Overview of the CMB and CMB Polarization
The first observable photons emitted by our young universe were released about 380,000 years after the big bang (Bennett et al., 2003). Before this time the photons were tightly coupled to electrons in a photon-baryon fluid consisting primarily of electrons, photons, and H and He nuclei. However, as the universe expanded it became cool enough for the electrons to bind to nuclei, (referred to as the time of recombination), and the photons were able to free stream out from the surface of last scattering. These photons were initially emitted in the infrared, but due to the expansion of the universe they are now microwaves with a corresponding 3 K blackbody temperature. The CMB radiation was first detected by Penzias and Wilson in the mid 1960s (Penzias and Wilson, 1965), however it took until the early 1990s for the COBE satellite to measure the tiny fluctuations in the temperature of the radiation at different points in the sky at the level of 1 in 10^5. Subsequent missions have further developed our understanding of the CMB temperature anisotropies. Notably, the WMAP satellite data released in 2003 produced a precise full-sky map and a power spectrum of the temperature anisotropies down to a scale of l=400 in frequencies ranging from 22 to 90 GHz (Spergel et al., 2003). A close study of the anisotropies of both the temperature and polarization of the CMB yields an immense wealth of cosmological information about the universe at the surface of last scattering as well as the conditions at reionization, the initial conditions for large scale structure, and the validity of various models of the early universe such as the energy scale of inflation.
Although there is still much to learn from the temperature anisotropies in the CMB, we are instead focusing our effort on the much younger and less developed field of CMB polarization. Primary polarization of the CMB is generated by Thomson scattering of the CMB photons off of free electrons in an anisotropic radiation field. The only epochs in our evolutionary history that provide the appropriate conditions for significant CMB polarization are recombination and reionization. The polarization pattern of the CMB (represented by the linear Stokes vector components Q and U) can be broken into divergence-free and curl-free components.
The E-mode, or curl-free component, of the CMB polarization resulted from the scattering of CMB photons off of electrons in the anisotropic velocity field which was generated by quadrupolar fluctuations in density, and thus temperature, at the surface of last scattering and during reionization. E-mode polarization data used alone (the EE auto-correlation) and with temperature data (the TE cross-correlation) can improve our estimates of various cosmological parameters, in some cases breaking the degeneracies between parameters measured using CMB temperature anisotropy data alone. Although the E-mode polarization signal is weaker than that from the temperature anisotropies by roughly an order of magnitude, it has already been detected by a number of experiments including DASI (Kovac et al., 2002), CBI (Readhead et al., 2004), WMAP (Kogut et al., 2003), and BOOMERANG (MacTavish et al., 2005). The figure above shows the expected EBEX sensitivity to E mode polarization anisotropy.
The B-mode polarization of the CMB is a divergence-free polarization pattern. It is a predicted result of the stochastic background of gravitational waves thought to have been generated during inflation, resulting in an anisotropic radiation field at the surface of last scattering and during reionization. A determination of the amplitude of the primordial B-mode signal, expected to peak at l=6 and 100, will be especially remarkable since it will provide a measure of the energy scale of inflation (Hu 2002). B-modes are also predicted to be generated at small scales when the primordial E-mode CMB radiation is lensed by intervening matter between the surface of last scattering and the present epoch (Zaldarriaga and Seljak, 1998). The lens generated B-mode signal is expected to be much weaker than the primordial signal at large scales, however the it is expected to completely dwarf the primordial signal on small scales; the figure above shows the anticipated B-mode amplitude resulting from gravity waves and lensing separately and summed together. To distinguish these signals we need to observe over a wide range of l. The lensing signal will help us to develop a better understanding of the distribution of matter and the evolution of large scale structure, complementing other methods like the Sunyaev-Zel'dovich Effect and other non-CMB galaxy cluster surveys as well as large-scale galaxy weak lensing surveys. The B-mode signal has thus far not been detected, however the current upper limit on r is 0.5, where r is the ratio of the strength of the temperature anisotropy generated by gravitational waves and by perturbations in density at l=2 (Peiris, 2003 and Tegmark, 2003). |
