Antipodal Angular Correlations of Inflationary Stochastic Gravitational Wave Background

TL;DR

This paper investigates the potential to detect antipodal correlations in the inflationary stochastic gravitational-wave background, proposing that statistical anisotropy could enable such detection, distinguishing it from other SGWB sources.

Contribution

It demonstrates that antipodal correlations are detectable only in anisotropic inflation models, providing a new method to identify the inflationary SGWB.

Findings

Antipodal correlations are undetectable in isotropic inflation models.

Statistical anisotropy allows for a non-vanishing estimator of antipodal correlations.

Detection of these correlations can distinguish inflationary SGWB from other backgrounds.

Abstract

The measurement of the inflationary stochastic gravitational-wave background (SGWB) is one of the main goals of future GW experiments. In direct GW experiments, an obstacle to achieving it is the isolation of the inflationary SGWB from the other types of SGWB. In this paper, as a distinguishable signature of the inflationary SGWB, we argue the detectability of its universal property: antipodal correlations, i.e., correlations of GWs from the opposite directions, as a consequence of the horizon re-entry. A phase-coherent method has been known to be of no use for detecting the angular correlations in SGWB due to a problematic phase factor that erases the signal. We thus investigate whether we can construct a phase-incoherent estimator of the antipodal correlations in the intensity map. We found that the conclusion depends on whether the inflationary GWs have statistical isotropy or not. In the standard inflationary models with statistical homogeneity and isotropy, there is no estimator that is sensitive to the antipodal correlations but does not suffer from the problematic phase factor. On the other hand, it is possible to find a non-vanishing estimator of the antipodal correlations for inflationary models with statistical anisotropy. SGWB from anisotropic inflation is distinguishable from the other components.