Primordial clocks within stochastic gravitational wave anisotropies

Abstract

A first order phase transition in the early universe can give an observable stochastic gravitational background (SGWB), which will necessarily have primordial anisotropies across the sky. In multi-field inflationary scenarios, these anisotropies may have a significant isocurvature component very different from adiabatic fluctuations, providing an alternate discovery channel for high energy physics at inflationary scales. Here, we consider classically oscillating heavy fields during inflation that can imprint distinctive scale-invariance-breaking features in the power spectrum of primordial anisotropies. While such features are highly constrained in the cosmic microwave background, we show that their amplitude can be observably large in isocurvature SGWB, despite both probing a similar period of inflation. Measuring SGWB multipoles at the required level, ℓ ∼ 𝒪(10-100), will be technologically challenging. However, we expect that early detection of a strong isotropic SGWB, and the guarantee of anisotropies, would spur development of next generation detectors with sufficient sensitivity, angular resolution, and foreground discrimination.

Back to the features: assessing the discriminating power of future CMB missions on inflationary models

“…These findings clearly demonstrate that the study of primordial features should be considered as one of the main tools to understand the primordial universe in the next decades. With the addition of the large-scale structure data from various galaxy surveys [66][67][68][69][70][71][72][73][74][75][76][77][78] in a similar time frame, as well as 21 cm hydrogen line surveys [79,80] and stochastic gravitational wave background mapping [37,81,82] in a more distant future, we can hope for discoveries that will shake our understanding of the origin of the Universe.…”

“…Specifically, sharp features generate a correction of the form ∆P sharp (k) ∝ P 0 sin(k/k 0 ) where k 0 is the mode that crosses the horizon at the time of the sharp feature, while resonant features take the form ∆P res (k) ∝ P 0 sin(ω ln k + phase), where the running is linear in ln k space. Some models of inflation, such as the so-called Classical Primordial Standard Clocks (CPSC) predict a complicated superposition of the two kinds of features [20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38]. 1 Measuring the parameters describing ∆P would give access to the fundamental physics at the origin of the signal, necessarily beyond the simple SFSR paradigm.…”

Enhancing gravitational wave anisotropies with peaked scalar sources

“…The effect of primordial isocurvature perturbations on the GW anisotropy has also been studied recently in [54,87]. The isocurvature perturbations modify the GW anisotropy as [54],…”

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.…”

“…Similarly, one can also consider a significant violation of scale invariance in the cosmological anisotropy component through certain 'primordial features' [64,65] as studied in ref. [23]. For such scenarios, we expect our methodology can be adapted appropriately.…”