Keisuke Inomata

Several papers have recently calculated the contribution to pulsar timing array overlap reduction functions (ORFs) induced by our peculiar velocity with respect to the rest frame of the stochastic gravitational-wave background. Here we show that a harmonic-space calculation confirms the most recent result. We note that, with the harmonic-space calculation, the ORFs for spin-1 GWs and the correlations with astrometry measurements are also easily obtained.

A bstract: Gravitational waves (GWs) provide a powerful probe of the early universe due to their ability to free-stream across cosmic history. We study GW production in a compelling scenario where a rotating axion(-like) field becomes relevant for a brief period in the early universe before transitioning into a kination fluid and rapidly dissipating its energy through cosmic expansion. During this short epoch, the curvature perturbation can be predominantly sourced by the rotating axion and may significantly exceed the adiabatic component. Moreover, axion field perturbations grow on superhorizon scales during this phase. These effects can generate a strong stochastic background of induced GWs. This GW background also exhibits a pronounced large-scale anisotropy inherited from the axion fluctuations, serving as a distinctive signature of the scenario. Importantly, the transient nature of axion relevance enables this scenario to evade stringent bounds on large-scale perturbations. We analyze various observational constraints and find that both the amplitude and anisotropy of the resulting GW signal could be accessible to future detectors.

Gravitational waves (GWs) provide a powerful probe of the early universe due to their ability to free-stream across cosmic history. We study GW production in a compelling scenario where a rotating axion(-like) field becomes relevant for a brief period in the early universe before transitioning into a kination fluid and rapidly dissipating its energy through cosmic expansion. During this short epoch, the curvature perturbation can be predominantly sourced by the rotating axion and may significantly exceed the adiabatic component. Moreover, axion field perturbations grow on superhorizon scales during this phase. These effects can generate a strong stochastic background of induced GWs. This GW background also exhibits a pronounced large-scale anisotropy inherited from the axion fluctuations, serving as a distinctive signature of the scenario. Importantly, the transient nature of axion relevance enables this scenario to evade stringent bounds on large-scale perturbations. We analyze various observational constraints and find that both the amplitude and anisotropy of the resulting GW signal could be accessible to future detectors.

For the first time, we investigate the nonperturbative dynamics of single field inflation with a departure from slow roll. Using simulations, we find that oscillatory features in the potential can drastically alter the course of inflation, with major phenomenological implications. In certain cases, the entire Universe gets trapped in a forever inflating de Sitter state. In others, only some regions get stuck in a false vacuum, offering an alternative channel for primordial black hole formation. Analogous to the flap of a butterfly, these results show that small-scale phenomena can have profound consequences on the evolution of the entire Universe. More generally, our work shows the power of simulations in the exploration of the small-scale physics of inflation, particularly in the regime relevant for gravitational-wave astronomy.

We examine one-loop corrections from small-scale curvature perturbations to the superhorizon-limit ones in single-field inflation models, which have recently caused controversy. We consider the case where the Universe experiences transitions of slow-roll (SR)→intermediate period→SR. The intermediate period can be an ultra-slow-roll period or a resonant amplification period, either of which enhances small-scale curvature perturbations. We assume that the superhorizon curvature perturbations are conserved at least during each of the SR periods. Within this framework, we show that the superhorizon curvature perturbations during the first and the second SR periods coincide at one-loop level in the slow-roll limit.