Dark Energy, Dark Matter, and Cosmology
Cosmology with Clusters and Groups of Galaxies
Recent studies of the baryonic mass fraction in clusters of galaxies, the evolution of clusters of galaxies, type Ia supernovae (SN), and the cosmic microwave background (CMB) have revealed a "preposterous Universe" (Carroll 2005) in which ~70% of the energy density of the Universe today is in the form of "dark energy", 26% is in the form of dark matter, and the rest is the sum of normal matter and neutrinos. Strikingly, the observed value of the dark energy density today is many orders of magnitude smaller than the most natural values predicted by the standard model of particle physics.
Understanding cosmic acceleration and the nature of dark energy is one of the most important goals in physics and astronomy today, and these results must be checked by a variety of precise cosmological tests over a wide range of astrophysical objects with small statistical and systematic errors.
There are several near-term, non-X-ray programs to study dark energy including CMB data, ground and HST-based SN studies, gravitational lensing studies, and studies of large-scale structure. The equation of state of dark energy can be defined in terms of w, the ratio of pressure to density. The combined multiwavelength data of these upcoming studies will constrain the constant equation-of-state dark energy models at the σw ~ 0.1-0.15 level. However, each technique has its own limits and systematic errors. One major systematic uncertainty of the SN-based studies is the unknown evolution of the standard candles with redshift. Other systematic concerns include the nature and subtraction of the host galaxy light and the effects of gravitational lensing. For an individual supernova event at z~1, the difference in SN Ia optical magnitude between a closed universe model and a ΩCDM model is only 0.25 mag. Thus, extreme care and precision are required in the analysis and interpretation of the SN Ia data.
In large-scale galaxy cluster surveys the difference between a closed model and a ΩCDM model is an order of magnitude. For this method, the major systematic uncertainty lies in connecting the observable (e.g. X-ray luminosity or optical number counts) to cluster masses. These surveys take advantage of redundant cosmological information in the spatial clustering of the sources and the evolution of the mass function with redshift (observed as a luminosity function). At the core, this technique relies on reliable and direct mass measurements, which are possible with the high resolution, high signal-to-noise spectroscopy of Constellation-X.
Constellation-X will perform two independent sets of cosmological tests using X-ray measurements of clusters of galaxies. The first is measuring the absolute distances to clusters via direct and indirect means, thereby determining the transformation between redshift and true distance, d(z), which is a strong function of cosmological parameters. The second is measuring the growth of structure by combining Constellation-X measurements with theoretically informed models for baryon population evolution over redshift.
There are two other planned projects to study dark energy which will be comlpementary to Cosntellation-X and will be carried out on timescales similar to Constellation-X. The Large Synoptic Survey Telescope (LSST) mission will focus on measuring cosmic shear and producing samples of 105 SN distances to z~0.8. One possible incarnation of Joint Dark Energy Mission (JDEM) will measure the distances to ~3,000 SN to z~1.7 and map cosmic shear over a small portion of the sky. Both projects claim constraints on the dark energy equation-of-state parameter at the level of a few percent, similar to that achievable with Constellation-X.
The following articles contain more information on how Constellation-X will study clusters and groups of galaxies to constrain cosmological parameters:
- Dark Energy Constraints from Absolute Cluster Distances
- Constraining the Growth of Cosmic Structure via the Cluster Mass Function
- The Entropy Problem in Groups
- Dark Matter
