Dark Energy: A Short Review

Understand

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 ($w=-1$).