Suppressing the lower multipoles in the CMB anisotropies

Abstract

The Cosmic Microwave Background (CMB) anisotropy power on the largestangular scales observed both by WMAP and COBE DMR appears to be lowerthan the one predicted by the standard model of cosmology with almostscale free primordial perturbations arising from a period of inflation. One can either interpretthis as a manifestation of cosmic variance or as a physical effectthat requires an explanation. We discuss various mechanisms that couldbe responsible for the suppression of such low ℓmultipoles. Features in the late time evolution of metric fluctuationsmay do this via the integral Sachs–Wolfe effect. Another possibilityis a suppression of power at large scales in the primordial spectruminduced by a fast rolling stage in the evolution of the inflaton fieldat the beginning of the last 65 e-folds of inflation. We illustratethis effect in a simple model of inflation and fit the resulting CMBspectrum to the observed temperature–temperature (TT) powerspectrum. We find that the WMAP observations suggest a cutoff atkc = 4.9+1.3−1.6 × 10−4 Mpc−1 at 68% confidence,but only an upper limit of kc < 7.4 × 10−4 Mpc−1 at95%. Thus, although it improves the fit of the data, the presence ofa cutoff in power spectrum is only required at a level close to2σ. This is obtained with a prior which corresponds to equaldistribution w.r.t. kc. We discuss how other choices (such as an equaldistribution w.r.t. lnkc, which is natural in the context ofinflation) can affect the statistical interpretation.