Dark Energy Science

Two separate and highly complementary Constellation-X dark energy experiments will each, individually, in combination with Planck data, obtain uncertainties on the time-averaged dark energy equation of state w to ±0.05. In combination with other, contemporary constraints (such as those from Planck) Constellation-X will provide an order of magnitude improvement in our knowledge of the key dark energy parameters.

Determining the nature of the "Dark Energy" that appears to dominate the energy budget of the Universe and is driving the acceleration of its expansion remains a major goal of both fundamental physics and astrophysics. To constrain Dark Energy we require multiple, independent means of testing its nature so that we may rule out some of the many competing theories. There are important tools available in the X-ray bandpass that provide extremely important tests of dark energy. This is thanks to the nature of the largest gravitationally bound structures in the Universe – galaxy clusters.

X-ray observations of galaxy clusters are crucial since ~85% of the baryons within them are in the hot X-ray emitting gas. Detailed measurements of the temperature and density profiles of this hot gas permit two types of tests of Dark Energy using galaxy clusters, one based upon the observationally-verified baryon mass fraction "standard candle" (a geometric measurement) and the other based on the evolution of the cluster mass function (a "growth of structure" measurement). Conveniently, the key measurements for both tests can be made using the same set of large, relaxed clusters of galaxies. Constellation-X will observe large samples of clusters of galaxies (> 500 objects) over a wide redshift range (0 < z < 2; median redshift z~1) with high precision to constrain Dark Energy parameters.

The strategy for dark energy work with Constellation-X will involve an initial snapshot program (1-ks exposures) to observe the 3000-5000 most X-ray luminous (or highest integrated Sunyaev-Zeldovich flux) clusters known at that time. These snapshots will identify the 500 most relaxed systems, based on their X-ray morphology. These 500 clusters will then be followed up with deeper exposures of, on average, 20 ks each, which will be sufficient to measure fgas and predict the Sunyaev-Zeldovich (SZ) flux from the observed X-ray temperature and gas density profiles to 5% accuracy, corresponding to 3.3% in absolute distance. Similar constraints on dark energy should also be achievable by observing the best 250 clusters for 40 ks each, on average. This strategy may be useful if the fraction of relaxed, luminous clusters is found to drop significantly at the highest redshifts.