because lumps in the CMBR are believed to be the ancestors of lumpsof matter in our universe today. Making large surveys of the sky,astronomers are now able to locate the positions of thousands ofgalaxies in space and have found to their surprise that galaxiesare far from uniformly distributed. Enormous sheets of galaxies enclosehuge empty voids and form a structure that resembles a sponge orsoap foam. Similarly, large-scale studies of galaxy motions showthat huge regions of the universe are involved in high-speed bulkmotion relative to the CMBR. The complexity and enormous scale ofstructure in the universe surprised cosmologists. But an eleganttheory has been proposed to explain the formation of the recentlydiscovered large-scale structure. In this theory, puny irregularitiesin the CMBR, similar to those detected by the COBE satellite, areamplified over the eons by gravitational forces to become the lumpsand sheets of matter astronomers see in the universe today. As thistheory is tested by new data, new ideas, and new calculations, cosmologistsmay at long last understand, in broad outline, the mechanism forstructure formation in the universe.

The details of the statistical properties of the structure expectedin the universe are dependent on the type and quantity of “dark matter” that dominates the universe. Cosmologists look forward to anotherexciting discovery—the identification of this mysterious dark matter.Over the decades, astronomers have gathered evidence of unseen massbinding together galaxies and clusters of galaxies, but the natureof the dark matter still remains a mystery. Is it something thatwe already know about, like the stuff that makes up our Earth andSun? Or has nature concealed some completely new kind of matter fromour earthbound physics experiments? Theoretical particle physicsoffers many exotic candidates, raising the possibility of not onlydiscovering a major component of the universe but triggering a newera in particle physics as well. The search is on, in physics laboratoriesand at telescopes. A recent development is the search for dark matterby using its ability to bend light from distant stars or galaxies(gravitational lensing). Dark matter in clusters of galaxies andin the halo of our own galaxy is being studied with this elegantnew technique.

A dream of cosmologists is to be able to make detailed studies ofgalaxies at great distances and early times. What did galaxies looklike soon after formation? How do they evolve? When did they form?With the Hubble Space Telescope (HST) and giant (8- to 10-meter)new ground-based optical telescopes just commissioned and being built,the astronomical exploration of deep space (astronomers say “highredshift”) starts now. The repaired Hubble and the new Keck telescopehave racked up major discoveries in their first few months of operation,leaving no doubt that a new era is beginning for observational cosmology.

But what about even earlier, simpler cosmic times, before the galaxiesformed? The CMBR can bring us news from a time when the universewas only 150,000 years old, or 0.001 percent of its current age.To find out what happened at even earlier times cosmologists mustrely more on theoretical calculations, based on the physics we havelearned on Earth. Our current understanding of conditions in theuniverse penetrates to remarkably early times, because theoreticalpredictions of early events have measurable consequences today. Forexample, nuclear reactions predicted to occur when the universe wasonly about a minute old should have produced helium, deuterium, andother light elements. The predicted abundances agree exquisitelywith currently measured abundances of these elements! On less firmground, but even more amazing, is the idea that the same CMBR lumpsthat acted as seeds for the formation of the large-scale structurecan be traced back to quantum fluctuations occurring in the firstbillion billion billion billionth of a second after the Big Bang!If confirmed by measurements now under way, this knowledge will standamong the major triumphs of human ingenuity and imagination.

These are some of the current research frontiers that challenge andexcite cosmologists. The main body of this report discusses current



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