B1290_Chapter-09.qxd 1/10/2012 11:48 AM Page 444 B1290 Understanding the Universe 444 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 recreating the universe 445 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 446 understanding the universe 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 recreating the universe 447 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 448 understanding the universe 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.