Dark Energy

Present CMB datasets, combined with information about extragalactic or other cosmological observers, such as the SN 1a data sample, the gas fraction of galaxy clusters and the Hubble parameter, offer a powerful tool to probe the space of cosmological parameters. t allows, at the same time, the investigation of the properties, initial value and evolution of dark energy (hereafter DE), setting bounds on the w-parameter of the DE equation of state, which relates the cosmological fluid pressure and density, as PX = wρX [16].

CMB data are probably the most powerful and sensitive of these data, although subjected to substantial limitations (good sky coverage, systematic errors, very good sensitivity, etc.). The positions of the CMB acoustic peaks are mostly sensitive to variations of the DE equation of state. Most of that sensitivity arises from the dependence of the distance to the last-scattering surface upon the time history of ω.

Several authors have discussed different combinations of the abovementioned datasets to study the nature and properties of DE. A good starting reference is the paper from Huterer and Turner [10], where they compare a number of approaches to combine different observations to constrain w.

An example of combined CMB and LSS measurements was proposed by Hu & Scranton [8]. They correlate the integrated Sachs-Wolfe (ISW) effect and galaxy survey catalogs, claiming that a simple detection of the correlation can impose a ∼ 3% limit to changes in the total density fluctuation due to dark energy clustering on the Gpc scales. Measuring the correlation out to z ≈ 2 allows one to distinguish between dark energy models. Beyond the adiabatic model, clustering in the dark energy may have strong effects even if w → -1, offering another test to isocurvature models.

Balbi et al. [1] use only the Fisher information matrix (see [22] for the formalism description) with cosmological parameters as measured by CMB experiments (including temperature and polarization) to set stringent limits on the dark energy component. There are also attempts to estimate dark energy based only on CMB acoustic peaks measurements, with the interesting possibility to allow discrimination between the original cosmological constant (which starts dominating at very recent redshifts) and a quintessential model, which, in principle, would be present since recombination [6]. Both of the above methods are, in principle, immediately testable with current experiments and can become useful tools when the next generation of CMB experiments come to play in the upcoming years.

We recall that the use of CMB for any cosmological measurement, including dark energy, has the great advantage that CMB perturbations are linear and therefore relatively simple to describe. With any other observable from LSS, we need to fully understand the non-linear structure formation in dark energy cosmology. As a closing remark, we should stress that while in the near term CMB anisotropy will provide the first measurements of ω, supernovae and number counts appear to have the most potential to probe dark energy in the long run.