Clustering Fossil from Primordial Gravitational Waves in Anisotropic Inflation

TL;DR

This paper explores how anisotropic inflation models produce distinctive fossil imprints of primordial gravitational waves in large-scale structure, offering a new way to differentiate inflationary scenarios through galaxy and 21cm surveys.

Contribution

It demonstrates that anisotropic inflation models generate unique quadrupole anisotropies in the TSS bispectrum, which can be used to probe large-scale gravitational waves and inflation mechanisms.

Findings

Quadrupole anisotropy shape differs in anisotropic inflation models.

Amplitude of anisotropy is proportional to $g_* N_e$ and aligned with a preferred direction.

Potential detectability of signals in Euclid and 21cm galaxy surveys.

Abstract

Inflationary models can correlate small-scale density perturbations with the long-wavelength gravitational waves (GW) in the form of the Tensor-Scalar-Scalar (TSS) bispectrum. This correlation affects the mass-distribution in the Universe and leads to the off-diagonal correlations of the density field modes in the form of the quadrupole anisotropy. Interestingly, this effect survives even after the tensor mode decays when it re-enters the horizon, known as the fossil effect. As a result, the off-diagonal correlation function between different Fourier modes of the density fluctuations can be thought as a way to probe the large-scale GW and the mechanism of inflation behind the fossil effect. Models of single field slow roll inflation generically predict a very small quadrupole anisotropy in TSS while in models of multiple fields inflation this effect can be observable. Therefore this large scale quadrupole anisotropy can be thought as a spectroscopy for different inflationary models. In addition, in models of anisotropic inflation there exists quadrupole anisotropy in curvature perturbation power spectrum. Here we consider TSS in models of anisotropic inflation and show that the shape of quadrupole anisotropy is different than in single field models. In addition in these models the quadrupole anisotropy is projected into the preferred direction and its amplitude is proportional to $g_* N_e$ where $N_e$ is the number of e-folds and $g_*$ is the amplitude of quadrupole anisotropy in curvature perturbation power spectrum. We use this correlation function to estimate the large scale GW as well as the preferred direction and discuss the detectability of the signal in the galaxy surveys like Euclid and 21 cm surveys.