Unraveling cosmological anisotropies within stochastic gravitational wave backgrounds

Studying the primordial non-Gaussianity of inflationary perturbations is crucial for testing the inflation paradigm of the early universe. In this work, we conduct a comprehensive analysis of the angular bispectrum and trispectrum of scalar-induced gravitational waves (SIGWs) in the presence of local-type primordial non-Gaussianity parameterized by f NL and g NL, deriving their semi-analytical formulae for the first time. Our findings indicate that it is the presence of primordial non-Gaussianity that leads to a non-Gaussian SIGW background, suggesting that the angular bispectrum and trispectrum of SIGWs could serve as probes of the primordial non-Gaussianity. Our numerical results further illustrate that f NL and g NL exert significant impacts on the spectral amplitudes, potentially reaching up to 10-5 for the former and 10-8 for the latter. In particular, we demonstrate that the angular bispectrum and trispectrum exhibit characteristic dependence on the angular multipoles and frequency bands. They hold potentials to be measured by gravitational-wave detectors that may advance our understanding of the origin of the universe.

Section: Jcap05(2024)109 3 Angular Correlation Functionsmentioning

confidence: 99%

Angular bispectrum and trispectrum of scalar-induced gravitational waves: all contributions from primordial non-Gaussianity f _NL and g _NL

Li,

Wang,

Zhao

et al. 2024

“…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.

Enhancement of gravitational waves at Q-ball decay including non-linear density perturbations

Kawasaki,

Murai

2024

The existence of a stochastic gravitational wave background is indicated by the recent pulsar timing array (PTA) experiments. We study the enhanced production of second-order gravitational waves from the scalar perturbations when the universe experiences a transition from the early matter-dominated era to the radiation-dominated era due to Q-ball decay. We extend the analysis in previous work by including the frequency range where density perturbations go non-linear and find that the resultant gravitational wave spectrum can be consistent with that favored by the recent PTA experiment results.