The Perfect Theory (29 page)

In 1979, Stephen Hawking, along with a South African relativist named Werner Israel, put together a survey of relativity to celebrate Einstein's centenary. They brought together the leading researchers in cosmology, black holes, and quantum gravity. Bob Dicke and Jim Peebles contributed an essay titled “The Big Bang Cosmology—Enigmas and Nostrums.” It was a short essay. In a few pages, Dicke and Peebles laid out what they believed to be some fundamental problems in an incredibly successful theory.

So what was wrong? For a start, the universe seems far too smooth. Although there had been attempts to come up with an explanation in the past, Dicke and Peebles couldn't to their satisfaction identify one that worked. And there was more. Why does the geometry of space, as opposed to that of spacetime, look so simple? The geometry of space seems to have no overall curvature, and the rules of high-school-level Euclidean geometry apply. Such rules as
Parallel lines never intersect
and
The sum of the angles of a triangle is 180 degrees
seem invariably true. A universe with no spatial curvature is allowed in general relativity, but it is a very special case. Einstein's equations predict that the evolution of the universe is likely to push the curvature away from zero incredibly quickly. So, if the universe seems to have almost no curvature today, it must have had even
less
curvature in the past. The universe we live in is extremely unlikely. Finally, the galaxies and structures built up of galaxies spanning the heavens must have come from somewhere. Conditions had to be perfectly tuned for the universe to look as it does today. At the Big Bang, the tendency of the universe to expand had to be just enough to compensate for the pull of gravity and prevent the whole of spacetime from collapsing into itself, yet not so extreme that spacetime would fly apart in an empty void. Their article boiled down to a simple question: What happened in the very beginning?

Dicke and Peebles's article was followed by another short essay by Yakov Zel'dovich. In his article, Zel'dovich pondered the very early universe following the line of reasoning that the Abbé Lemaître had first taken when discussing his primordial atom. There was a whole plethora of interesting phenomena at play in the hot early universe that could impact its evolution and affect how it evolved into what we see today. Zel'dovich urged the community of particle physicists and relativists to figure out what these effects would be.

Dicke and Peebles's and Zel'dovich's papers were prescient. Just one year later, cosmology would be turned on its head by a simple proposal for how the early universe evolved. The idea had been floating around in an unformed way, but it took Alan Guth, a postdoc at the Stanford Linear Accelerator Center, to come up with the essence of cosmic inflation. Guth realized that in some grand unified theories—theories that attempted to unify the electromagnetic, weak, and strong forces into one overarching force—the universe could be trapped in a state in which the energy of one of the fields was incredibly high and dominated everything else. In that state, the universe would be driven to expand rapidly, or inflate, as Guth dubbed it. Although Guth's original idea turned out to be flawed—if the universe was trapped in such a state, there was no way of getting out of it—new ways of making the universe inflate were quickly proposed by others.

The idea of an inflating universe, or inflation, opened up a new avenue in cosmology, revealing a new period in the universe's past that could be explored. Now there was a theory that predicted exactly how the universe should be when structure started to form, and it seemed to address the problems raised by Dicke and Peebles. For a start, the theory of inflation pushed space to almost instantaneously have no curvature. Imagine taking a round balloon that you can hold in your hands and using a giant pump to blow it up so quickly that it almost instantly becomes the size of the Earth. From your perspective, the piece of balloon in front of you would now look pretty flat. Inflation would also drive the universe toward a tremendously smooth, pristine state. Any large lumps or voids that would naturally pepper the landscape of spacetime would have been pushed far out into the distance, invisible to our gaze. Inflation also brought with it a way to kick-start the growth of structure in the very early universe. During the period of intense inflation, the microscopic quantum fluctuations in the fabric of spacetime would be stretched and imprinted onto the largest scales.

Inflation, as astrophysicists in Chicago succinctly put it, established the link between
“inner space and outer space.” Inner space was the world of the quantum and the fundamental forces, and outer space encompassed the cosmos, where general relativity came into its own. And so, the program of research that Peebles had been developing over the previous decade, along with the work of Zel'dovich, Silk, and others, took on a new purpose: the large-scale structure of the universe, the distribution of galaxies, and relic light should hold the clues that link inner and outer space. People began to take notice.

 

In 1982, Peebles tried to construct a new universe. The old model he'd developed with Jer Yu, made of atoms and radiation, wasn't working out. When he compared the results of his model to the surveys of galaxies that had been mapped out in the sky, they didn't match. Reality simply didn't agree with his elegant calculation. Not only that, in the previous decade, galaxies themselves seemed to have become a whole lot more complicated. A strange picture was emerging of what was going on inside them.

The American astronomer Vera Rubin had found that galaxies seemed to spin far too quickly for their own good, like manic Catherine wheels held together by a mysterious force. Rubin focused her telescope on the Andromeda Galaxy, a swirl of stars and gas spinning at hundreds of kilometers per second. At least that's how it appears if you look at it with a telescope. There was much more light at the center where all the stars are concentrated, so Rubin expected that most of the gravitational pull keeping the galaxy together would come from its central core. But as she looked at nuggets of stars farther and farther away from the center of the galaxy, she found they were moving far too quickly. In fact, the stars were speeding around so quickly that Rubin simply couldn't understand how the gravitational pull of the galaxy's center could rein them in. It was as if the Earth suddenly doubled or tripled the speed of its orbit around the sun. Unless the sun somehow increased its gravitational pull, the Earth would simply fly out of the sun's orbit and shoot off into space. Something else, big and invisible, was holding the outer stars in their orbits.

Fritz Zwicky observed a similar phenomenon in the 1930s, but his results were ignored for almost forty years. Zwicky had looked at the Coma cluster of galaxies and added up the total amount of mass he could see there. He had then measured the speed with which the galaxies were moving around inside the cluster and found that they were moving far too quickly. As he said in a paper he published in Switzerland in 1937,
“The density of luminous matter in Coma must be minuscule compared with the density in some sort of dark matter.”

Jim Peebles was coming up against his own problems with galaxies. With a young collaborator from Princeton, Jerry Ostriker, he set about building simple computer models for how galaxies formed, representing them as a bunch of particles pulling each other through gravity and spinning around in a spiral. But whenever he set his models spinning, the galaxies would disintegrate. A blob would form at the center that would stretch out through the arms and tear the galaxy apart. Ostriker and Peebles tried to stabilize their models by immersing their spinning particles in a ball of invisible mass. This sphere of stuff—a halo, they called it—would bolster the gravity keeping the galaxy together. The halo had to be dark (that is, invisible) so as not to be detected by telescopes. Paradoxically, the model showed that this dark matter had to be much more abundant than the atoms that were seen in stars. In the late 1970s, Sandra Faber, working at Santa Cruz in California, and Jay Gallagher, working in Illinois, wrote a review in which they collated the odd findings that astronomers were getting when looking at galaxies and that Peebles and his colleagues were discovering when simulating them. They concluded that
“we think it likely that the discovery of invisible matter will endure as one of the major conclusions of modern astronomy.”

In 1982, when Peebles began building a new model of the universe, he decided to include atoms
and
dark matter. In fact, he assumed that almost
all
of the universe was made up of a mysterious form of matter composed of heavy particles, invisible to us because it didn't interact with light. Peebles's cold dark matter model was simple, and it enabled him to predict what the distribution of galaxies looked like and how large the ripples in the relic radiation would be. This approach would prove to have a momentous impact on the development of cosmology, but as Peebles recalled, “I didn't take it at all seriously . . . I wrote it down because it was simple and it could fit the observations.”

While Peebles didn't refer to the recently proposed inflationary era, his new model fit the Zeitgeist perfectly. It invoked a massive particle that could arise from fundamental physics, connecting inner and outer space. The cold dark matter model, or CDM model for short, was adopted by a growing army of astronomers and physicists who began to work out the fine details of how galaxies would actually form. Marc Davis at Berkeley allied himself with two British astronomers, George Efstathiou and Simon White, and the Mexican astronomer Carlos Frenk to build computer models to follow the formation of individual galaxies and clusters of galaxies in virtual universes. In their simulations, this gang of four, as they became known, would follow hundreds of thousands of particles as they interacted with one another, coming together to form the large-scale structure of the universe.

While CDM was popular and eagerly adopted, too many things seemed to go wrong. In the CDM model Peebles created, the universe could be only 7 billion years old, which was far too young. Astronomers had found dense pockets of stars known as globular clusters bobbing around in galaxies. These bright concentrations of light were full of old stars that must have formed early on in the history of the universe when it was mostly full of hydrogen and helium, which meant that the globular clusters had to be at least 10 billion years old. And there was more. If the universe was primarily made up of cold dark matter, the proportion of dark matter to atoms would be roughly 25 to 1. Yet, hard as they looked, astronomers couldn't figure out where that dark matter was. From the speed at which galaxies rotated or from the temperature of clusters of galaxies they observed they could infer how much gravity there was (the hotter they were, the more gravitational pull there had to be) and how much dark matter was necessary to generate that amount of gravity. The ratio of dark matter to atoms they kept on coming up with was closer to 6 to 1. True, the methods for weighing the dark matter were still crude and uncertain, but the deficit seemed too great to be explained by the margin of error. Almost immediately after creating the CDM model, Peebles felt compelled to give it up and look for alternative models.
“There was a lot of net casting in the eighties and early nineties,” as he put it.

The gang of four didn't fare any better. They used their computer models to create virtual universes and compared them with the real universe to see if they looked alike. They didn't. For a start, the real universe appeared to be much more structured and complex on large scales than the fake universes. In the CDM universe, the galaxies were much more clustered on small scales but smoothed out more quickly, once you zoomed out to look at the bigger picture, than in the real universe. It was possible to alleviate some of the problems in the virtual universes by slightly fudging the results, but the truth was that Peebles's simple model wasn't entirely working.

Despite the fact that it conflicted with basic observations, the cold dark matter model was embraced by the majority of astronomers and physicists. It was conceptually simple and fit nicely with inflation and the evidence for dark matter in galaxies. CDM's adherents looked for ways to further develop and somehow fix the model. One way of fixing the CDM involved resurrecting Einstein's cosmological constant. To many, that was anathema.

 

The case against the
cosmological constant had become stronger since Einstein first introduced it in 1917. While he had, with the discovery of the expanding universe, rapidly discarded the cosmological constant from his theory, a few of his colleagues clung to it. Both Eddington and the Abbé Lemaître chose to incorporate it in their models of the universe. Lemaître went so far as to conjecture that the cosmological constant was nothing more than the energy density of the vacuum. In 1967 Zel'dovich showed what a serious problem the cosmological constant could be. He added up the energy of all the virtual particles that would pop in and out of existence in the universe and found that the resulting energy density would look like a cosmological constant but should have a truly gigantic value. Strictly speaking, the resulting cosmological constant would be infinite, for exactly the same reasons that everything involving quantum gravity was infinite, but a little hand waving could make it finite. Even so, it was a huge number, orders of magnitude greater than any energy that had been measured in the cosmos.

Zel'dovich's calculation showed that if there was an energy of the vacuum in the universe—and therefore a cosmological constant—it would be far too big to be compatible with observations. The only way to proceed was to assume that some as-yet-undiscovered physical mechanism intervened to make the cosmological constant equal zero. In practice cosmologists chose to ignore the cosmological constant and pretend it didn't exist.

Yet, again and again, whenever anyone tried to resolve the problems with the cold dark matter model, the cosmological constant, known as lambda, always cropped up as one of the possible solutions. In 1984, Peebles himself found that a viable universe with cold dark matter would need lambda to make up about 80 percent of the total energy of the universe. When the gang of four—Davis, Efstathiou, Frenk, and White—tried simulating one of their universes with lambda in it, they found that many of the problems they came up against with the simple CDM scenario went away.