Primordial Clocks within Stochastic Gravitational Wave Anisotropies

TL;DR

This paper explores how primordial anisotropies in the stochastic gravitational wave background, caused by early universe phase transitions and oscillating fields, can reveal high-energy physics and inflationary features.

Contribution

It introduces the potential for detecting scale-invariance-breaking features in gravitational wave anisotropies as a new probe of inflationary physics and primordial universe conditions.

Findings

Primordial anisotropies can carry signatures of early universe phase transitions.

Scale-invariance-breaking features may be observable in gravitational wave anisotropies.

Detection of anisotropies could motivate development of advanced gravitational wave detectors.

Abstract

A first-order phase transition in the early universe can give an observable stochastic gravitational background (SGWB), which will necessarily have primordial anisotropies across the sky. In multi-field inflationary scenarios, these anisotropies may have a significant isocurvature component very different from adiabatic fluctuations, providing an alternate discovery channel for high energy physics at inflationary scales. Here, we consider classically oscillating heavy fields during inflation that can imprint distinctive scale-invariance-breaking features in the power spectrum of primordial anisotropies. While such features are highly constrained in the cosmic microwave background, we show that their amplitude can be observably large in isocurvature SGWB, despite both probing a similar period of inflation. Measuring SGWB multipoles at the required level, $\ell \sim {\cal O}(10-100)$, will be technologically challenging. However, we expect that early detection of a strong isotropic SGWB, and the guarantee of anisotropies, would spur development of next-generation detectors with sufficient sensitivity, angular resolution, and foreground discrimination.