Does the small CMB quadrupole moment suggest new physics?

TL;DR

This study investigates whether the anomalously low CMB quadrupole can be explained by a low-$k$ cutoff in the primordial power spectrum $P(k)$, arising from features in the inflaton potential, a pre-inflationary epoch, or coupling to an oscillating field. Using a complete six-parameter cosmological model plus a cutoff scale $k_*$, and incorporating WMAP TT and TE data (plus 2dFGRS), the authors perform a likelihood analysis to assess the evidence for such a cutoff. They find only marginal support for a nonzero $k_*$: the TT data alone prefer $k_*$, but TE polarization reduces this to about $1.4σ$, and inclusion of 2dF data further weakens the case, yielding $k_* ightarrow 0$ within uncertainties. The work highlights the non-Gaussian nature of the low-$ ext{l}$ likelihood and concludes that there is no strong evidence for new physics in the form of a low-$k$ cutoff, though the quadrupole anomaly remains a challenging puzzle with correlations to reionization parameters.

Abstract

Motivated by WMAP's confirmation of an anomalously low value of the quadrupole moment of the CMB temperature fluctuations, we investigate the effects on the CMB of cutting off the primordial power spectrum P(k) at low wave numbers. This could arise, for example, from a break in the inflaton potential, a prior period of matter or radiation domination, or an oscillating scalar field which couples to the inflaton. We reanalyze the full WMAP parameter space supplemented by a low-k cutoff for P(k). The temperature correlations by themselves are better fit by a cutoff spectrum, but including the TE temperature-polarization spectrum reduces this preference to a 1.4 sigma effect. Inclusion of large scale structure data does not change the conclusion. If taken seriously, the low-k cutoff is correlated with optical depth so that reionization occurs even earlier than indicated by the WMAP analysis.