Large primordial fluctuations in gravitational waves from phase transitions

Abstract

It is well-known that first-order phase transitions in the early universe can be a powerful source of observable stochastic gravitational wave backgrounds. Any such gravitational wave background must exhibit large-scale anisotropies at least as large as those seen in the CMB 10−5, providing a valuable new window onto the (inflationary) origins of primordial fluctuations. While significantly larger fractional anisotropies are possible (for example, in multi-field inflation) and would be easier to interpret, it has been argued that these can only be consistent with CMB bounds if the gravitational wave signal is correspondingly smaller. In this paper, we show that this argument, which relies on assuming radiation dominance of the very early universe, can be evaded if there is an era of early matter dominance of a certain robust type. This allows large gravitational wave anisotropies to be consistent with observable signals at proposed future gravitational wave detectors. Constraints from the CMB on large scales, as well as primordial black hole and mini-cluster formation on small scales, and secondary scalar-induced gravitational waves are all taken into account.

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.…”

“…Cosmological mechanisms leading to such scenarios have been recently constructed in ref. [24]. In the Fisher analysis, we pick a range of A c (as defined in eq.…”

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

“…In addition, our methodology does not account for distinct imprints that might be associated with specific sources. For example, as highlighted in [62,[84][85][86], certain sources of the CGWB, such as the phase transitions in the presence of isocurvature perturbations or the metric perturbations resulting from quantum fluctuations during inflation, may cause the standard adiabatic initial condition in eq. (2.28) not to hold and introduce additional anisotropies to the CGWB.…”