GW_CLASS: Cosmological Gravitational Wave Background in the cosmic linear anisotropy solving system

Abstract

The anisotropies of the Cosmological Gravitational Wave Background (CGWB) retain information about the primordial mechanisms that source the gravitational waves and about the geometry and the particle content of the universe at early times. In this work, we discuss in detail the computation of the angular power spectra of CGWB anisotropies and of their cross correlation with Cosmic Microwave Background (CMB) anisotropies, assuming different processes for the generation of these primordial signals. We present an efficient implementation of our results in a modified version of CLASS which will be publicly available. By combining our new code GW_CLASS with MontePython, we forecast the combined sensitivity of future gravitational wave interferometers and CMB experiments to the cosmological parameters that characterize the cosmological gravitational wave background.

On the anisotropies of the cosmological gravitational-wave background from pulsar timing array observations

“…Since the GW and the CMB photon share the same geodesics under the same metric perturbation during their propagation, we expect that CGWB anisotropies have a noticeable correlation with these two CMB tracers. The cross-correlation between CGWB and CMB has been previously studied in [47,75,76], we here give the result based on NANOGrav data and show that it significantly affects the low ℓ of CGWB×CMB angular power spectrum. The a ℓm s of CMB temperature fluctuations are…”

“…(2.30)(2.31), η in influences the transfer functions through the metric perturbations and projection effects from Spherical Bessel functions, potentially introducing model dependence to the anisotropies in the CGWB. However, in practice, we have observed that the varying specific value of η in does not appreciably alter any anisotropic signals noticeably provided that η in remains small (see also [47]). We therefore adopt η in = 10 −4 Mpc in the rest part of this paper.…”

“…The SGWB also exhibits anisotropies, and these anisotropies in the SGWB should have noticeable discrepancies in their spectra between the CGWB [41][42][43][44][45][46][47][48][49][50][51][52] and AGWB [53][54][55][56][57][58][59][60][61] due to their different sourcing and propagating mechanisms. For instance, compared to the AGWB, the CGWB generally began propagating at earlier times, involving more deeply with various cosmological perturbations during its propagation, leading to a distinct angular spectrum of anisotropies.…”

Does NANOGrav observe a dark sector phase transition?

“…An even clearer signature would be a broken powerlaw, but to test for such a feature will require even higher signal-to-noise ratios and hence take correspondingly longer time to probe observationally. A valuable source of information to further disambiguate between SMBHBs and GWBs of primordial origin may lie in the anisotropies of the GWB [121][122][123], in particular via the polarization of the GWB [124][125][126][127][128][129][130] and possibly the detection of singular GW sources as already searched for [131,132].…”

Primordial non-Gaussianity fNLand anisotropies in scalar-induced gravitational waves

“…[100], other related works can be found in refs. [94,[101][102][103][104][105][106][107].One of the leading aims of our present work is to establish the first complete analysis.…”