The most striking recent development in cosmology was the discovery that the expansion of the universe is speeding up. Initially discovered by observations of distant Type Ia supernovae, this effect has now been confirmed by observations of the CMB anisotropy, the large-scale galaxy distribution, and the evolution of the galaxy cluster abundance. While the existence of cosmic acceleration is now firmly established, its physical origin remains a deep mystery. Within General Relativity, Einstein's theory of gravity, vacuum energy (i.e. the Cosmological Constant) smooth compared with the dark matter could be responsible. Alternatively, General Relativity may need to be modified or replaced to correctly explain the acceleration.

Identifying the physical cause of the cosmic acceleration is a key challenge for fundamental physics in the coming decade. Answering this question requires multiple astrophysical and cosmological techniques, analysis of multiple, large-volume data sets, and synergy between theory and observations. The Dark Energy MA was created to take on this challenge. The goals of the MA are to address whether or not cosmic acceleration requires a modification of Einstein's theory of gravity, General Relativity, and if not, to probe the nature of the dark energy that is driving acceleration and that makes up 75% of the universe.

In order to meet these goals, the MA will seek to measure the dark energy equation of state parameter to high precision and accuracy and to test the consistency of General Relativity with cosmic acceleration by measuring the cosmic expansion history and growth rate of large-scale structure. The MA is engaging in the following specific activities to achieve its plans:

• The MA is leading two major experimental efforts that will deliver the next major steps in probing dark energy:
  
  • The SPT Sunyaev-Zel'dovich survey, which will measure the abundance of galaxy clusters out to high redshift.
  • The Dark Energy Survey, which will probe cosmic acceleration through four complementary techniques.
• The Dark Energy MA has formed the Joint Analysis Hub which will coordinate the analyses of cosmic acceleration probes. This Hub will initially focus upon data from SPT and DES as they relate to cluster cosmology. In the longer term, data from LSST and the proposed DESI spectroscopic follow-up survey will be incorporated into the analyses.
• The Supernova Hub has been formed. This Hub brings together observers and theorists to improve supernovae as cosmological distance indicators by employing existent and upcoming supernova surveys as well as state-of-the-art three-dimensional simulations of SN Ia explosions.