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. Answering this question requires multiple astrophysical and cosmological techniques, analysis of multiple, large-volume data sets, and synergy between theory and observations. The KICP has been a leader in addressing whether or not cosmic acceleration requires a modification of General Relativity, and, if not, in probing the nature of the dark energy that is driving acceleration and that makes up 75% of the universe.
To meet these goals, the KICP, has led efforts 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.
- In preparation for the next steps, the KICP led two major experimental efforts whose results continue to probe dark energy:
- The SPT Sunyaev-Zel'dovich survey, measured the abundance of galaxy clusters out to high redshift and produced an unbiased catalog of great use for studies of the growth of clusters
- The Dark Energy Survey (DES), probed cosmic acceleration through four complementary techniques and the data continues to be analyzed to hone in on dark energy parameters with greater precision.
- The KICP led efforts to jointly analyze the data from multiple surveys, such as DES and the SPT-SZ and continues its efforts to bring together complementary surveys to increase their precision. Numerous techniques were developed which will play important roles as future data from CMB-S4 and the LSST (and others) are analyzed together to tease out cosmological information about Dark Energy.
- The KICP plays a leading role in reducing the systematic errors inherent in using Type Ia Supernovae as cosmological probes and in using these techniques on the DES Supernova survey to learn more about cosmic acceleration.