Primordial non-Gaussianity f_NLand anisotropies in scalar-induced gravitational waves

Abstract

Primordial non-Gaussianity encodes vital information of the physics of the early universe, particularly during the inflationary epoch. To explore the local-type primordial non-Gaussianity f NL, we study the anisotropies in gravitational wave background induced by the linear cosmological scalar perturbations during radiation domination in the early universe. We provide the first complete analysis to the angular power spectrum of such scalar-induced gravitational waves. The spectrum is expressed in terms of the initial inhomogeneities, the Sachs-Wolfe effect, and their crossing. It is anticipated to have frequency dependence and multipole dependence, i.e., Cℓ (ν) ∝ [ℓ(ℓ+1)]-1 with ν being a frequency and ℓ referring to the ℓ-th spherical harmonic multipole. In particular, the initial inhomogeneites in this background depend on gravitational-wave frequency. These properties are potentially useful for the component separation, foreground removal, and breaking degeneracies in model parameters, making the non-Gaussian parameter f NL measurable. Further, theoretical expectations may be tested by space-borne gravitational-wave detectors in future.

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

“…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.…”

“…The production and propagation of the CGWB in the perturbed universe shares the similar physics with the CMB photons, making these two signals correlated. This correlation serves as another important observable and can be useful in multiple ways, such as testing primordial [50] or isocurvature perturbations [62], examining CMB anomalies [63], or improving the signal-to-noise ratio [64]. Therefore, it is worth estimating the anistropic information based on existing PTA observations and examining the level of cross-correlations with other cosmological tracers.…”

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

“…Theoretically, they were produced through non-linear processes by the linear inflationary JCAP05(2024)109 perturbations that reentered the Hubble horizon in the early universe [18][19][20][21][22][23][24]. Taking into account f NL and g NL can lead to significant alterations by several orders of magnitude in both the energy-density fraction spectrum and the angular power spectrum [25], especially within the projected sensitivity regimes of future or futuristic gravitational-wave detection programs . This suggests the potential for measuring primordial non-Gaussianity through the search for an anisotropic SIGW background.…”

“…[51][52][53][54][55][56][57][58][59][60][61][62][63][64][65][66][67][68][69] and refs. [70][71][72], respectively. Additionally, in conjunction with the production of scalar-induced gravitational waves, these scalar perturbations may gravitationally collapse into primordial black holes (PBHs) [73,74], which are believed to be a reasonable candidate for cold dark matter (e.g., see review in refs.…”

“…In this work, we will perform a comprehensive analysis of the angular bispectrum and trispectrum of SIGWs by simultaneously taking into account the local-type primordial non-Gaussianity parameterized by f NL and g NL . Following a diagrammatic approach [25], we will study the non-Gaussianity of SIGW background via including higher-order energy-density perturbations in SIGWs. We will derive semi-analytic formulae for the angular bispectrum and trispectrum and further find the consistency relations between them with the angular power spectrum, respectively.…”