CMB hemispherical asymmetry from non-linear isocurvature perturbations

Abstract

We investigate whether non-adiabatic perturbations from inflation could produce an asymmetric distribution of temperature anisotropies on large angular scales in the cosmic microwave background (CMB). We use a generalised non-linear δN formalism to calculate the non-Gaussianity of the primordial density and isocurvature perturbations due to the presence of non-adiabatic, but approximately scale-invariant field fluctuations during multi-field inflation. This local-type nonGaussianity leads to a correlation between very long wavelength inhomogeneities, larger than our observable horizon, and smaller scale fluctuations in the radiation and matter density. Matter isocurvature perturbations contribute primarily to low CMB multipoles and hence can lead to a hemispherical asymmetry on large angular scales, with negligible asymmetry on smaller scales. In curvaton models, where the matter isocurvature perturbation is partly correlated with the primordial density perturbation, we are unable to obtain a significant asymmetry on large angular scales while respecting current observational constraints on the observed quadrupole. However in the axion model, where the matter isocurvature and primordial density perturbations are uncorrelated, we find it may be possible to obtain a significant asymmetry due to isocurvature modes on large angular scales. Such an isocurvature origin for the hemispherical asymmetry would naturally give rise to a distinctive asymmetry in the CMB polarisation on large scales.

Large-scale anomalies in the cosmic microwave background as signatures of non-Gaussianity

“…(A multiple-field model of inflation, or one that otherwise breaks the usual consistency relation, is required because superhorizon perturbations in the standard inflaton field cannot generate an asymmetry [50].) Dubbed the Erickcek-Kamionkowski-Carroll (EKC) mechanism [11], this approach has been expanded upon and refined several times since its inception [12,[14][15][16][17][18][19][20][22][23][24][25][26][27][28][29][30][31][32][33][34]. In these analyses, WMAP and Planck bounds on local-type non-Gaussianity forced the amplitude of the superhorizon perturbation to be much larger than predicted by an extrapolation of the observed primordial power spectrum to larger scales.…”

“…Although possible origins for this large-amplitude fluctuation have been proposed, such as a supercurvature perturbation in an open universe [19], a bounce prior to inflation [31], a non-vacuum initial state [26], topological defects [18,27], or deviations from slowroll inflation [20,22], it is largely taken as an ad hoc addition to the inflationary landscape. The scale dependence of the asymmetry often requires an additional elaboration to the theory, either in the form of scale-dependent non-Gaussianity [12,14,18,26,[30][31][32] or isocurvature fluctuations [12,20,25].…”

Analytical study of classic models of Hybrid Inflation

“…On the other hand, Planck and WMAP have observed a ∼ 10% hemispherical asymmetry in the power level of CMB at large angular scales [11][12][13]. This feature attracts several explanations and suggestions, one of which is that it could originate from inflation due to multiple fields with non-adiabatic and non-linear isocurvature perturbations from inflation [14][15][16][17].…”

Finslerian dipolar modulation of the CMB power spectra at scales $$2<l<600$$ 2 < l < 600

“…Several anisotropic inflationary models have been built to solve the power asymmetry problem [23][24][25][26][27][28][29][30]. Usually, the background spacetime of the anisotropic inflation model is described by Bianchi spacetime [31].…”