The Cosmological Constant

This is a review of the physics and cosmology of the cosmological constant. Focusing on recent developments, I present a pedagogical overview of cosmology in the presence of a cosmological constant, observational constraints on its magnitude, and the physics of a small (and potentially nonzero) vacuum energy.

Solving the coincidence problem: tracking oscillating energy

Recent cosmological observations strongly suggest that the Universe is dominated by an unknown form of energy with negative pressure. Why is this dark energy density of order the critical density today? We propose that the dark energy has periodically dominated in the past so that its preponderance today is natural. We illustrate this paradigm with a model potential and show that its predictions are consistent with all observations.

The dearth of halo dwarf galaxies: is there power on short scales?

N-body simulations of structure formation with scale-invariant primordial perturbations show significantly more virialized objects of dwarf-galaxy mass in a typical galactic halo than are observed around the Milky Way. We show that the dearth of observed dwarf galaxies could be explained by a dramatic downturn in the power spectrum at small distance scales. This suppression of small-scale power might also help mitigate the disagreement between cuspy simulated halos and smooth observed halos, while remaining consistent with Lyman-alpha-forest constraints on small-scale power. Such a spectrum could arise in inflationary models with broken-scale invariance.