Testing the Early Universe with Anisotropies of the Gravitational Wave
  Background
  In this work we analyse in detail the possibility of using small and
intermediate-scale gravitational wave anisotropies to constrain the
inflationary particle content. First, we develop a phenomenological approach
focusing on anisotropies generated by primordial tensor-tensor-scalar and
purely gravitational non-Gaussianities. We highlight the quantities that play a
key role in determining the detectability of the signal. To amplify the power
of anisotropies as a probe of early universe physics, we consider
cross-correlations with CMB temperature anisotropies. We assess the size of the
signal from inflationary interactions against so-called induced anisotropies.
In order to arrive at realistic estimates, we obtain the projected constraints
on the non-linear primordial parameter $F_{\rm NL}$ for several upcoming
gravitational wave probes in the presence of the astrophysical gravitational
wave background. We further illustrate our findings by considering a concrete
inflationary realisation and use it to underscore a few subtleties in the
phenomenological analysis.
