Measurement of Parity Violation in the Early Universe using Gravitational-wave Detectors

Abstract

A stochastic gravitational-wave background (SGWB) is expected to arise from the superposition of many independent and unresolved gravitational-wave signals, of either cosmological or astrophysical origin. Some cosmological models (characterized, for instance, by a pseudo-scalar inflaton, or by some modification of gravity) break parity, leading to a polarized SGWB. We present a new technique to measure this parity violation, which we then apply to the recent results from LIGO to produce the first upper limit on parity violation in the SGWB, assuming a generic power-law SGWB spectrum across the LIGO sensitive frequency region. We also estimate sensitivity to parity violation of the future generations of gravitational-wave detectors, both for a power-law spectrum and for a model of axion inflation. This technique offers a new way of differentiating between the cosmological and astrophysical sources of the isotropic SGWB, as astrophysical sources are not expected to produce a polarized SGWB.

Figures (2)

• Figure 1: Overlap reduction functions for detector pairs H1-L1 and L1-K1.
• Figure 2: Top-left: 95% CL limit curves based on the most recent LIGO result S5stoch, for several values of $\alpha$. Top-right: Expected sensitivities (at $2\sigma$ level) for the advanced LIGO H1-L1 pair and for an example of a third-generation detector pair (see text for more detail). Bottom-left: 95% CL contours are shown for several examples of simulated SGWB with parity violation, assuming $\alpha=0$, standard strain sensitivities, and 1 year of observation. The x's denote the signal simulation parameter values. The lightest-gray line corresponds to the recovery with H1-L1, medium-gray line to the recovery with H1-L1-V1, and the black line to the recovery with H1-L1-V1-K1 network. Bottom-right: assuming a SGWB with maximal parity violation ($\Pi=+ 1$), the lines denote $\Omega_{\alpha}$ needed for a given $\alpha$ to detect the SGWB and exclude $\Pi=0$ at 95% confidence, using two second-generation detector networks and the example of a third-generation detector pair.