Abstract
The Stochastic Gravitational Wave Background (SGWB) is expected to be a key observable for Gravitational Wave (GW) interferometry. Its detection will open a new window on early universe cosmology and on the astrophysics of compact objects. Using a Boltzmann approach, we study the angular anisotropies of the GW energy density, which is an important tool to disentangle the different cosmological and astrophysical contributions to the SGWB. Anisotropies in the cosmological background are imprinted both at its production, and by GW propagation through the large-scale scalar and tensor perturbations of the universe. The first contribution is not present in the Cosmic Microwave Background (CMB) radiation (as the universe is not transparent to photons before recombination), causing an order one dependence of the anisotropies on frequency. Moreover, we provide a new method to characterize the cosmological SGWB through its possible deviation from a Gaussian statistics. In particular, the SGWB will become a new probe of the primordial non-Gaussianity of the large-scale cosmological perturbations.
Highlights
Operating ground-based interferometers is not far from reaching the sensitivity to detect the stochastic gravitational wave background (SGWB) from unresolved astrophysical sources [1,2]
Its detection will open a new window to early Universe cosmology and to the astrophysics of compact objects
Using a Boltzmann approach, we study the angular anisotropies of the GW energy density, which is an important tool to disentangle the different cosmological and astrophysical contributions to the SGWB
Summary
Operating ground-based interferometers is not far from reaching the sensitivity to detect the stochastic gravitational wave background (SGWB) from unresolved astrophysical sources [1,2]. As shown in [27,28], two effects make such a measurement unfeasible: (i) the GW propagation in the perturbed Universe destroys any hh3i correlation possibly present in the primordial signal, and (ii) modes of nearby frequencies get confused with one another due to the finite duration of the experiment, resulting in a large phase decorrelation There is, another type of nonGaussianity that can be observed, and it is the one present in the spatial distribution of the GW energy density. Notice that Planck set the tightest limits from CMB data on deviations from Gaussian statistics for cosmological fluctuations [29] Still, this does not rule out the possibility of primordial non-Gaussian signatures: the observable we discuss (the angular bispectrum of GW energy density anisotropies) relies on future measurements that might be sensitive enough to probe primordial non-Gaussian signals. In a companion paper [30], we shall present the details of these computations, extend them to include the GW propagation to second order in perturbations, and develop a more extended analysis of the GW bispectrum
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