, it is like looking at older and older snapshots.Each photo reveals something of an earlier era. Using such instru-ments as the Hubble Space Telescope (HST) and the Sloan DigitalSky Survey (SDSS), scientists have been able to image galaxies a merebillion or so years after the Big Bang. This was not long after the firststars formed and began to burn with their bright nuclear fire. As onelooks back in time, galaxies begin to take on a different shape … onemore primitive. In this way, cosmologists interested in the physics ofgalaxy formation can view examples at all stages of development. Inthis, they are luckier than paleontologists. Cosmologists can see ear-lier “living and breathing” galaxies, while their dinosaur-huntingfriends must content themselves studying dry bones.

While the study of the evolution of galaxies is interesting and acrucial effort for one wanting to understand the fate of the universe,in some sense, it doesn’t address the question of why the universe isthe way it is. A billion years after the Big Bang, the laws of physics hadlong since been determined. Well-understood nuclear and gravita-tional processes were shaping the stars and galaxies, but the questionof why the nuclear fires burn as they do was still a mystery. To answerthat question will require a journey further back in time. We’ll con-tinue that journey in a moment.

However, before we do, I’d like to take a moment to address aquestion raised by the observations by both the HST and the SDSS.This question involves the distribution of matter across the universe.One could imagine that matter was all lumped together, surrounded byan unimaginably vast void. Alternatively, matter could be distributedthroughout the universe or spaced periodically like a giant honeycomb.So, what is the truth?

For nearly 100 years, astronomers have been doing three-dimensional maps of the universe. Even early astronomers could mapthe positions of objects on the surface of the sphere that is the heavens.With Hubble’s insight, astronomers could determine an object’s dis-tance as well, thereby locating the object uniquely in space. On a




purely stellar level, clearly matter isn’t distributed uniformly. Each starcontains a great concentration of matter, surrounded by the vastnessof nearly empty interstellar space. One can expand the question fur-ther and ask if the stars are spread uniformly throughout space. On adistance scale of some few hundreds or thousands of light years, onefinds that the stars are spread relatively uniformly. The situationchanges when the entire galaxy is considered. Our own Milky Waygalaxy is a spiral or barred spiral galaxy, with stars concentrated inlong graceful arms that spiral out from a dense core. Other galaxiesreveal different structures.

If one simply thinks of galaxies as clumps of matter, without toomuch thought going into the details of their structure, one can begin toask questions that are more relevant to the structure of the universe.How are the galaxies arranged in the universe? It turns out that galaxiescluster together on the size scale of a few million light years. While sucha distance is truly vast, it’s one ten thousandth of the size of the visibleuniverse as a whole. In 1989, Margaret Geller and John Huchra pub-lished a study in which they revealed a most marvelous map of the sky.Locating the galaxies out to a distance of 500 million light-years, theyfound the most delicate structure. This map of the universe showedgalaxies arranged in long filaments across the sky, surrounding vast voidsin which very little matter was found. Their data is shown in Figure 9.4.On the distance scales that they explored, the universe looked like soapbubbles with the galaxies arranged along the soap’s film.

By the mid 1990s, several experiments redid Geller and Huchra’smeasurements, this time extending the distance investigated by a fac-tor of ten. On this much larger distance scale, the bubbles look verysmall and the universe is much more uniform. Careful perusal of theimages in Figure 9.4 indicates that the size of the voids in Geller andHuchra’s measurements is the largest that the voids get. There do notappear to be even larger structures. The conclusion one must drawfrom this is the following. On the largest distance scales, roughly thesize of the visible universe itself, matter is distributed uniformlythroughout the cosmos. At the smaller scales of ribbons and bubbles




of galaxy clusters, down through galaxies and a more stellar environ-ment, gravitational interactions have made the universe more clumpy.The clumpiness, although interesting and incidentally crucial to life,does not reflect the beginnings of the universe. For that, the uniformdistribution of matter is what must be explained. A newer idea calledcosmological inflation has been suggested to explain how the universecould be so uniform on such a large scale. Inflation suggests that atiny fraction of a second after the Big Bang, the universe expandedextremely rapidly. We’ll revisit this idea later when we talk about theconditions of the universe just fractions of a second after the Big Bang.

The Big Whisper

While observational astronomy using the electromagnetic spectrum(light, infrared, ultraviolet, x-rays, radio waves, etc.) to view heavenly



Figure 9.4 Experimental data from the Cfa and Las Campanas experiments.In both pictures, each dot represents an entire galaxy. The Cfa experimentlooked out to a distance of 500 million light-years, looking for structure.The Las Campanas experiment greatly expanded that range. There appearsto be clusters of galaxies, as well as spots where no galaxies exist. The largeststructures in the universe seem to be about 100 million light-years in size.(Figure courtesy of John Huchra, for the Cfa Collaboration and DougTucker, for the Las Campanas Collaboration.)


objects has impressively contributed to our understanding of theuniverse at an earlier time, so far it has only been able to contributefor times more than a billion years after the Big Bang. To push ourunderstanding even earlier requires a different approach. In 1945,Ukrainian émigré George Gamow took on a student, Ralph Alpher,who was to attempt to quantify the conditions immediately followingthe Big Bang. Joined shortly thereafter by another student namedRobert Herman, Alpher set out to calculate the relative ratios of theelements that would be produced early in the universe. Like typicalstudents, they followed their mentor’s lead. Gamow had realized thatin order for nuclear fusion to be able to produce elements other thanhydrogen, the early universe had to be hot. What Gamow missed, buthis students realized, was that if the universe was once a hot and densefireball, in the intervening years it should have cooled considerablyand it should be possible to view remnants of the original energy bylooking out at the cosmos. While there was some question as to whatthe signature might be, something of a consensus arose that perhapsone would see a uniform radio or microwave background.

In 1964, Arno Penzias and Robert Wilson (this is a differentRobert Wilson than Fermilab’s first director) were working at BellLaboratory in New Jersey. They were trying to make an absolutemeasurement of the radio emission of a supernova remnant calledCas A. Cas A is located in the constellation Cassiopeia and is, mostlydue to its relative proximity, the brightest radio source in the sky.Making an absolute measurement is just about the hardest thing onecan do. Making a relative measurement is much easier. In a relativemeasurement, one tries to compare two things. For instance, if onelooks at two light bulbs, a 40-watt one and a 150-watt one, it’s prettyeasy to say that the 150-watt bulb is brighter. But to say exactly howmany lumens the light is emitting (lumens are a unit of light likepounds are a unit of weight) is much harder.

Other people had measured the various radio sources in the skyand concluded that Cas A was the brightest source in the heavens.They also were able to even say how much brighter it was than its




nearest competitor. However, in order to be able to compare theirmeasurement to a calculation dealing with supernova, they needed anabsolute number. They needed to be able to say unequivocally thatCas A put out so many units of radio energy. So the idea seems easy.One simply points an antenna at Cas A and records the radio energyreceived. There’s only one problem. The fact is that everything emitsradio waves. In the case of Penzias and Wilson, they were receivingradio waves from not only Cas A, but also from the antenna itself, theatmosphere, stray sources from those secret government labs in Area51 that cause my Uncle Eddy to put tinfoil in his baseball cap, etc.Penzias and Wilson had a tough job ahead of them. They were able tocalculate the amount of radio waves from all known sources and theysubtracted out these effects, again aiming their telescope at the sky.This time they looked not at Cas A, but at empty space. They expectedto see nothing, yet an unwanted radio hiss remained. In order to maketheir measurement, they needed to understand the source of this mys-tery. They calibrated and recalibrated their equipment. They climbedup into their antenna, evicted two pigeons, and cleaned up piles ofbird poop. (Which goes to show you that the life of a research physi-cist is even more exciting than you think. Not only do we get the fastcars and beautiful women (or gray-eyed Counts for my more femininecolleagues), but sometimes we get the lucky bit of bird poop thrownin too.) Penzias and Wilson’s efforts were appreciated by the custodialstaff, but they didn’t get rid of their mysterious hiss.

Penzias and Wilson were a bit depressed, as this unexplained radionoise would make their measurement a failure. As is usual at this pointin an experiment, they started asking people for ideas. What did theymiss? In January of 1965, Penzias was talking to Bernard Burke, whowas a radio astronomer in his own right. Burke was aware of an effortby Jim Peebles at Princeton to find Gamow, Alpher and Herman’sradio signal from the Big Bang. Finally, the pieces clicked into place.In 1965, Penzias and Wilson published an article in AstrophysicalJournal, detailing their experimental results. This paper was accom-panied by another paper, written by Peeble’s Princeton group that




interpreted their result. For the discovery of the radio signal remain-ing from the Big Bang, Penzias and Wilson received the 1978 NobelPrize. Incidentally, they eventually published a measurement of theradio emissions of Cas A as well, although not to the same generalacclaim as their serendipitous discovery.

It turns out that it is possible to convert the Big Bang’s back-ground signal into a temperature. The temperature of outer space is2.7 degrees Kelvin or �455�F. An important question was “how uni-form was this temperature?” Penzias and Wilson were able to scan thesky and they found that the radio emissions were remarkably uniform;any variation from perfectly uniform was less than 0.1%. This was theprecision of their equipment, thus they couldn’t say that this “back-ground” radiation was nonuniform at the 0.01% level, but they couldsay that the uniformity was better than 99.9%. The temperature ofthe universe was everywhere 2.7 degrees Kelvin (K). Rounding thetemperature upwards, we call this remnant radio radiation “the 3 Kbackground.” As is usually the case, earlier scientists had studied thecyanogen molecule in the interstellar environment and noted thatit appeared to be surrounded by a bath of radiation between 2 and3 degrees Kelvin. They missed the significance and yet more physicistsjoined the “If only …” club.

So why is this measurement interesting? The theory of the Big Bangsuggests that at one time, the universe was much hotter and highly ener-getic photons were ubiquitous. At about 300,000 years after the BigBang, the universe was a relatively cool 3,000 degrees Kelvin (about5,000�F). All vestiges of quarks were gone and the universe was com-posed of the non-interacting neutrinos and the much more interestingprotons, electrons, photons and the rare alpha particle (helium nuclei).Protons and electrons have the opposite electrical charge and thus theyfeel an attractive force. Get a proton and electron together and theyreally want to combine and become a hydrogen atom. Similarly, an alphaparticle wants to grab two electrons and become a helium atom.However, highly energetic photons can knock the electrons away fromthe proton and thus electrically neutral atoms don’t form. The photons




jump from electrons to other electrons and back again like a hyperactiveseven year old interfering with his older sister’s date.

However, as the temperature drops below the 3,000 degreesKelvin temperature, suddenly everything changes. The energy carriedby the photons is no longer enough to separate the electrons and theprotons. Now instead of a universe of separated electrical charge, theuniverse is full of neutral hydrogen and helium atoms. Since photonsonly interact with charged objects, the photons stop interacting andmarch undisturbed across the cosmos, much as their distant cousins,the neutrinos, were already doing. Thus, and this is the importantpart, these photons last interacted with matter 300,000 years after thecosmos came into being. These photons then are a snapshot of theuniverse only 300,000 years after the Big Bang. This pushes ourunderstanding of the origins of the universe quite a bit closer to thebeginning as compared to the studies of galaxies discussed earlier.

In a sense, the view that the 3K background radiation is a highlyuniform bath, recording the conditions of a much earlier epoch, has notchanged much in the intervening years, although this is not to say thatother measurements have not been made. In fact, in 1990 the COsmicBackground Explorer (or COBE) satellite re-measured the 3K back-ground with exquisite precision. The full story of the significance oftheir results is beyond the scope of this book, but they are clearlydescribed in George Smoot and Keay Davidson’s book Wrinkles inTime. Smoot was a leading member of the group that measured the 3Kbackground radiation, while Davidson is a talented science writer, andthis book is well worth your time. In the simplest terms, the COBE col-laboration determined that the 3K background did have a slight non-uniformity at the 0.001% level. To give you a sense of the magnitudeof the accomplishment, they needed to measure the temperature witha precision of one part in 100,000. To give a concrete example, it’s asif they accurately measured the length of a football field and found thatit was off by one millimeter. These small variations in temperaturereflect early variations in the density of the universe. These littlevariations in density have been amplified in the ensuing years to become




Documents

Recreating the Universe 10,000,000 Times a Second · field that studies the entire cosmos, across billions of light years and the 10–15 billion years since the creation of the universe,