Grantee: University of Chicago, Chicago, IL, USA
Researcher: John E. Carlstrom, Ph.D.
Grant Title: Taking a Snapshot of the Early Universe
Program Area: Centennial Fellowship
Grant Type: Research Award
Year Awarded: 1999Taking a Snapshot of the Early Universe
Starting as children, we are driven to explore our environment and constantly expand our horizons. The field of cosmology coordinates this exploration, allowing each generation to start its exploration at the frontier of the previous generation. Cosmology strikes a fundamental chord in all of us; we are struck with the enormity of the universe and are humbled by our place within it. At the same time we are empowered with our ability to obtain answers to fundamental questions of the universe. Peering back to the early universe and taking a snapshot does more than let us measure cosmological parameters, it enriches us and satisfies a fundamental desire in all of us.
Copernicus at the beginning of the Renaissance set in motion a fundamental change in our understanding of our place in the universe by showing the Earth circles the sun. Today, cosmologists are also setting in motion a fundamental change, by showing that the matter we know about, the stuff that makes up the sun, Earth, and even ourselves, only accounts for a small fraction, perhaps as little as 10 percent, of the mass of universe.
Cosmology today is at a crossroads where theory and experiment are coming together for a major increase in our understanding of the universe. Astrophysicists are narrowing in on the Hubble constant, the rate of expansion of the universe. Theorists have developed a standard cosmological model for the origin of structure in the universe. They have also shown that the key to understanding the universe is contained in the cosmic microwave background radiation, the relic radiation of the Big Bang, motivating experimentalists to build ever more sensitive experiments. Through studies of the microwave background, theorists and experimentalists are on the cusp of testing rigorously the standard model and determining precise values of cosmological parameters, which will soon tell us the age of the universe, whether it will expand forever or close in upon itself, and how much mass is in a form still unknown to physicists.
The cosmic microwave background photons (particles of light) travel to us from across the universe, from a time when the universe was in its infancy, less than a ten-thousandth of its present age. Small differences, of order a few hundred-thousandths, in the intensity of the microwave background carry information about the structure of the infant universe to us today. The properties of the cosmic microwave background also make it an incredible backlight with which to explore the universe; it is strong, even your radio can pick it up, and it is remarkably isotropic. If your eyes were sensitive to microwave radiation, you would see a uniform glow in all directions. However, just as the stained glass of a cathedral window creates a beautiful display of color and shape from a common backlight, large scale structure in the universe alters the intensity and spectrum of the microwave background. The results can be stunning and, moreover, much can be learned about the universe from detailed studies of the display. Your microwave eyes, however, would have to be very sensitive to see it, as the display has very little contrast, with only a tenth of a percent or less deviation from the uniform glow of the background.
My collaborators and I have built a sensitive system to image the display created by the passage of the cosmic microwave background radiation through the gas contained within clusters of galaxies. The effect is known as the Sunyaev Zel'dovich effect after the two Russian astrophysicists who predicted it in 1972. Using the powerful technique of radio interferometry, our system has allowed us to make, for the first time, detailed images of the effect. With these images we are able to measure distances to clusters, and therefore obtain a completely independent measure of Hubble constant, using a technique based solely on the physics of the cluster gas. Our Sunyaev Zel'dovich effect data also allow us to estimate the ratio of ordinary matter to the total mass of a galaxy cluster. Combined with results from primordial nucleosynthesis calculations and elemental abundance measurements, our results allow an estimate of the mass density of the universe. The mass density we find is close to the critical value which marks the transition between a universe which expands forever and one which eventually will collapse upon itself due to the gravitational attraction of the matter.
Based on the success of our Sunyaev Zel'dovich effect imaging system, my collaborators and I have begun a much more ambitious experiment. We plan to take a detailed snapshot of the early universe. I am working with collaborators at the University of Chicago and at Caltech to build a pair of dedicated interferometers to image the small intensity differences, of order a few hundred-thousandths, in the cosmic microwave background radiation itself. Because this small anisotropy in the background radiation provides the key to understanding the origin and evolution of structure in the universe, these new instruments will open a new window on the universe. While the instruments are designed to provide answers to some of the most pressing questions in modern cosmology, we recognize that discoveries in astrophysics are driven by new techniques and instrumentation. We fully expect the new window on the universe opened by these instruments will provide fascinating and unexpected views.
The promise of using the cosmic microwave background radiation to explore the early universe and to unlock the secrets of the origin of structure in the universe was immediately appreciated when Arno Penzias and Robert Wilson discovered the background over 30 years ago. Since that time theorists have shown exactly how the information may be encrypted in the background radiation as small differences in the brightness of the background from one part of the sky to another. And, experiments have shown that these fluctuations in the otherwise smooth background are indeed tiny. Only now, however, are we developing sufficiently sensitive and carefully optimized instruments with which to unveil the wealth of information contained in these small fluctuations. I firmly believe that dedicated, specially designed interferometers will play a major role in this exciting field, and I plan to continue to play an active role in their development and use.
In trying to answer our questions, we are led to new ones. What is the matter that makes up most of the universe? If inflation is shown to be the correct model for the early universe, then we must revise drastically our notion of the big bang. Inflation holds that our universe is the result of a small inflated bubble, and that countless bubbles are possible. Our understanding of our place in the universe, or even the place of the universe as we currently envision it, must once again undergo a fundamental change.