Did the universe undergo inflation, a rapid period of accelerating expansion within its first moments? If so, what drove this early acceleration, when exactly did it occur, and how did it end? When introduced in the early 1980s, the inflationary paradigm made a number of generic observational predictions: we live in a flat universe seeded by nearly scale-invariant, adiabatic, Gaussian scalar fluctuations. Over the past decade, cosmological observations have confirmed these predictions. Over the coming decade, it may be possible to detect the gravitational waves produced by inflation, and thereby infer the inflationary energy scale, through measurements of the polarization of the microwave background. It may also be possible to test the physics of inflation and distinguish among models by precisely measuring departures from the predictions of the simplest models.

Is the acceleration of the universe the signature of a breakdown of general relativity on the largest scales, or is it due to dark energy? The current evidence for the acceleration of the universe rests primarily on measurements of the relationship between distance and redshift based on observations of supernovae, the CMB, and LSS. Improved distance measurements can test whether the distance-redshift relationship follows the form expected for vacuum energy or whether the dark energy evolves with redshift. Measurements of the growth rate of LSS provide an independent probe of the effects of dark energy. The combination of these measurements tests the validity of general relativity on large scales. The evidence for cosmic acceleration provides further motivation for improving tests of general relativity on laboratory, interplanetary, and cosmic scales, and for searching for variations in fundamental parameters.

Astronomical observations imply that the dark matter is nonbaryonic. Particle theory suggests several viable dark matter candidates, including weakly interacting massive particles (WIMPs)¹ and axions. Over the coming decade, the combination of accelerator experiments at the Large Hadron Collider (LHC), direct and indirect dark matter searches, and astrophysical probes are poised to test these and other leading candidates and may identify the particles that constitute dark matter.