  The accelerating expansion of the universe is the most surprising
cosmological discovery in many decades. In this short review, we briefly
summarize theories for the origin of cosmic acceleration and the observational
methods being used to test these theories. We then discuss the current
observational state of the field, with constraints from the cosmic microwave
background (CMB), baryon acoustic oscillations (BAO), Type Ia supernovae (SN),
direct measurements of the Hubble constant ($H_0$), and measurements of galaxy
and matter clustering. Assuming a flat universe and dark energy with a constant
equation-of-state parameter $w = P/\rho$, the combination of Planck CMB
temperature anisotropies, WMAP CMB polarization, the Union2.1 SN compilation,
and a compilation of BAO measurements yields $w = -1.10^{+0.08}_{-0.07}$,
consistent with a cosmological constant ($w=-1$). However, with these
constraints the cosmological constant model predicts a value of $H_0$ that is
lower than several of the leading recent estimates, and it predicts a parameter
combination $\sigma_8(\Omega_m)^{0.5}$ that is higher than many estimates from
weak gravitational lensing, galaxy clusters, and redshift-space distortions.
Individually these tensions are only significant at the ~$2\sigma$ level, but
they arise in multiple data sets with independent statistics and distinct
sources of systematic uncertainty. The tensions are stronger with Planck CMB
data than they were with WMAP because of the smaller statistical errors and the
higher central value of $\Omega_m.$ With the improved measurements expected
from the next generation of data sets, these tensions may diminish, or they may
sharpen in a way that points towards a more complete physical understanding of
cosmic acceleration.
