make predictions that can be tested are deeply interesting but very open questions.


The inflationary paradigm is a bold attempt to extend the big bang model backward to the first moments of the universe. It uses some of the most fundamental ideas in particle physics (e.g., symmetry breaking and vacuum energy) to answer many of the basic questions of cosmology. Because of these deep connections, advances in elementary particle physics and cosmology will come hand in hand.

A number of opportunities are now ripe for answering some of the central questions of the inflationary picture. Is the inflationary picture correct? If so, what is the physics of inflation? Were there phase transitions in the early universe associated with changes in the symmetries of the underlying physics? Are the gravitational whispers detectable? It may well be possible to answer he boldest question we can ask—How did the universe begin?

How Did the Universe Begin?

In the coming years, observations will provide more stringent tests of the inflationary model. Experiments that map the fluctuations of the microwave background on finer angular scales, together with weak gravitational lensing surveys, can measure the size and the rate of expansion of the universe, the density of ordinary and dark matter, and the basic parameters of inflation. Measurements of microwave background polarization fluctuations will be sensitive to primordial gravitational waves, possibly yielding clues to the physics that underlies inflation.

What Are the New States of Matter at Exceedingly High Density and Temperature?

The direct detection of long-wavelength gravitational waves will enable scientists to “listen to” phase transitions in the early universe. This ability will require new experiments, probably space-based, designed to look for the background of gravitational waves produced in the early universe. If successful, these observations could reveal exotic states of matter in the hot, early universe, including quark-gluon plasma.

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