Suppressing CMB low multipoles with ISW effect

Abstract

Recent results of Planck data reveal that the power [1,2] in the low multipoles ofthe CMB angular power spectrum, approximately up to l = 30, is significantly lower thanthe theoretically predicted in the best fit ΛCDM model.There are different known physical effectsthat can affect the power at low multipoles, such as features in the primordial powerspectrum (PPS) in some models of inflationand ISW effect. In this paper we investigate the possibility of invoking the IntegratedSachs-Wolfe (ISW) effect to explain thepower deficit at low multipoles. The ISW effect thatoriginates from the late time expansion history of theuniverse is rich in possibilities given the limited understanding of the origin ofdark energy (DE). It is a common understandingthat the ISW effect adds to the power at the low multipoles of the CMB angular power spectrum. In thispaper we carry out an analytic study to show that there are some expansion historiesin which the ISW effect, instead of adding power, provides negativecontribution to the power at low multipoles.Guided by the analytic study, we present examples ofthe features required in the late time expansion history of theuniverse that could explain the power deficiency through the ISW effect.We also show that an ISW origin of power deficiency is consistent, at present, with othercosmological observations that probethe expansion history such as distance modulus, matter power spectrum and the evolution of clusternumber count. We also show that the ISW effect may be distinguished frompower deficit originating from features in the PPS using the measurements of the CMB polarizationspectrum at low multipoles expected from Planck. We conclude that the power at low multipolesof the CMB anisotropy could well be closely linked to Dark Energy puzzle in cosmology and thisobservation could be actually pointing to richer phenomenology of DE beyond the cosmologicalconstant Λ.