Suppressing the lower Multipoles in the CMB Anisotropies

The Cosmic Microwave Background (CMB) anisotropy power on the largest angular scales observed both by WMAP and COBE DMR appears to be lower than the one predicted by the standard model of cosmology with almost scale free primordial perturbations arising from a period of inflation \cite{cobe,Bennett:2003bz,Spergel,Peiris}. One can either interpret this as a manifestation of cosmic variance or as a physical effect that requires an explanation. We discuss various mechanisms that could be responsible for the suppression of such low $\ell$ multipoles. Features in the late time evolution of metric fluctuations may do this via the integral Sachs-Wolfe effect. Another possibility is a suppression of power at large scales in the primordial spectrum induced by a fast rolling stage in the evolution of the inflaton field at the beginning of the last 65 e-folds of inflation. We illustrate this effect in a simple model of inflation and fit the resulting CMB spectrum to the observed temperature-temperature (TT) power spectrum. We find that the WMAP observations suggest a cutoff at $k_c=4.9^{+1.3}_{-1.6}\times 10^{-4}$Mpc$^{-1}$ at 68% confidence, while only an upper limit of $k_c < 7.4\times 10^{-4}$Mpc$^{-1}$ at 95%. Thus, although it improves the fit of the data, the presence of a cutoff in power spectrum is only required at a level close to $2\sigma$. This is obtained with a prior which corresponds to equal distribution wrt $k_c$. We discuss how other choices (such as an equal distribution wrt $\ln k_c$ which is natural in the context of inflation) can affect the statistical interpretation.