Circular Polarization of the Astrophysical Gravitational Wave Background

We build an analytical framework to study the observability of anisotropies and a net chiral polarization of the Stochastic Gravitational Wave Background (SGWB) with a generic network of ground-based detectors. We apply this formalism to perform a Fisher forecast of the performance of a network consisting of the current interferometers (LIGO, Virgo and KAGRA) and planned third-generation ones, such as the Einstein Telescope and Cosmic Explorer. Our results yield limits on the observability of anisotropic modes, spanning across noise- and signal-dominated regimes. We find that if the isotropic component of the SGWB has an amplitude close to the current limit, third-generation interferometers with an observation time of 10 years can measure multipoles (in a spherical harmonic expansion) up to ℓ = 8 with 𝒪(10-3 – 10-2) accuracy relative to the isotropic component, and an 𝒪(10-3) amount of net polarization. For weaker signals, the accuracy worsens as roughly the inverse of the SGWB amplitude.

Section: Jcap08(2023)053supporting

confidence: 90%

“…Some cosmological mechanisms, for instance, a coupling between a pseudo-scalar inflaton and a gauge field [21], can result in a fully circularly polarized SGWB [22,23]. A degree of polarization might also be present in the astrophysical SGWB due to Poisson fluctuations in the (finite) number of unresolved sources [24].…”

Section: Introductionmentioning

confidence: 99%

Prospects for detecting anisotropies and polarization of the stochastic gravitational wave background with ground-based detectors

Mentasti,

Contaldi,

Peloso

2023

“…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].…”

Section: Comparison With Other Gw Sourcesmentioning

confidence: 99%

Does NANOGrav observe a dark sector phase transition?

Bringmann,

Depta,

Konstandin

et al. 2023

Gravitational waves from a first-order cosmological phase transition, at temperatures at the MeV-scale, would arguably be the most exciting explanation of the common red spectrum reported by the NANOGrav collaboration, not the least because this would be direct evidence of physics beyond the standard model. Here we perform a detailed analysis of whether such an interpretation is consistent with constraints on the released energy deriving from big bang nucleosynthesis and the cosmic microwave background. We find that a phase transition in a completely secluded dark sector with sub-horizon sized bubbles is strongly disfavoured with respect to the more conventional astrophysical explanation of the putative gravitational wave signal in terms of supermassive black hole binaries. On the other hand, a phase transition in a dark sector that subsequently decays, before the time of neutrino decoupling, remains an intriguing possibility to explain the data. From the model-building perspective, such an option is easily satisfied for couplings with the visible sector that are small enough to evade current collider and astrophysical constraints. The first indication that could eventually corroborate such an interpretation, once the observed common red spectrum is confirmed as a nHz gravitational wave background, could be the spectral tilt of the signal. In fact, the current data already show a very slight preference for a spectrum that is softer than what is expected from the leading astrophysical explanation.

“…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.…”

Section: Introductionmentioning

confidence: 99%

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

Ding,

Tian

2024

Significant evidence for a stochastic gravitational-wave background has recently been reported by several Pulsar Timing Array observations. These studies have shown that, in addition to astrophysical explanations based on supermassive black hole binaries (SMBHBs), cosmological origins are considered equally important sources for these signals. To further explore these cosmological sources, in this study, we discuss the anisotropies in the cosmological gravitational wave background (CGWB) in a model-independent way. Taking the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) 15-year dataset as a benchmark, we estimate the angular power spectra of the CGWB and their cross-correlations with cosmic microwave background (CMB) fluctuations and weak gravitational lensing. We find that the NANOGrav 15-year data implies suppressed Sachs-Wolf (SW) effects in the CGBW spectrum, leading to a marginally negative cross-correlation with the CMB at large scales. This procedure is applicable to signals introduced by different early universe processes and is potentially useful for identifying unique features about anisotropies of CGWB from future space-based interferometers and astrometric measurements.