Dark Energy: A Short Review

TL;DR

This review summarizes current theories and observational constraints on dark energy, highlighting tensions between data sets and the potential for future measurements to clarify the nature of cosmic acceleration.

Contribution

It provides a comprehensive overview of observational constraints on dark energy and discusses current tensions and future prospects in understanding cosmic acceleration.

Findings

Constraints on dark energy parameter w are consistent with a cosmological constant.

Current data show tensions in H_0 and sigma_8 measurements at about 2 sigma.

Future data may resolve or deepen existing tensions in dark energy studies.

Abstract

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.