New universal property of cosmological gravitational wave anisotropies

Abstract

The anisotropies of the stochastic gravitational wave background, as produced in the early phases of cosmological evolution, can act as a key probe of the primordial universe particle content. We point out a new universal property of gravitational wave anisotropies of cosmological origin: for adiabatic initial conditions, their angular power spectrum is insensitive to the equation of state of the cosmic fluid driving the expansion before big bang nucleosynthesis. Any deviation from this universal behavior points to the presence of nonadiabatic sources of primordial fluctuations. Such scenarios can be tested by gravitational wave detectors operating at a frequency range which is fully complementary to cosmic microwave background (CMB) experiments. In this work, we prove this general result, and we illustrate its consequences for a representative realization of initial conditions based on the curvaton scenario. In the case of the simplest curvaton setup, we also find a significant cross-correlation between gravitational wave anisotropies and the CMB temperature fluctuations. There is a fourfold enhancement vis-à-vis the purely adiabatic scenario. We discuss the implications of our findings for identifying the origin of the (cosmological) gravitational wave background when, as is often the case, this cannot be determined solely on the basis of its spectral shape.

Unraveling cosmological anisotropies within stochastic gravitational wave backgrounds

“…An observation of these anisotropies would provide us with a new map of the primordial, inhomogeneous Universe, potentially different from the CMB. Thus a non-trivial cross-correlation (or lack thereof) between CMB and SGWB anisotropies, can reveal the existence of new degrees of freedom or isocurvature perturbations in the early Universe [15,[22][23][24][25]. In fact, even without cross-correlating, if we confirm a cosmological origin of an SGWB and find that anisotropies of such an SGWB are much larger than 10 −5 , that alone would provide strong evidence that isocurvature fluctuations are generated in the early Universe.…”

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

“…Therefore, measuring the anisotropies in SIGWs and probing PBHs can serve as complementary approaches to break the sign degeneracy of f NL . In addition, the theoretical formalism could be straightforwardly generalized to study SIGWs produced during other epochs [143] or other CGWBs. The theoretical predictions of this work may be tested by space-borne GW detectors or networks in future.…”

GW_CLASS: Cosmological Gravitational Wave Background in the cosmic linear anisotropy solving system

“…In the limit n gwb → 0, the factor on the right-hand side is independent of the number of relativistic and decoupled degrees of freedom (or of the equation of state of the universe at early times, as pointed out by [53]). However, we will explicitly show in further sections that, in the majority of the frequency bins accessible by future interferometers, the tensor tilt n gwb is different from zero (in particular it must be larger than zero if we focus on inflationary mechanisms which respect CMB bounds) for signals that are detectable by current and future GW interferometers.…”

“…The CGWB is sensitive not just to the relativistic and decoupled degrees of freedom at high temperatures, f dec (η in ), but in principle also to any ingredient changing the evolution of metric perturbations in the very early universe. Another example is provided by the equation of state of the cosmic fluid driving the expansion of the Universe at η prod [53]. More specifically, deviations from the standard equation of state of a relativistic fluid at early times, w = 1/3, would affect both the SW and the primordial ISW with expressions analogous to eqs.…”