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chapter 9 ❖

Recreating the Universe 10,000,000 Times a Second

I want to know how God created this world. I am not interested in this or that phenomenon, in the spectrum of this or that element. I want to know His thoughts; the rest are 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, the field that studies the entire cosmos, across billions of light years and the 10–15 billion years since the creation of the universe, stands hand in hand with particle physics, which is concerned with the behavior of unstable particles with the most fleeting of lifetimes, many of which have not been generally present in the universe since the first instants following the Big Bang. Given that these fields are seemingly so dissimilar, how is it that the study of particle physics can reveal so much about the birth and B1290_Chapter-09.qxd 1/10/2012 11:48 AM Page 445 B1290 Understanding the Universe

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the ultimate fate of the universe? First, one must recall that in the tiny fractions of a second after the Big Bang, the universe was unimagin- ably hot. When matter (e.g. particles) is so hot, it is moving extremely quickly; that is to say, the matter (the particles) has (have) a lot of energy. And the study of highly energetic subatomic particles is exactly the topic that elementary particle physicists pursue. In the huge 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. Cosmology is fundamentally an observational science—in that we can only look out and see the universe—but we can’t really do experiments (after all, creating and destroying universes is pretty exhausting work … conventional wisdom is that each one takes a week). We have but one universe and we learn about it by staring at it with ever more sophisticated instruments, trying to winnow out its secrets. In con- trast, in particle physics we do experiments. We can change the energy of the particles. We collide baryons, mesons and leptons. We have control over the experimental conditions and directly observe the behavior of our experiments. Cosmologists can only infer the initial conditions of the universe by observation literally billions of years after the fact. Particle physics experiments can directly observe the behavior of matter under the conditions of the primordial inferno, thus the knowledge obtained from particle physics experiments is directly applicable to the study of cosmology. In addition to the creation of the universe, cosmologists use the known laws of physics to describe the behavior of heavenly bodies. In general, they are very successful, yet they do occasionally experience failure. The rotation rates of the outer arms of galaxies are much too rapid 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 world are 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 gravitational effects and is somehow not observable in the traditional meaning of B1290_Chapter-09.qxd 1/10/2012 11:48 AM Page 446 B1290 Understanding the Universe

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the word). Particle physicists potentially have something to say about this 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 that we may discover massive particles that interact, not through the strong or electromagnetic force, but through only the weak force and perhaps not even that. Recall that after the primordial Big Bang was complete (a whole second after it began), the laws of physics and the populations of subatomic particles were frozen. As discussed in Chapter 7, by that time, there were essentially no antimatter particles and for every matter quark or lepton, there were about one billion (109) neutrinos and photons. If each neutrino had a small mass, this would contribute to the mass of the universe and perhaps explain the mystery. The discovery of neutrino oscillations, also discussed in Chapter 7, shows that neutrinos do have a mass and so perhaps the conundrum is solved. We’ll talk more about this soon, but we believe that 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 we learned in Chapter 8, the LSP is thought to be massive, stable and does not interact with matter via any of the known forces except, con- veniently, gravity. So the discovery of supersymmetry could directly contribute 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 exciting developments and avenues of research followed by modern cosmolo- gists. There are entire books, many listed in the bibliography, which do 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 experiments performed in particle physics laboratories, past even that field’s fron- tier and on to some of the ideas discussed in the previous chapter. By the end, I hope to have convinced you that the study of the very small and the highly energetic will supplement much of the beautiful vistas B1290_Chapter-09.qxd 1/10/2012 11:48 AM Page 447 B1290 Understanding the Universe

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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 to understand the particles and forces described in earlier chapters, to understand the universe in its cosmological or astronomical sense, it is gravity that reigns supreme. Even though in the particle physics realm gravity is the mysterious weak cousin of the better understood other forces, in the realm of the heavens, gravity’s infinite range and solely attractive nature gives it the edge it needs to be the dominant force. The strong and weak force, both much larger than gravity at the 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 the electromagnetic force, with its own infinite range, has both attractive and repulsive aspects. Averaged over the large number of subatomic particles that comprise a star, planet or asteroid, the attractive and repulsive contributions cancel out, yielding no net electromagnetic force at all. So gravity finally gets the attention that our senses suggest that it should. For centuries, Newton’s universal law of gravity was used to describe the motion of the heavens. It was only unseated in 1916 by the ideas of another great man, Albert Einstein. Einstein postulated his law of general relativity, which described gravity as a warping of space itself. Regardless of the theory used, we must focus on the fact that gravity is an attractive force. An attractive force makes objects tend to come closer together. Thus, after a long time, one would expect the various bits of matter that comprise the universe (i.e. the galaxies) would have all come together in a single lump. Given that we observe this not to be true, if we know the mindset of the astronomers of the early 1920s (during which time this debate raged), we can come to only one conclusion. While there certainly was discussion on 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 called a “cosmological constant.” B1290_Chapter-09.qxd 1/10/2012 11:48 AM Page 448 B1290 Understanding the Universe

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The Shape of the Universe

The cosmological constant was designed with a single purpose … to counteract 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 field that had a repulsive character. Because of its repulsive nature, it spreads out across the universe, filling it completely. (If you think about it, if every object repels every other object, the only way they can have the maximum distance between each other (in a universe of finite size) is to spread uniformly across the cosmos.) Essentially, the cosmological constant can be thought of as a uniform field, consist- ing of energy that is “self-repulsive.” In a steady state universe, the strength of the repulsive cosmological constant is carefully tuned to counteract the tendency of gravity to collapse the universe, a point illustrated in Figure 9.1. In 1929, Edwin Hubble presented initial evidence, followed by an improved result in 1931, which suggested that the universe was not static, but rather was expanding very rapidly. After much debate, an explanation emerged. In a cataclysmic explosion, termed the Big Bang, 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 cosmological constant provides an outwards pressure. In Einstein’s early vision of the uni- verse, the two forces were balanced, providing a static and non-changing universe. B1290_Chapter-09.qxd 1/10/2012 11:48 AM Page 449 B1290 Understanding the Universe

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Starting from a single spot in a place that can’t even properly be called space, the matter that constitutes the universe was flung by the Big Bang outwards at great velocities. In an explosion of a house, like you might see in a war movie, the roof is blown off and ejected upwards very rapidly. As the explosive fireball expands, it cools off and it no longer forces the roof upwards. Eventually, the effects due to the force of gravity become dominant and the bits of the roof fall back to the ground. Similarly, the effect of the Big Bang is to fling the matter that makes up the beautiful stars and galaxies you see under a clear midnight sky across the universe. (In fact, the reality is more compli- cated, as the expansion of matter actually creates the universe as it goes. In addition, strictly speaking the Big Bang is still ongoing, as the universe continues to expand … essentially we are in the later stages of the explosion. We’ll gloss over these points right now and instead use the 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 cause the initial expansion to slow down and possibly even stop and crash the matter of the universe back together, like the bits of the roof crashing back to Earth. The fact that the universe was not static caused Einstein to remove from his equations the cosmological con- stant, calling it “the greatest blunder in his life.” Ironically, nearly 80 years later the cosmological constant is making a comeback. More on this later. As astronomers understood the phenomenon of the Big Bang and the slowing effect of the universe’s self gravity, naturally a question arose. What happens to the matter in the universe after the initial explosion? Does the universe expand forever, slowing while it goes? Does it expand and eventually stop? Does the force of gravity cause it to eventually contract, making the matter of the universe race together in a “Big Crunch?” How can we resolve these questions? Before we talk about these questions within the context of the fate of the universe itself, let’s discuss a somewhat simpler example. Suppose you have a giant slingshot and you want to launch an object B1290_Chapter-09.qxd 1/10/2012 11:48 AM Page 450 B1290 Understanding the Universe

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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 choose your launch speed, you realize that three things can happen. Launch your object too slowly and it will crash back to Earth. Launch the object with too much energy and you’ll fling it off into the dark depths of space. However, at a single particular velocity, which we call the “critical velocity,” we are able to attain the desired orbit. One velocity among all possibilities is special. In determining the fate of the expansion of the universe, whether it will expand forever or not, the critical parameter is the density of matter 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 the infinite future. The whole thing has a very “Goldilocks” quality to it … too much, too little or just right. We call the “magic” amount of mass needed to just stop the expansion of the universe in the far future the “critical density.” Density in this context has the usual meaning, so one takes the ratio of the mass (or equivalently energy) of the universe to its volume. In order to easily communicate about this whole question, cosmologists have defined a quantity called ⍀ (omega), which is simply the ratio of the mass density of our universe (denoted ␳) to the critical mass den- ␳ ⍀ϭ␳ ␳ sity of the universe (denoted c). Mathematically, we say / c. If the mass density of our universe is equal to the critical density, then ⍀ϭ1. If our density is greater, then omega is greater than 1 (⍀Ͼ1), while obviously too low a mass density will make omega less than one (⍀Ͻ1). Thus, the determination of ⍀ will reveal the ultimate fate of the universe. While our discussion thus far has been relatively intuitive, when the whole question is cast in Einstein’s theoretical framework, the dis- cussion becomes a bit murkier. Since many of the accounts you will read 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 B1290_Chapter-09.qxd 1/10/2012 11:48 AM Page 451 B1290 Understanding the Universe

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general relativity cast gravity in a geometrical framework, describing gravity as a curvature of space itself. It should not surprise you that the question of the critical mass of the universe has a geometric ana- log. Since we are discussing the mass that permeates the universe, this mass gives the universe its shape. The concept of curved space is a pretty tricky one, requiring that one understands the distortion of our familiar 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 ⍀ϭ1, we can say that on average, the universe is “flat” like a plane. If the mass density of the universe is too high (⍀Ͼ1), the universe has a spherical shape, while if the density is too low (⍀Ͻ1), the universe has a “saddle” or “hyperbolic” shape. In Figure 9.2, we see the three shapes that space can take. Think of two ants walking along two perpendicular lines in the grids of each type 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 place that 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, you are a curious person, with a deep-seated interest in the structure of the universe. I expect that you are becoming impatient. I can imagine what’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 universe was not known. Recent work suggests that our universe is flat. B1290_Chapter-09.qxd 1/10/2012 11:48 AM Page 452 B1290 Understanding the Universe

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able to make the relevant measurements and they find … a drum roll please … space is flat. We know this because of subtle variations in the radio waves emitted by space itself. This measurement is somewhat beyond the scope of this book, although it is described in some of the suggested 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 to fully 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 ⍀ϭ1), we know what the mass (technically energy) density of the universe must be … it must be equal to the critical density discussed earlier. As a crosscheck, astronomers can look out at the universe and catalog the matter that they observe. They do this by looking out at the cosmos and cata- loging stars. From what is known of stellar evolution, they can convert the brightness and color of each star they observe into a stellar mass. The visible mass of galaxies can be determined by similar studies and through the application of statistical techniques. What they find is that 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 realized that the combination of the observed distribution of luminous matter and Einstein’s law of gravity could not explain the rotation rates of galaxies. The rates at which a star orbits the center of an extended object like a galaxy is determined by two things. The first is the amount of matter (other stars and gas and such) contained within the spherical volume circumscribed by a star’s orbit. The second parameter is the distance 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 extent of the central bulge, it’s the size of the orbit that dominates. When astronomers measured the speed of stars at various orbital radii in our own Milky Way galaxy (and other nearby galaxies), they B1290_Chapter-09.qxd 1/10/2012 11:48 AM Page 453 B1290 Understanding the Universe

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found that the galaxies rotate differently than Einstein’s theory would predict. The Milky Way rotates in a complex way, but essentially one expects the stars in the spiral arms to revolve more slowly as the radius of the orbit increases (much in the same way that Pluto moves much more slowly than Mercury). However, as shown in Figure 9.3, what one finds instead is that the rotational velocity of stars in the arms is independent of radius. The favored (although not unique) explanation for this discrep- ancy is the idea that perhaps there exists matter throughout the galaxy that is not luminous. Luminous, in this context, means giving off electromagnetic energy. An object that we can detect, whether it emits visible light, infrared, ultraviolet, microwaves, radio, x-rays or other electromagnetic energy, is luminous. Such a hypothesis is fairly arresting, if not exactly new. In the mid 1930s, Caltech astronomer Fritz Zwicky proposed non-luminous or dark 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 predicted from conventional gravitational theory and the observed distribution of matter in a galaxy. In contrast to predictions, in which the outer stars of the galaxy are expected to revolve more slowly, the revolution speed of the galaxy appears to be independent of radius. Observation of this fact has led to the idea of dark matter. B1290_Chapter-09.qxd 1/10/2012 11:48 AM Page 454 B1290 Understanding the Universe

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find it? Many options have been proposed which we introduce in increasing degrees of exoticness. Hydrogen gas within the galaxy, but not tied up in a star, can be excluded as it emits radio waves and is therefore luminous. The next most plausible explanation is the so- called “brown dwarfs.” Brown dwarfs are essentially stars too small to ignite and burn. Somewhat larger than our own Jupiter, they can’t quite make up their mind whether they are large planets or small, failed stars. There’s nothing that forbids such objects from forming, indeed recent attempts to find planets around nearby stars have revealed objects that would qualify as being small brown dwarfs. However, since they are so small on the stellar scale, in order to make up the invisible mass that seems to permeate our galaxy, there needs to be a lot of them. So how would you find invisible brown dwarfs? Essentially, you see 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 a brown dwarf would wander across of the line of sight between you and 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 to be eclipsed in any reasonable amount of time, consequently astronomers simultaneously observe many stars. The usual approach is to look towards the center of our galaxy, which has the greatest concentration of stars and see if any of these are ever eclipsed. Long studies have seen very few such events, conclusively proving that a preponderance of brown dwarfs is not the explanation for dark mat- ter, although the amount of matter tied up in brown dwarfs and related 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 fairly easily. While black holes are, by definition, black (i.e. non-luminous), they play havoc with the matter that surrounds them. As matter encounters a black hole, it accelerates inwards. Accelerating matter usually radiates electromagnetic energy. Thus while the black holes are invisible, the lack of disturbances in the interstellar medium rules out the existence of so many black holes. B1290_Chapter-09.qxd 1/10/2012 11:48 AM Page 455 B1290 Understanding the Universe

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You may have heard of a super massive black hole at the center of our galaxy. While a consensus seems to have arisen that a black hole with the mass of millions of times greater than our Sun probably gov- erns the galaxy’s overall rotational dynamics, to explain the uniform rotational speed observed in the galactic arms requires a spherical and extended 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 matter are called MACHO’s (for MAssive Compact Halo Objects). The name stems from the fact that these objects have significant mass, are compact (like brown dwarfs, rather than gas clouds) and make their presence felt most strongly in the galactic halo (i.e. periphery) of the galaxy. All of the matter mentioned thus far is called baryonic, as it is made of common baryons (protons and neutrons). As the leptonic electrons contribute little to an atom’s mass, their presence is ignored in the name. Leaving the traditional explanations for dark matter, we now turn to our particle physics knowledge for options. If the Big Bang idea is true, neutrinos were produced copiously in the primordial inferno. One can calculate the number of neutrinos that should be present in the universe. It turns out that for every stable baryon (i.e. protons or neutrons) in the universe, there should be about 109 (one billion) neutrinos. While we don’t know the mass of neutrinos, Chapter 7 suggests that we have enough information to make a reasonable guess as to their mass. If one combines the current best guess of the mass of the various flavors of neutrinos with the number of neutrinos inferred from the Big Bang model, one finds that the neutrinos can account for only about 1–4% of the mass needed to make the universe flat 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 only about 5% of the matter necessary to make space flat and about 15% needed to solve the galactic rotation problem. So now what? We need to find enough matter, first to explain the rotation of galaxies (which B1290_Chapter-09.qxd 1/10/2012 11:48 AM Page 456 B1290 Understanding the Universe

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would be about 5 times the total potentially visible matter, i.e. all baryonic matter, even the dark stuff as it would emit light if it were heated 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 of galaxies. 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 could see it interact with ordinary matter). This form of matter may feel the weak 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 a hypothetical particle that might fit the bill. When we were discussing supersymmetry, we talked about the lightest supersymmetric particle or LSP. Because the LSP is the lightest of its brethren, there are no lighter supersymmetric particles into which it could decay. In addi- tion, because supersymmetry is “conserved,” these particles cannot decay into ordinary (and luminous) matter and therefore are stable. Further, since we have not detected the particle yet, if it exists, it must be electrically neutral, impervious to the strong force and massive. The LSP, while wholly theoretical, would prove to be an attractive candidate 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 these particles 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 will keep a close eye on these experiments, in the event that they provide cosmologically 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 necessary to successfully describe galactic rotation measurements, can only account for about 30% of the matter needed to make space flat (or equivalently ⍀ϭ1). So now what? Cosmologists have now proposed B1290_Chapter-09.qxd 1/10/2012 11:48 AM Page 457 B1290 Understanding the Universe

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another idea … dark energy. The dark energy can take several forms, one provided by Einstein’s resurgent cosmological constant and another related idea called quintessence. The important point is that the dark energy provides a repulsive force. The cosmological constant is sort of a vacuum energy … something analogous to the Higgs field of Chapter 5. Basically, the vacuum itself is permeated by an energy field of a repulsive nature. Quintessence is somewhat more analogous to the discredited idea of the aether, the non-existent material which physicists once thought was needed to allow light to propagate. Quintessence, if true, would also be an energy field that permeates the universe. It can be disturbed and interact with itself. It is this aspect that distinguishes it from the cosmological constant, which is … well … constant. These two ideas make similar predictions and will require 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 is research in progress. In research, confusion is good. It means that something doesn’t hang together and you’re about to learn something new. New equipment, like the Dark Energy Survey in Chile, plus a planned orbital mission that might fly in the future will add crucial new data to our understanding. This is an exciting time in cosmology. The whole idea of what constitutes the matter of the universe is a rather complicated one. The matter that makes up the beautiful and sparkling night sky is only responsible for 0.5–1.0% of the energy of the universe. The luminous matter is a very thin icing on a very dark cake. Table 9.1 shows the contribution of the various components to the 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 simpler explanation … one we have not yet formulated. The possible solutions proposed here are not unique. For instance about 20 years ago, Mordehai Milgrom at the Weizmann Institute in Israel proposed a solution to the galactic rotation problem that did not invoke dark B1290_Chapter-09.qxd 1/10/2012 11:48 AM Page 458 B1290 Understanding the Universe

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Table 9.1 Various components of the universe.

Percentage of Likely Source of Material the Mass of Composition Information the Universe Luminous Visible matter, stars Telescopes, etc. ϳ0.5% baryonic gas, etc. Hydrogen and Normal, but helium Dark dark matter abundances in ϳ5% baryonic (brown dwarfs, the universe, planets, etc.) “eclipse” experiments Non-baryonic matter (no Rotation speeds protons and of galaxies, neutrons) LSPs Exotic dark motion of and other ϳ25% matter galaxies within unusual matter galactic to be discovered clusters by particle physicists Cosmological Observed constant, Dark energy flatness of ϳ70% quintessence, space etc.

matter. He proposed a rather small modification to the laws of physics. His proposal would have no effect except in situations in which the acceleration was very small, as is the case in the outer arms of the galaxy. So, which of the two explanations is right? I don’t know. Nobody does, although studies of the Bullet Cluster published in 2006, are strongly supportive of the dark matter hypothesis. Indeed some researchers have claimed this evidence to be definitive, although modifications of Milgrom’s original hypothesis were quickly pub- lished, keeping the debate alive for at least a little while. These debates are what make the whole question so much fun. Luckily, experiments B1290_Chapter-09.qxd 1/10/2012 11:48 AM Page 459 B1290 Understanding the Universe

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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 of cosmological 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 interwoven with the question of what makes up the universe is the story of its birth and evolution. In this, there is one clearly favored explanation. The Big Bang cosmology was originally suggested in 1922 by Aleksandr Friedmann at the University of Petrograd and developed independently about 5 years later by Georges Henri LeMaître, a Catholic priest turned astronomer. LeMaître later said that he had an advantage 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 the observed expansion of the universe, first discovered by Edwin Hubble in 1929. The term “Big Bang” was not offered by the proponents of the theory, but was intended to be a denigrating term, first suggested by a key opponent. Fred Hoyle was an architect of a competing theory, the so-called Steady State hypothesis (initially so beloved of Einstein). The Steady State theory postulated that the universe was in a … well … steady state, that is to say that matter was being created and consumed in equal quantities and thus on average nothing was changing. Hoyle, in a criticism of the competition, was unimpressed with the need for a unique event and thus offered the disparaging term “Big Bang” as a way to show how silly the theory was. Much to his chagrin, proponents loved the term and the Big Bang cosmology was named. The Big Bang cosmology is the only one that clearly agrees with the observational evidence, as we will discuss in the following pages. Competing scientific theories and all ancient myths, including biblical ones, have been discredited. This is not to say that the Big Bang cos- mology is without its mysteries. Details of what the universe looked B1290_Chapter-09.qxd 1/10/2012 11:48 AM Page 460 B1290 Understanding the Universe

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like earlier, and how it got to be so smooth and homogeneous, are still topics of research and debate. Unfortunately, the popular press sometimes uses that debate to report sensational stories, “The Big Bang is Dead” being my particular favorite. Adherents of competing theories use these reports to try to convince others that the scientific community is in a much greater turmoil than it is. Biblical literalists insist that at best, the Big Bang cosmology be taught as a theory, on par with, but less true than their own Genesis-based ideas. Such an approach is nonsense. While the Big Bang cosmology is not without its own internal debates, no reputable scientist can dispute the evi- dence that the universe was once much smaller and hotter and that it is now expanding at great speed. The evidence for this is simply over- whelming. Theology based counterarguments must now join the same debate as scientists; Why are the laws of physics what they are? While an unlikely explanation, a deist answer to this question remains tenable. When one considers how one might ascertain the nature of the universe immediately after the Big Bang, one is struck by the magni- tude of the task. The Big Bang occurred between 10–15 billion years ago at an unknown point probably many billions of light years away (and quite possibly in a now-inaccessible dimension). Given that the primordial explosion was such a long time ago, it is difficult to infer any details. One might as well take air pressure measurements today and infer from them the details of that first nuclear detonation at the Trinity site in New Mexico.

Where Are the Galaxies?

As hard as the task may seem, astronomers actually have had an impressive success rate. Edwin Hubble found that other galaxies tend to 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 and greatly improved the accuracy of his measurement. Scientists can use B1290_Chapter-09.qxd 1/10/2012 11:48 AM Page 461 B1290 Understanding the Universe

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exquisitely precise telescopes, the Hubble Space Telescope being the most famous of them, and measure the speed of a galaxy. By knowing the relationship between speed and distance, they can determine the galaxy’s distance. While the precise number assigned to the distance still has some experimental and theoretical uncertainties, at present we 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 a year, it can travel 6 ϫ 1012 (six trillion) miles and, in 10 billion years travel a whopping 6 ϫ 1022 miles. These distances, while impres- sive, are not the most useful fact. The important thing to remember is 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 takes light a little over 8 minutes to travel from the Sun to our eyes. So the light you see from the Sun shows us not the Sun as it is right now, but rather as it was 8 minutes ago. The nearest star, Proxima Centauri, is 4.3 light years away. If you were to see it go nova the night you read this book (a highly unlikely prospect), any hypothetical people living there would be already dead for 4.3 years, as that’s the amount of time it takes for the news to get here. The most important consequence of this observation is the following. The farther away an object is from Earth, the longer it takes for light to get here. When it does get here, you see the object not as it appears now, but as it appeared in the past. If you rigged three cameras on Earth to simultaneously record the Sun, Proxima Centauri and the nearest “real” galaxy to our own (M31, also called Andromeda), you’d be taking pictures of objects 8 minutes, 4.3 years and 2.2 million years in the past. Once we realize this fact, it becomes obvious how to study the evolution of the universe. Take your most powerful telescopes and train them outwards, looking at ever more distant objects. The farther away you look, the farther back in time you see. If you’re interested in 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 B1290_Chapter-09.qxd 1/10/2012 11:48 AM Page 462 B1290 Understanding the Universe

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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 Digital Sky Survey (SDSS), scientists have been able to image galaxies a mere billion or so years after the Big Bang. This was not long after the first stars formed and began to burn with their bright nuclear fire. As one looks back in time, galaxies begin to take on a different shape … one more primitive. In this way, cosmologists interested in the physics of galaxy formation can view examples at all stages of development. In this, they are luckier than paleontologists. Cosmologists can see ear- lier “living and breathing” galaxies, while their dinosaur-hunting friends must content themselves studying dry bones. While the study of the evolution of galaxies is interesting and a crucial effort for one wanting to understand the fate of the universe, in some sense, it doesn’t address the question of why the universe is the way it is. A billion years after the Big Bang, the laws of physics had long since been determined. Well-understood nuclear and gravita- tional processes were shaping the stars and galaxies, but the question of why the nuclear fires burn as they do was still a mystery. To answer that 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 a question 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 by an unimaginably vast void. Alternatively, matter could be distributed throughout 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 map the 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 B1290_Chapter-09.qxd 1/10/2012 11:48 AM Page 463 B1290 Understanding the Universe

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purely stellar level, clearly matter isn’t distributed uniformly. Each star contains a great concentration of matter, surrounded by the vastness of nearly empty interstellar space. One can expand the question fur- ther and ask if the stars are spread uniformly throughout space. On a distance scale of some few hundreds or thousands of light years, one finds that the stars are spread relatively uniformly. The situation changes when the entire galaxy is considered. Our own Milky Way galaxy is a spiral or barred spiral galaxy, with stars concentrated in long graceful arms that spiral out from a dense core. Other galaxies reveal different structures. If one simply thinks of galaxies as clumps of matter, without too much thought going into the details of their structure, one can begin to ask questions that are more relevant to the structure of the universe. How are the galaxies arranged in the universe? It turns out that galaxies cluster together on the size scale of a few million light years. While such a distance is truly vast, it’s one ten thousandth of the size of the visible universe 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, they found the most delicate structure. This map of the universe showed galaxies arranged in long filaments across the sky, surrounding vast voids in 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 soap bubbles with the galaxies arranged along the soap’s film. By the mid 1990s, several experiments redid Geller and Huchra’s measurements, this time extending the distance investigated by a fac- tor of ten. On this much larger distance scale, the bubbles look very small and the universe is much more uniform. Careful perusal of the images in Figure 9.4 indicates that the size of the voids in Geller and Huchra’s measurements is the largest that the voids get. There do not appear to be even larger structures. The conclusion one must draw from this is the following. On the largest distance scales, roughly the size of the visible universe itself, matter is distributed uniformly throughout the cosmos. At the smaller scales of ribbons and bubbles B1290_Chapter-09.qxd 1/10/2012 11:48 AM Page 464 B1290 Understanding the Universe

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

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 uniform distribution of matter is what must be explained. A newer idea called cosmological inflation has been suggested to explain how the universe could be so uniform on such a large scale. Inflation suggests that a tiny fraction of a second after the Big Bang, the universe expanded extremely rapidly. We’ll revisit this idea later when we talk about the conditions 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 B1290_Chapter-09.qxd 1/10/2012 11:48 AM Page 465 B1290 Understanding the Universe

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objects has impressively contributed to our understanding of the universe at an earlier time, so far it has only been able to contribute for times more than a billion years after the Big Bang. To push our understanding 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 following the Big Bang. Joined shortly thereafter by another student named Robert Herman, Alpher set out to calculate the relative ratios of the elements that would be produced early in the universe. Like typical students, they followed their mentor’s lead. Gamow had realized that in order for nuclear fusion to be able to produce elements other than hydrogen, the early universe had to be hot. What Gamow missed, but his students realized, was that if the universe was once a hot and dense fireball, in the intervening years it should have cooled considerably and it should be possible to view remnants of the original energy by looking out at the cosmos. While there was some question as to what the signature might be, something of a consensus arose that perhaps one would see a uniform radio or microwave background. In 1964, Arno Penzias and Robert Wilson (this is a different Robert Wilson than Fermilab’s first director) were working at Bell Laboratory in New Jersey. They were trying to make an absolute measurement of the radio emission of a supernova remnant called Cas A. Cas A is located in the constellation Cassiopeia and is, mostly due to its relative proximity, the brightest radio source in the sky. Making an absolute measurement is just about the hardest thing one can do. Making a relative measurement is much easier. In a relative measurement, one tries to compare two things. For instance, if one looks at two light bulbs, a 40-watt one and a 150-watt one, it’s pretty easy to say that the 150-watt bulb is brighter. But to say exactly how many lumens the light is emitting (lumens are a unit of light like pounds are a unit of weight) is much harder. Other people had measured the various radio sources in the sky and 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 B1290_Chapter-09.qxd 1/10/2012 11:48 AM Page 466 B1290 Understanding the Universe

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nearest competitor. However, in order to be able to compare their measurement to a calculation dealing with supernova, they needed an absolute number. They needed to be able to say unequivocally that Cas 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 energy received. There’s only one problem. The fact is that everything emits radio waves. In the case of Penzias and Wilson, they were receiving radio waves from not only Cas A, but also from the antenna itself, the atmosphere, stray sources from those secret government labs in Area 51 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 to calculate the amount of radio waves from all known sources and they subtracted out these effects, again aiming their telescope at the sky. This time they looked not at Cas A, but at empty space. They expected to see nothing, yet an unwanted radio hiss remained. In order to make their measurement, they needed to understand the source of this mys- tery. They calibrated and recalibrated their equipment. They climbed up into their antenna, evicted two pigeons, and cleaned up piles of bird 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 fast cars and beautiful women (or gray-eyed Counts for my more feminine colleagues), but sometimes we get the lucky bit of bird poop thrown in too.) Penzias and Wilson’s efforts were appreciated by the custodial staff, but they didn’t get rid of their mysterious hiss. Penzias and Wilson were a bit depressed, as this unexplained radio noise would make their measurement a failure. As is usual at this point in an experiment, they started asking people for ideas. What did they miss? In January of 1965, Penzias was talking to Bernard Burke, who was a radio astronomer in his own right. Burke was aware of an effort by Jim Peebles at Princeton to find Gamow, Alpher and Herman’s radio signal from the Big Bang. Finally, the pieces clicked into place. In 1965, Penzias and Wilson published an article in Astrophysical Journal, detailing their experimental results. This paper was accom- panied by another paper, written by Peeble’s Princeton group that B1290_Chapter-09.qxd 1/10/2012 11:48 AM Page 467 B1290 Understanding the Universe

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interpreted their result. For the discovery of the radio signal remain- ing from the Big Bang, Penzias and Wilson received the 1978 Nobel Prize. Incidentally, they eventually published a measurement of the radio emissions of Cas A as well, although not to the same general acclaim 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 is 2.7 degrees Kelvin or Ϫ455ЊF. An important question was “how uni- form was this temperature?” Penzias and Wilson were able to scan the sky and they found that the radio emissions were remarkably uniform; any variation from perfectly uniform was less than 0.1%. This was the precision of their equipment, thus they couldn’t say that this “back- ground” radiation was nonuniform at the 0.01% level, but they could say that the uniformity was better than 99.9%. The temperature of the universe was everywhere 2.7 degrees Kelvin (K). Rounding the temperature upwards, we call this remnant radio radiation “the 3 K background.” As is usually the case, earlier scientists had studied the cyanogen molecule in the interstellar environment and noted that it appeared to be surrounded by a bath of radiation between 2 and 3 degrees Kelvin. They missed the significance and yet more physicists joined the “If only …” club. So why is this measurement interesting? The theory of the Big Bang suggests that at one time, the universe was much hotter and highly ener- getic photons were ubiquitous. At about 300,000 years after the Big Bang, the universe was a relatively cool 3,000 degrees Kelvin (about 5,000ЊF). All vestiges of quarks were gone and the universe was com- posed of the non-interacting neutrinos and the much more interesting protons, electrons, photons and the rare alpha particle (helium nuclei). Protons and electrons have the opposite electrical charge and thus they feel an attractive force. Get a proton and electron together and they really want to combine and become a hydrogen atom. Similarly, an alpha particle wants to grab two electrons and become a helium atom. However, highly energetic photons can knock the electrons away from the proton and thus electrically neutral atoms don’t form. The photons B1290_Chapter-09.qxd 1/10/2012 11:48 AM Page 468 B1290 Understanding the Universe

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jump from electrons to other electrons and back again like a hyperactive seven year old interfering with his older sister’s date. However, as the temperature drops below the 3,000 degrees Kelvin temperature, suddenly everything changes. The energy carried by the photons is no longer enough to separate the electrons and the protons. Now instead of a universe of separated electrical charge, the universe is full of neutral hydrogen and helium atoms. Since photons only interact with charged objects, the photons stop interacting and march undisturbed across the cosmos, much as their distant cousins, the neutrinos, were already doing. Thus, and this is the important part, these photons last interacted with matter 300,000 years after the cosmos came into being. These photons then are a snapshot of the universe only 300,000 years after the Big Bang. This pushes our understanding of the origins of the universe quite a bit closer to the beginning as compared to the studies of galaxies discussed earlier. In a sense, the view that the 3 K background radiation is a highly uniform bath, recording the conditions of a much earlier epoch, has not changed much in the intervening years, although this is not to say that other measurements have not been made. In fact, in 1990 the COsmic Background Explorer (or COBE) satellite re-measured the 3 K back- ground with exquisite precision. The full story of the significance of their results is beyond the scope of this book, but they are clearly described in George Smoot and Keay Davidson’s book Wrinkles in Time. Smoot was a leading member of the group that measured the 3 K background radiation, while Davidson is a talented science writer, and this book is well worth your time. In the simplest terms, the COBE col- laboration determined that the 3 K background did have a slight non- uniformity at the 0.001% level. To give you a sense of the magnitude of the accomplishment, they needed to measure the temperature with a precision of one part in 100,000. To give a concrete example, it’s as if they accurately measured the length of a football field and found that it was off by one millimeter. These small variations in temperature reflect early variations in the density of the universe. These little variations in density have been amplified in the ensuing years to become