Induced gravitational waves from ultra slow-roll inflation and pulsar timing arrays observations

Abstract

The stochastic gravitational wave background (SGWB) detected recently by the pulsar timing arrays (PTAs) observations may have cosmological origins. In this work we consider a model of single field inflation containing an intermediate phase of ultra slow-roll. Fixing the amplitude of the peak of curvature perturbations by the PBHs bounds we calculate the gravitational waves (GWs) induced from the curvature perturbations enhanced during USR. The spectrum of the induced GWs depends on the sharpness of the transition from the USR phase to the final attractor phase as well as to the duration of the USR period. While the model can accommodate the current PTAs data but it has non-trivial predictions for the induced GWs on higher frequency ranges which can be tested by future observations.

Induced gravitational waves from non-attractor inflation and NANOGrav data

“…Our setup comprises three stages: two slow-roll inflation stages at the beginning and end, and a non-attractor phase in the middle. This model is a generalization of the work presented in [56], where the middle phase allows for an arbitrary sound speed, c s . Specifically, the Lagrangian in the middle phase is given by…”

“…In an alternative scenario, the perturbations can experience growth during the nonattractor phase, even with an arbitrary sound speed c s [48,49]. In this study, we extend the approach proposed in [56] by considering the arbitrary sound speed c s as an additional degree of freedom. Our objective is to investigate how this model can account for the NANOGrav data.…”

“…In our model, the Lagrangian of the inflaton is an arbitrary function of the kinetic term X, denoted as P (X), with a constant potential. The sound speed can have non-trivial effects on the generalization of the results obtained in [56]. Specifically, our model comprises three JCAP04(2024)009 inflationary phases: two slow-roll phases at the beginning and end, and a non-attractor phase in between.…”

“…The paper is structured as follows. In section 2, we present our model and generalize the results of [56] to the case with a general sound speed. We calculate the non-Gaussianity for this model and study its consistency in confrontation with the observations of PBHs.…”

Can the Gravitational Wave Background Feel Wiggles in Spacetime?

“…Due to the weakness of the gravitational interaction, gravitational waves (GWs), and the GWB as well, are thought to be insensitive to any environmental physics they propagate through, unless such physics has strength beyond the linear regime, which is typically only true near the GW source. As a result, most theoretical studies of the PTA signal have focused on the possible sources such as supermassive black holes (see Agazie et al 2023b;Antoniadis et al 2023b for some up-todate constraints, and also Huang et al 2023;Konoplya & Zhidenko 2023;Yang et al 2023b), inflation (Starobinsky 1979;Rubakov et al 1982;Guzzetti et al 2016;Vagnozzi 2021Vagnozzi , 2023Borah et al 2023;Choudhury 2023;Datta 2023;Firouzjahi & Talebian 2023;Niu & Rahat 2023;Unal et al 2023), scalar-induced GWs (Tomita 1967;Matarrese et al 1993Matarrese et al , 1994Domènech 2021; Abe & Tada 2023;Cai et al 2023;Ebadi et al 2023;Franciolini et al 2023;Liu et al 2023a;Wang et al 2023a;Yi et al 2023;Zhu et al 2023), and collision of bubbles of first-order phase transitions (Kosowsky et al 1992;Caprini et al 2008;Huber & Konstandin 2008;Arzoumanian et al 2021;Li et al 2021;Ashoorioon et al 2022;Addazi et al 2023;Bringmann et al 2023;…”