Cosmologia

Dark Matter

The combination of X-ray, infrared and CMB millimetric and submillimetric data can be used to study the distribution and possible evolution of dark matter (DM) in low- and intermediate- redshift universe. CMB contributes to this effort with spectral, anisotropy and polarization measurements.

Low frequency CMB spectrum measurements can probe the Universe at z ∼ 10 - 30, where the bulk of structure formation occurs. Some processes (such as partial reionization, at 11 ≲ z_r ≲ 30) can distort the almost perfect Planckian CMB spectrum in the low frequency tail (below ∼ 15 GHz). It generates free electrons capable of polarize the CMB photons in large angular scales and can be used as a DM tracer at these redshifts.

This frequency interval is also sensitive to earlier distortions ( 10⁴ ≲ z ≲ 10⁷) described by a Bose-Einstein distribution with dimensionless chemical potential μ = 1.4ΔE/E. Current DM models predict a non-zero chemical potential for the CMB as a result of exotic particle decay. Measurements in this frequency range would provide valuable information for high energy and neutrino physics [11], since the details of the decay of DM candidates can be associated to the decrement profile in the CMB spectrum.

CMB temperature fluctuations can explore two interesting and complementary directions: Sunyaev-Zeldovich (SZ) and lensing measurements. These are quite difficult to separate from one another since all of them appear as ΔT_CMB. An elegant way to study CMB lensing is through its effect in the CMB anisotropy power spectrum [19]. The "lensed" CMB power spectrum shows an increase in amplitude if compared to the "unlensed" one at large l (l ≳ 1000 where the coherent gravitational light deflection starts to become important. This happens because lensing mixes power from larger scales into the CMB power spectrum damping tail. The present generation of experiments are already able to explore the high -l tail of the power spectrum, so effects of lensing contamination should appear in the next set of hig resolution CMB power spectra. Other noticeable effects of lensing on CMB observations are the induction of non-Gaussian features on the measured temperature map (see, e.g., [24]) and the mixing of different types of CMB polarisation (E and B modes) [25]. The later, however, is fairly small in typical cosmological models and will only marginally affect future CMB polarisation measurements.

The SZ effect is responsible for the up-scattering of about 1 - 2% of CMB photons, when they cross the hot intracluster medium. Its amplitude is ∼ a few mK for the thermal SZ effect and about 10% of it for the kynematic SZ effect. SZ's most important feature, however, is its redshift independence, allowing measurements of clusters as soon as they are formed. The SZ effect also allows DM estimates, when combined with other cosmological datasets. For a good review on the importance of SZ for cosmological research see, e.g., [5]. For instance, combined with X-ray estimates of the gas temperature, provides a powerful probe of the total baryonic mass in a galaxy cluster and, indirectly, one can estimate the total matter density Ω_Mh². Combined with an estimate of the X-ray temperature, observations of the thermal and kinetic SZ effects can be used to determine the peculiar velocity of a distant cluster. Although it will be challenging to determine the peculiar velocities of individual clusters, it should be possible to make accurate determinations of bulk flows involving many clusters. Finally, due to its redshift independence, observations of the SZ effect, teamed with X-ray flux determination, have the potential to be a uniquely powerful method of locating distant clusters.