Published in the UCD Physics Dept Newsletter, Fall 1998

It is worth noting that the recent coming of age of cosmology has come at an important time in the evolving relationship between science and society. Citizens and politicians are questioning with increasing frequency the role of basic research and the extent to which they should support it. The old mix of elitism and cold war paranoia no longer can be counted on to provide the steady stream of funding it once did. I feel that cosmology has a special role to play in these discussions. First of all, the field generates a very natural interest—it does not take much work to persuade non-experts that the questions are interesting. Secondly, because the field is presently making such rapid scientific progress, it makes an ideal showcase of what science has to offer. People’s natural curiosity about the universe can lead them, unwittingly in some cases, into a first rate introduction to the excitement and rewards of frontier research. I have always taken a strong interest in advancing this role for cosmology, and I expect my efforts in this direction to continue here at UC Davis.

This special role for cosmology takes on a more substantial form when it comes to university level teaching (both undergraduate and graduate). In this context, I have often seen that the passion for the fundamental questions of cosmology causes students to stretch and challenge themselves intellectually in ways that they might not otherwise have achieved. As often as not, once hooked on the thrill of conquering intellectual challenges, these students discover they can find equal (if not greater) personal satisfaction from taking on challenges in a wide variety of other areas, both in academia and industry. I believe this is probably one of the most important spin-offs from the field of cosmology.

But what are the new ingredients that are making cosmology such a success, and what are the tools we need to realize the great promise of the field? There is no doubt that a key element of current progress in cosmology is the new data. Modern technology is completely revolutionizing observational astrophysics. One of the crucial astronomical measurements is the redshift of an object. From the redshift we can determine an object’s radial motion relative to us. For distant objects this radial motion is dominated by the cosmic Hubble expansion, and even the remaining motion can have deep cosmological significance. Not long ago redshifts had to be measured one at a time. Now an automated spectrometer exists which can measure 200 redshifts at once. In a few years the Sloan Digital Sky Survey will have measured the redshifts of well over a million galaxies, using a spectrometer now under construction that can measure 640 objects at once. These advances are being repeated across the board, with rapid progress at all wavelengths, and covering a wide range of astronomical objects.

A particularly important example is the mapping of the sky at microwave frequencies. We expect to extract from such a map information about light that has interacted very little over the last 10 billion years, giving us an image of the edge of the observable universe. (The early universe was so hot and dense that it was opaque, so there is only so far back we can look before hitting this opaqueness, the so-called "last scattering surface.") This Cosmic Microwave Background (CMB) has been observed since the ‘60’s, and one of its most striking features is its tremendous isotropy. The isotropy of the CMB provides crucial evidence supporting the standard Big Bang model of the universe. But almost all models also predict tiny deviations from perfect isotropy, and in most cases the details of these tiny deviations (or fluctuations) are relics of earlier events that provide direct links with the high energy physics that describes the earliest stages of the Big Bang.

When I was a graduate student in the early ‘80’s, fluctuations in the CMB had not been measured, although steadily decreasing upper bounds on their amplitude were being determined. Today, thanks to the COBE satellite, we have a map of these fluctuations over the entire sky, down to a resolution of seven degrees. Furthermore, numerous smaller patches of the sky have been measured to much higher resolutions. Satellites now being built (NASA’s MAP and the European Space Agency’s PLANCK, of which I am a member of the Science Team) will measure the microwave sky at a large number of frequencies with a resolution of fractions of a degree. The advent of the COBE data was already revolutionary. There is no question that the new CMB data will completely transform our understanding of the universe.

The example of the CMB and redshift data are but two items on a long and impressive list of new information about the universe which is flowing in at a tremendous rate. Much of the current theoretical work reflects the theorists’ natural interest in getting as close as possible to the new data. For example, some of my recent work includes developing new statistics that can be used to extract crucial pieces of information from the new data, as well as studies of how our uncertainties about the last scattering surface will affect our interpretation of the CMB data. There has also been a lot of work identifying the observable signals that will have the most impact on our understanding of the early universe. Recent work of mine has emphasized the significance of certain features in the angular power spectrum of CMB anisotropies. This kind of phenomenological work is the bread and butter of modern cosmology. There is a lot to be done, and it is certain that this kind of work will be rewarded by steady progress as the data continues to flow in.

But ultimately, what will all this concrete, steady progress tell us? Behind fairly straightforward questions like "what was the spectrum of primordial perturbations" lurk more difficult questions, like "how could we possibly claim to have a theory of initial conditions for the universe, which could explain these primordial perturbations?" Perhaps surprisingly, we are not at an utter loss on such questions either. A big part of the reason cosmologists can be so ambitious traces back to the isotropy of the CMB that I mentioned earlier. Because we do not believe we are at some special central location in the universe, the isotropy we observe in the CMB (the temperature looks the same in all directions) is interpreted as homogeneity (the universe was the same temperature at all locations at the time of last scattering). Of course, the homogeneity is not perfect due to the perturbations, which give small spatially varying corrections to the temperature (at the 0.001 percent level). Still, the inferred homogeneity allows us to construct a remarkably simple model of the universe, the standard Big Bang model, in which the universe is an expanding body in nearly perfect local thermal equilibrium for most periods of its history.

Against the backdrop of this simple model, the field of particle cosmology has developed. In an expanding and cooling homogeneous quasi-thermal state (starting with the ultra-hot singularity of the Big Bang) one can use models of the fundamental constituents of matter to generate a detailed description of the matter as it expanded and cooled. The standard analysis of nucleosynthesis gives a good illustration of what is possible: Known laboratory measurements teach us enough about nuclear reactions to know that the nuclei were in chemical equilibrium at sufficiently early stages. As the universe expanded and cooled, one can trace (using computer models of the relevant equations) the ultimate freezing out of particular nuclear species, thus providing a prediction of their primordial abundance. These predictions are broadly confirmed by observations, leading to one of the great successes of cosmology.

But physicists have gone further. At high enough temperatures no experiments have probed the nature of matter directly, but there is a great deal of speculation by high energy physicists as to what the laws of nature could be like in these regimes. Thus, at sufficiently early times, the universe becomes a laboratory in which one can test ideas in high energy physics. One can work out the observable consequences of a given model and check if it is consistent with the observations. One of the great recurring ideas in high energy physics is spontaneous symmetry breaking. It is the only known way of giving fundamental particles a non-zero mass. Almost every instance of spontaneous symmetry breaking in particle physics will result in a cosmological phase transition, as the universe cools from a high temperature symmetric phase to the symmetry-broken phase. Cosmic phase transitions are a source of a wide range of interesting observable consequences. In fact this sub-field made its debut in the form of the famous monopole problem. It was shown that the phase transitions associated with essentially all known grand unified theories produced magnetic monopoles in such quantities that they would be the dominant form of matter today. Since not a single magnetic monopole has been observed to date, all those models were ruled out.

So against the background of a homogeneous expanding universe, the field of particle cosmology has flourished. It is interesting to note that Newton used his cosmos (the motions of the planets) as a laboratory where sufficiently simple conditions prevailed to test his big ideas. To us the cosmos encompasses much more. But even so it offers us an excellent laboratory in which to test our big ideas. The special relationship with astrophysics is an interesting one: Astrophysics is not particle physics, but it is a necessary tool to do cutting edge particle physics, much like one must understand the physics of particle detectors if one is to interpret a laboratory experiment. One of the links with astrophysics that deserves special mention is the question of the dark matter. It is clear from astrophysical observation that most of the matter in the universe is dark and is thus sufficiently hidden not to be clearly identified. The most popular explanation of the dark matter says that it is weakly interacting frozen out fundamental particles, whose abundances can be calculated in specific models using methods similar to the nucleosynthesis calculations. This provides a particularly profound link between particle physics and astrophysics, since the entire picture of galaxy formation hinges crucially on the nature of the dark matter.

So thanks to the field of particle cosmology, numerous links can be made between current observations and events in the very early universe. Thus, the new cosmological data will teach us not only about astrophysics, but about high energy physics as well. But do these links really help us understand the most fundamental questions? Even if we do think of galaxies as being seeded by topological defects formed in a cosmic phase transition, and having halos of dark matter composed of specific fundamental particles, the nature of the galaxies is also affected by the initial conditions given to the universe before the phase transition. It might seem natural to assume perfect homogeneity, but that is only one possible initial state out of an infinity of possibilities. (It turns out, in fact, that given gravity’s natural tendency to clump things, a homogeneous initial state looks like an extremely unreasonable starting point.) Surely our universe simply had one set of initial conditions and there is nothing more one can say about it.

But modern day cosmologists still do not give up! We believe we can even explain the initial conditions of the universe. The basic idea on which we pin our hopes was already present in the discussion of nucleosynthesis. Why did we think we could predict the abundances of the elements, without reference to the initial conditions? The key was the local chemical equilibrium that existed at the beginning. The process of equilibration wiped out information about any chemical initial conditions from an earlier epoch, and gave a clean predictable starting point for the process of nucleosynthesis. In this picture, the impact of different pre-equilibrium initial conditions is hidden in subtle details of the microscopic motions—things which mean nothing to us.

The theory of cosmic inflation implements the same idea to explain the homogeneity of the universe. It can be shown that under the right conditions, a special potential dominated state of matter can be achieved, for which gravity is repulsive. In this state, cosmic expansion would be ultra-rapid, and many different initial conditions would be drawn toward an attractor which is the homogeneous expanding universe of the standard Big Bang. In fact, it turns out that variations on the inflationary theme predict small perturbations, and inflation is actually the account of the origin of these fluctuations favored by most cosmologists. I should mention that the idea of a potential dominated state of matter is also a product of the fundamental role of spontaneous symmetry breaking in particle physics. Not only does modern cosmology help us test models of particle physics, it draws some of its key ideas from particle physics as well.

Inflation is a relatively new arrival, and I certainly feel there are many loose ends to be resolved. For what sorts of pre-inflation initial conditions is the convergence really effective? Can one work out a model in which the probability in the space of all possibilities is really peaked around the universe in which we live? These questions still have not been given satisfactory answers. In fact, in the face of so much promise of concrete progress on other questions at the phenomenological end of the field, these deeper questions look like just so much navel gazing. However, once we have processed all our new data, and hopefully made sense of it, these deeper questions will still demand attention.

I hope I have conveyed some of the scope and excitement of the field of cosmology. I am really looking forward, with Bob Becker and other members of the department, to building an impressive cosmology group here. We will keep you posted of new developments, as our group and the field continues to grow.