Foreground biases on primordial non-Gaussianity measurements from the CMB temperature bispectrum: Implications forPlanckand beyond

Abstract

The cosmic microwave background (CMB) temperature bispectrum is currently the most precise tool for constraining non-Gaussianity (NG) in the primordial curvature perturbations. The Planck temperature data tightly constrain the amplitude of local-type NG: f loc NL = 2.5 ± 5.7. In this paper, we compute previously-neglected foreground biases in temperature-based f loc NL measurements. We consider signals from the integrated Sachs-Wolfe (ISW) effect, gravitational lensing, the thermal (tSZ) and kinematic Sunyaev-Zel'dovich (kSZ) effects, and the cosmic infrared background (CIB). In standard analyses, a significant foreground bias arising from the ISW-lensing bispectrum is subtracted from the f loc NL measurement. However, a number of other terms sourced by the ISW, lensing, tSZ, kSZ, and CIB fields are also present in the temperature bispectrum. We compute the dominant biases on f loc NL arising from these signals, focusing on "squeezed" bispectrum shapes. Most of the biases are non-blackbody in nature, and are thus reduced by multifrequency component separation methods; however, recent analyses have found that extragalactic foregrounds are present at nonnegligible levels in the Planck component-separated maps. Moreover, the Planck FFP8 simulations do not include the correlations amongst components that are responsible for these biases. We compute the biases for individual Planck frequencies, finding that some are comparable to the statistical error bar on f loc NL , even for the main CMB channels (100, 143, and 217 GHz). For future experiments, they can greatly exceed the statistical error bar (considering temperature data only). Alternatively, the foreground contributions can be marginalized over, but without strong priors this leads to a non-negligible increase in the error bar on f loc NL . A full assessment for Planck and other experiments will require calculations in tandem with component separation, ideally using simulations. We also compute these biases for equilateral and orthogonal NG, finding large effects for the latter. Similar calculations must be performed for trispectrum NG. We conclude that the search for primordial NG using Planck data may not yet be over.

CMB B -mode non-Gaussianity: Optimal bispectrum estimator and Fisher forecasts

“…The main justification for this choice is the associated extra cosmic variance due to the lack of a B-mode component. Additionally, it should be noted that the squeezed hTTTi bispectrum is expected to be relatively strongly contaminated by a secondary non-Gaussian signal [125]. It is expected to be of limited use for our purpose; see the discussion in Sec.…”

The bispectra of galactic CMB foregrounds and their impact on primordial non-Gaussianity estimation

“…However, in this paper we are primarily interested in seeing how much these foregrounds contaminate a determination of primordial non-Gaussianity, which is in general isotropic and for which the bispectrum is then an optimal tool. tant extra-galactic foregrounds templates already exist in analytic form [22,23] (theoretically or heuristically determined), and can for example be used to compute biases [24]. But no such templates exist for galactic foregrounds.…”

The integrated angular bispectrum

“…The two cases also differ for the choice of cosmological parameters. In H18 we use the best-fit cosmology of [48] as in [15] (h 0 = 0.702, σ 8 = 0.817, n s = 0.962, Ω m = 0.277, Ω b = 0.0459, τ = 0.088), whereas in Pl18 we use the updated cosmological parameters from [36] (h 0 = 67.32, σ 8 = 0.812, n s = 0.96605, Ω m = 0.3158, Ω b = 0.04939, τ = 0.0543). Notice that in [45] it has been shown that the Planck SZ measurement prefer lower Ω m and σ 8 than the one used here, if we assume as we do that b H = 0.2.…”