Abstract
Abstract
The initial conditions on the anisotropies of the stochastic gravitational-wave background
of cosmological origin (CGWB) largely depend on the mechanism that generates the gravitational
waves. Since the CGWB is expected to be non-thermal, the computation of the initial conditions
could be more challenging w.r.t. the Cosmic Microwave Background (CMB), whose interactions with
other particles in the early Universe lead to a blackbody spectrum. In this paper, we show that
the initial conditions for the cosmological background generated by quantum fluctuations of the
metric during inflation deviate from adiabaticity. These primordial gravitational waves are indeed
generated by quantum fluctuations of two independent degrees of freedom (the two polarization
states of the gravitons). Furthermore, the CGWB plays a negligible role in the Einstein's
equations, because its energy density is subdominant w.r.t. ordinary matter. Therefore, the only
possible way to compute the initial conditions for inflationary gravitons is to perturb the
energy-momentum tensor of the gravitational field defined in term of the small-scale tensor
perturbation of the metric. This new and self-consistent approach shows that a large,
non-adiabatic initial condition is present even during the single-field inflation. Such a
contribution enhances the total angular power spectrum of the CGWB compared to the standard
adiabatic case, increasing also the sensitivity of the anisotropies to the presence of
relativistic and decoupled particles in the early Universe. In this work we have also proved that
our findings are quite general and apply to both single-field inflation and other scenarios in
which the CGWB is generated by the quantum fluctuations of the metric, like the curvaton.