Knocking on Heaven's Door (55 page)

Wilson told me how lucky they were in the timing of their discovery. They didn’t know about the Big Bang, but Robert Dicke and Jim Peebles at Prince ton University did. The physicists there had just realized that one implication of the theory would be a relic microwave radiation. They were in the process of designing an experiment to measure this radiation when they discovered they had been scooped—by the Bell Lab scientists who hadn’t yet realized what they had discovered. Luckily for Penzias and Wilson, the MIT astronomer Bernie Burke, who Robert Wilson described to me as the early version of the Internet, knew about the Princeton research and also the Penzias and Wilson discovery. He put two and two together and brought the connection to fruition by bringing the relevant players into contact.

This was a lovely example of science in action. The research was done for a specific scientific purpose that could also have ancillary technological and scientific benefits. The astronomers weren’t looking for what they found, but they were extremely technologically and scientifically skilled. When they discovered something, they knew not to dismiss it. Their research—while looking for relatively small phenomena—resulted in a discovery with tremendously deep implications, which they found because they and others were thinking about the big picture at the same time. The discovery by the Bell Lab scientists was accidental, but it forever changed the science of cosmology.

The cosmic radiation has proved to be a tremendous tool—not just for confirming the Big Bang but for turning cosmology into a detailed science. The cosmic microwave background (CMB) radiation gives us a very different way of observing the past than traditional astronomy measurements.

In the past, astronomers would observe objects in the sky, try to determine their age, and attempt to deduce the evolutionary history that produced them. With the CMB, scientists can now also look directly back in time before structure such as stars and galaxies were even formed. The light they observe was emitted long ago—very early in the universe’s evolution. When the microwave background we now observe was emitted, the universe was only about one-thousandth its current size.

Although the universe was originally filled with all types of particles—both charged and uncharged—once it cooled sufficiently, 400,000 years into its evolution, charged particles combined together into neutral atoms. Once this happened, light no longer scattered. Observed CMB radiation therefore arrives directly from about four hundred thousand years into the universe’s evolution—unhindered and uninterrupted—to telescopes on Earth and on satellites. The background radiation Penzias and Wilson discovered was the same radiation present in the earlier stages of the universe’s history, but it has been diluted and cooled through its expansion. The radiation traveled directly to the telescopes that detected it with no hindrances from scattering off any intervening charged particles en route. This light gives us a direct and precise window into the past.

The Cosmic Microwave Background Explorer (COBE), a four-yearlong satellite mission launched in 1989, measured this background radiation extremely accurately, and the mission scientists found that their measurements agreed with predictions to better than one part in 1,000. But COBE measured something new as well. By far, the most interesting thing that COBE measured was a tiny bit of nonuniformity in temperature across the sky. Although the universe is extremely smooth, tiny in-homogeneities at the level of less than one in 10,000 in the early universe grew bigger and were essential to the development of structure. The in-homogeneities originated on minuscule length scales, but were stretched to sizes relevant to astrophysical measurements and structure. Gravity caused the denser regions where the perturbations were especially large to become more concentrated and form the massive objects we currently observe. The stars, galaxies, and clusters of galaxies that we discussed earlier are all the result of these initial tiny quantum mechanical fluctuations and their evolution through the gravitational force.

The microwave background measurement continues to be critical to our understanding of the universe’s evolution. It’s role as a direct window into the early universe cannot be underestimated. More recently, along with more traditional methods, CMB measurements have provided experimental insights into several other more mysterious phenomena—cosmological inflation, dark matter, and dark energy—subjects that we turn to next.

WHAT’S SO LARGE TO YOU IS SO SMALL TO ME

When I was an MIT professor, the department ran out of office space on the third floor where the particle physicists worked. So I relocated to the open office next door to Alan Guth’s on the floor below, which at the time housed theoretical astronomers and cosmologists. Although Alan started his career as a particle physicist, he is known today as one of the best cosmologists around. At the time of my office move, I had already explored some connections between particle physics and cosmology. But it’s a lot easier to continue such research when your neighbor shares those interests—and is as messy as you are so that in his office you feel right at home.

Many particle physicists have gone further afield than a single floor and crossed over into a wide variety of other research areas. Wally Gilbert, a cofounder of Biogen, started life as a particle physicist but left to do biology and Nobel Prize–winning chemistry research. Many since have followed in his footsteps. On the other hand, many of my graduate student friends left particle physics to be “quants” on Wall Street where they could bet on changes in future markets. They chose just the right time to make such a move since the new financial instruments to hedge such bets were only just being developed at the time. In the crossover to biology, some ways of thinking and organizing problems carried over, whereas in finance some of the methods and equations did.

But the overlap between particle physics and cosmology is of course far deeper and richer than either of the above. Close examination of the universe on different scales has exposed the many connections between elementary particles on the smallest scales and the universe itself at the largest. After all, the universe is by definition unique and encompasses everything within it. Particle physicists, who look inward, ask what type of fundamental matter exists at the core of matter, and cosmologists, who look outward, study how whatever it is that is out there has evolved. The universe’s mysteries—most not ably what it is made of—matter to cosmologists and particle physicists alike.

Both types of researchers investigate basic structure and employ fundamental physical laws. Each needs to take into account the results of the other. The content of the universe that is studied by particle physicists is an important research subject for cosmologists too. Furthermore, the laws of nature that incorporate both general relativity and particle physics describe the universe’s evolution, as they must if both theories are correct and apply to a single cosmos. At the same time, the known evolution of the universe constrains what properties matter can have if it is to avoid disrupting the observed history. The universe was in some respects the first and most powerful particle accelerator. Energies and temperatures were very high in the early stages of its evolution, and the high energies that accelerators currently achieve aim to reproduce some aspects of those conditions today on Earth.

Recent attention to this convergence of interests has led to many fruitful investigations and major insights and will hopefully continue to do so. This chapter considers some of the big open questions in cosmology that particle physicists and cosmologists both explore. The overlapping arenas include cosmological inflation, dark matter, and dark energy. We’ll consider aspects we understand about each of these phenomena and—more important for active research—those that we don’t.

COSMOLOGICAL INFLATION

Even though we can’t yet say what happened at the very beginning of the universe, since we would need a comprehensive theory that incorporates both quantum mechanics and gravity, we can assert with reasonable certainty that at some time very early on (perhaps as early as 10
⁻³⁹
seconds into the universe’s evolution), a phenomenon called
cosmological inflation
occurred.

In 1980, Alan Guth first suggested this scenario, which says that the very early universe essentially exploded outward. Interestingly, he was initially trying to solve a problem for particle physics involving the cosmological consequences of Grand Unified Theories. Coming from a particle background, he used methods rooted in field theory—the theory combining special relativity and quantum mechanics that particle physicists employ for our calculations. But he ended up deriving a theory that revolutionized our thinking about cosmology. How and when inflation occurred is still a matter of speculation. But a universe that underwent this explosive expansion would leave clear evidence, and much of it has now been found.

In the standard Big Bang scenario, the early universe grew calmly and steadily—for example, doubling in size when its age increased by a factor of four. But in an inflationary epoch, a patch of the sky underwent a phase of incredibly rapid expansion, growing exponentially with time. The universe doubled in size in a fixed time and then doubled again in that same time and then kept doubling at least 90 times in a row until the inflationary epoch ended and the universe was as smooth as we see it today. This exponential expansion means, for example, that when the universe’s age had multiplied by 60 times, the size of the universe would have increased by more than a trillion trillion trillions in size. Without inflation, it would have increased by a mere factor of eight. In some sense, inflation was the beginning of our story of evolving from the small to the large—at least the part that we can potentially understand through observations. The initial enormous inflationary expansion would have diluted the matter and radiation content of the universe to practically nothing. Everything we observe today in the universe must therefore have arisen right after inflation, when the energy that drove the inflationary explosion converted into matter and radiation. At this point in time, conventional Big Bang evolution took over—and the universe began its further expansion into the huge structure we see today.

We can think of the inflationary explosion as the “bang” that was the precursor to the universe’s evolving according to the standard Big Bang theory. It’s not truly the beginning—we don’t know what happened when quantum gravity played a role—but it’s when the Big Bang stage of evolution, with matter cooling and eventually aggregating, began.

Inflation also partially answers why there is something rather than nothing. Some of the enormous energy density stored during inflation was converted (consistently with
E = mc
²
) to matter, and that is the matter that evolves to what we see today. As I discuss at the close of this chapter, we physicists still would like to know why there is more matter than antimatter in the universe. But whatever the answer to that question, the matter we know began evolving according to Big Bang theory predictions as soon as cosmological inflation ended.

Inflation was derived as a bottom-up theory. It solved important problems for the conventional Big Bang explosion, but only a few really believed any of the actual models for how it came about. No compelling high-energy theory seemed to obviously imply inflation. Since it was so challenging to make a credible model, many physicists (including those at Harvard when I was a graduate student) doubted the idea could be right. On the other hand, Andrei Linde, a Russian-born physicist now at Stanford, and one of the first to work on inflation, thought it had to be correct simply because no one had found any other solution to the puzzles about the size, shape, and uniformity of the universe that inflation addressed.

Inflation was an interesting example of the truth-beauty connection—or lack thereof. Whereas the exponential expansion of the universe beautifully and succinctly explained many phenomena about how the universe started, the search for a theory that naturally yields the exponential expansion led to many not-so-pretty models.

Recently, however, most physicists—even though not yet satisfied with most models—have become convinced that inflation, or something very similar to inflation, did occur. Observations of the last several years have confirmed the cosmological picture of Big Bang cosmology preceded by inflation. Many physicists now trust that Big Bang evolution and inflation have occurred because predictions based on these theories have been confirmed with impressive precision. The true model underlying inflation is still an open question. But the exponential expansion has a lot of evidence supporting it at this point.

One type of evidence for cosmological inflation has to do with the deviations from perfect uniformity in the cosmic microwave background radiation that the previous chapter introduced. The background radiation tells us much more than just that the Big Bang occurred. The beauty of it is that because it is essentially a snapshot of the universe very early on—before stars had time to form—it lets us look back directly into the beginnings of structure at the time when the universe was still very smooth. Cosmic microwave background (CMB) measurements also revealed tiny departures from perfect homogeneity. Inflation predicts this because quantum mechanical fluctuations caused inflation to end at slightly different times in different regions of the universe, giving rise to tiny deviations from absolute uniformity. The satellite-based Wilkinson Microwave Anisotropy Probe (WMAP), named for the Prince ton physicist David Wilkinson who pioneered the project, made detailed measurements that distinguished inflationary predictions from other possibilities. Despite the fact that inflation happened long ago at incredibly high temperatures, theory based on inflationary cosmology nonetheless predicts the exact statistical properties of the pattern of temperature variations that should be imprinted on the radiation in the sky today. WMAP measured the small inhomogeneities in temperature and energy density with more accuracy and on smaller angular scales than had been done before, and the pattern conformed to inflationary expectations.