Leverage on small-scale primordial non-Gaussianity through cross-correlations between CMB E-mode and μ-distortion anisotropies

Abstract

Multi-field inflation models and non-Bunch-Davies vacuum initial conditions both predict sizeable non-Gaussian primordial perturbations and anisotropic μ-type spectral distortions of the cosmic microwave background (CMB) blackbody. While CMB anisotropies allow us to probe non-Gaussianity at wavenumbers k ≃ 0.05 Mpc−1, μ-distortion anisotropies are related to non-Gaussianity of primordial perturbation modes with much larger wavenumbers, k ≃ 740 Mpc−1. Through cross-correlations between CMB and μ-distortion anisotropies, one can therefore shed light on the aforementioned inflation models. We investigate the ability of a future CMB satellite imager like LiteBIRD to measure μT and μE cross-power spectra between anisotropic μ-distortions and CMB temperature and E-mode polarization anisotropies in the presence of foregrounds, and derive LiteBIRD forecasts on ${f_{\rm NL}^\mu (k\simeq 740\, {\rm Mpc^{-1}})}$. We show that μE cross-correlations with CMB polarization provide more constraining power on $f_{\rm NL}^\mu$ than μT cross-correlations in the presence of foregrounds, and the joint combination of μT and μE observables adds further leverage to the detection of small-scale primordial non-Gaussianity. For multi-field inflation, we find that LiteBIRD would detect ${f_{\rm NL}^\mu }=4500$ at 5σ significance after foreground removal, and achieve a minimum error of ${\sigma (f_{\rm NL}^\mu =0) \simeq 800}$ at 68% CL by combining CMB temperature and polarization. Due to the huge dynamic range of wavenumbers between CMB and μ-distortion anisotropies, such large $f^\mu _{\rm NL}$ values would still be consistent with current CMB constraints in the case of very mild scale-dependence of primordial non-Gaussianity. Anisotropic spectral distortions thus provide a new path, complementary to CMB B-modes, to probe inflation with LiteBIRD.

Spectro-spatial evolution of the CMB. Part I. Discretisation of the thermalisation Green's function

“…For the fluctuating part, no spectral evolution has to be considered at the late stages (redshift z 10 4 ), just like for the SZ effect -a linear perturbation description of the problem is furthermore possible, yielding y-parameter transfer functions that are excited by first order temperature perturbations [33,34]. If the amplitude of the small-scale curvature perturbations is modulated by large-scale modes this can furthermore lead to correlated µ × T and y × T fluctuations [35][36][37][38][39][40][41][42], which can be directly constrained using CMB imagers [see [43][44][45], for most recent forecasts and constraints]. Note that at the largest angular scales, the corresponding transfer problem was simplified by neglecting details of the distortion evolution in the perturbed Universe [35,40,46].…”

CMB spectral distortions revisited: a new take on $μ$ distortions and primordial non-Gaussianities from FIRAS data

“…The work of Khatri & Sunyaev [56] was, to our knowledge, the first one that attempted a reconstruction of the fluctuating part of the µ distortions using a component separation method applied to imager data, namely those from the high-frequency instrument on board of the Planck satellite. However, this approach can be more affected by contaminations from residual primary CMB and other astrophysical foregrounds [e.g., 80,81]. In addition, we stress that knowledge of the µ monopole is needed to break the degeneracy between the average level of µ distortions, µ , and f NL (see [44,56,70,80,81] for discussions on measuring µ fluctuations with a relatively calibrated experiment).…”

“…However, this approach can be more affected by contaminations from residual primary CMB and other astrophysical foregrounds [e.g., 80,81]. In addition, we stress that knowledge of the µ monopole is needed to break the degeneracy between the average level of µ distortions, µ , and f NL (see [44,56,70,80,81] for discussions on measuring µ fluctuations with a relatively calibrated experiment). This possibility is only allowed by instruments such as spectrometers that are sensitive to the absolute sky temperature.…”

CMB spectral distortions revisited: A new take on μ distortions and primordial non-Gaussianities from FIRAS data

“…The work of Khatri & Sunyaev [64] was, to our knowledge, the first one that attempted a reconstruction of the fluctuating part of the µ distortions using a component separation method applied to imaging data, namely those from the high-frequency instrument on board the Planck satellite. However, this approach can be more affected by contaminations from residual primary CMB and other astrophysical foregrounds [e.g., 89,90]. In addition, we stress that knowledge of the µ monopole is needed to break the degeneracy between the average level of µ distortions, ⟨µ⟩, and f NL (see [52,64,79,89,90] for discussions on measuring µ fluctuations with a relatively calibrated experiment).…”

“…However, this approach can be more affected by contaminations from residual primary CMB and other astrophysical foregrounds [e.g., 89,90]. In addition, we stress that knowledge of the µ monopole is needed to break the degeneracy between the average level of µ distortions, ⟨µ⟩, and f NL (see [52,64,79,89,90] for discussions on measuring µ fluctuations with a relatively calibrated experiment). This possibility is only allowed by instruments such as spectrometers that are sensitive to the absolute sky temperature.…”