Primordial stochastic gravitational wave backgrounds from a sharp feature in three-field inflation. Part I. The radiation era

Abstract

The detection of a primordial stochastic gravitational wave background has the potential to reveal unprecedented insights into the early universe, and possibly into the dynamics of inflation. Generically, UV-complete inflationary models predict an abundance of light scalars, so any inflationary stochastic background may well be formed in a model with several interacting degrees of freedom. The stochastic backgrounds possible from two-field inflation have been well-studied in the literature, but it is unclear how similar they are to the possibilities from many-field inflation. In this work we study stochastic backgrounds from more-than-two field inflation for the first time, focusing on the scalar-induced background produced during the radiation era by a brief turn in three-field space. We find an analytic expression for the enhancement in the power spectrum as a function of the turn rate and the torsion, and show that unique signatures of three-field dynamics are possible in the primordial power spectrum and gravitational wave spectrum. We confirm our analytic results with a suite of numerical simulations and find good agreement in the shape and amplitude of the power spectra. We also comment on the detection prospects in LISA and other future detectors. We do not expect the moderately large growth of the inflationary perturbations necessary for detection to cause a breakdown of perturbation theory, but this must be verified on a case-by-case basis for specific microphysical models to make a definitive claim.

(Slow-)twisting inflationary attractors

“…Note that, although we only plot P (k , N ), to measure n s we compute the scalar perturbations at a range of scales around the pivot scale, see appendix E for details on our numerical procedure. approximation, following [82]. Because the torsion is so small, we can neglect the third field's effects on the perturbations, and recover the two-field result that sub-horizon growth will occur when k/k < Ω √ 1 − ξ nn , where ξ nn ≡ M nn /Ω 2 , assumed to be constant in the WKB approximation [83].…”

“…On sub-Hubble scales, the torsion and the masses interact to allow for the perturbations' growth in various scenarios. We defer to the analyses of [82,84] in this case.…”

“…We study explicit models and confirm our analytic understanding of the perturbations' superhorizon behavior. In subsequent works by the authors it was shown that slow-or rapid-twist inflation has observational consequences as it leads to distinctive features in the power spectrum; we refer the reader to [82,84] for more details. Lastly, as an application, we study in detail explicit many-field models from supergravity and elsewhere, and give the reader an explicit walk-through of the application of our methods.…”

One-loop infrared rescattering by enhanced scalar fluctuations during inflation

“…[21] considered the specific case of an enhancement due to the (ϕ − ϕ 0 ) 2 χ 2 interaction between the inflaton ϕ and another field χ, which is suddenly produced when the inflaton reaches the value ϕ 0 during inflation, we point out here that this IR production is a much more general effect, that can take place just by gravitational interactions (even in the absence of any other direct coupling) whenever a field is suddenly excited at sub-horizon scales at some given time τ out during inflation. This can be the result of a variety of physical processes, such as a momentary change in the speed of the inflaton [26][27][28][29][30][31][32][33][34][35] or a sharp turn in multifield space [36][37][38][39][40][41][42][43] or also multiple-stage inflation [44][45][46][47][48][49]. Depending on the specific model, the excited field could be an entropic (i.e.…”

The cosmological tree theorem

“…On a cosmological (time-dependent) spacetime background, things are even worse: loop diagrams contain potentially divergent integrals both over momenta and over time. Since loop corrections can JHEP12(2023)076 play an important role in a wide variety of cosmological settings -including precise calculations of the primordial power spectrum [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] and non-Gaussianity [19][20][21][22][23][24][25][26][27][28][29][30][31]; inflationary features that may seed primordial black holes [32][33][34][35][36][37][38][39][40][41] or produce a stochastic gravitational wave background [42][43][44][45][46]; de Sitter holography …”