a new one waiting to be discovered. Astronomers feel confident in using the universe as a laboratory to explore natural phenomena that are inaccessible to Earth-based laboratories. The study of how the universe and its constituent objects and phenomena work continues to yield unique insight into fundamental science.

The Nature of Inflation

As described previously, the inflation hypothesis proposes that the universe began to expand exponentially some 10−3 seconds after the big bang. This hypothesis explains why the present universe has almost the same temperature everywhere we look, as measured by the microwave background radiation, over the entire sky. Despite the power of the hypothesis, the mechanism by which inflation happened—its origin—remains a great mystery. Directly confirming inflation and understanding its fundamental underlying mechanism lie at the frontier of particle physics, because inflation probes scales of energy far beyond anything that can be achieved in accelerators on Earth. Inflation is central to astrophysics: the quantum fluctuations present during inflation formed the seeds that grew into the CMB fluctuations and the large-scale structure of the universe we see around us today. Perhaps the most profound reason to understand inflation is that its nature and duration might have spelled the difference between a universe of sufficient vastness to house galaxies, planets, and life, and a “microverse” so small that matter as we know it could not be contained therein. To understand the origin of our macroscopic universe—why we exist—requires understanding inflation.

The last decade was one of stunning progress in our understanding of the first moments of the universe. NSF-supported South Pole and Chilean ground-based work, and NASA’s balloon-based studies and the Wilkinson Microwave Anisotropy Probe Explorer mission, mapped the spatial pattern of temperature fluctuations that occur in the relic cosmic microwave background from the big bang. The state of the young universe during the epoch of inflation, prior to the existence of stars or galaxies, is imprinted as minute fluctuations in the CMB, and the character of these fluctuations is broadly consistent with the theory of inflation. Armed with theoretical advances and complementary balloon-borne and ground-based measurements, we are now ready to move beyond foundational knowledge of the very early universe and apply increasingly more precise measurements of the CMB to new questions. One important test of inflation involves making highly detailed measurements of the structure of the universe by mapping the distribution of hundreds of millions of galaxies. Inflation makes very specific predictions about the spatial distribution of the dark matter halos that host these galaxies.

However, the most exciting quest of all is to hunt for evidence of gravitational waves that are the product of inflation itself. Just as the light we see with our own eyes can be polarized, the CMB radiation may also carry a pattern of polarization—



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