Keisuke Inomata

Primordial black holes (PBHs) are one of the candidates to explain the gravitational wave (GW) signals observed by the LIGO detectors. Among several phenomena in the early universe, cosmic inflation is a major example to generate PBHs from large primordial density perturbations. In this paper, we discuss the possibility to interpret the observed GW events as mergers of PBHs that are produced by cosmic inflation. The primordial curvature perturbation should be large enough to produce a sizable am

Following a new microlensing constraint on primordial black holes (PBHs) with $\ensuremath{\sim}1{0}^{20}--1{0}^{28}\text{ }\text{ }\mathrm{g}$ [H. Niikura et al., arXiv:1701.02151.], we revisit the idea of PBH as all dark matter (DM). We have shown that the updated observational constraints suggest the viable mass function for PBHs as all DM to have a peak at $\ensuremath{\simeq}1{0}^{20}\text{ }\text{ }\mathrm{g}$ with a small width $\ensuremath{\sigma}\ensuremath{\lesssim}0.1$, by imposing ob

Compared to primordial perturbations on large scales, roughly larger than 1 Mpc, those on smaller scales are not severely constrained. We revisit the issue of probing small-scale primordial perturbations using gravitational waves (GWs), based on the fact that, when large-amplitude primordial perturbations on small scales exist, GWs with relatively large amplitudes are induced at second order in scalar perturbations, and these induced GWs can be probed by both existing and planned gravitational-w

We discuss the footprint of evaporation of primordial black holes (PBHs) on stochastic gravitational waves (GWs) induced by scalar perturbations. We consider the case where PBHs once dominated the Universe but eventually evaporated before the big bang nucleosynthesis. The reheating through the PBH evaporation could end with a sudden change in the equation of state of the Universe compared to the conventional reheating caused by particle decay. We show that this ``sudden reheating'' by the PBH ev

A primordial black hole (PBH) is one of the leading nonparticle candidates for dark matter (DM). Although several observations severely constrain the amount of PBHs, it was recently pointed out that there is an uncertainty on the microlensing constraints below $\ensuremath{\sim}{10}^{\ensuremath{-}10}\text{ }\text{ }{M}_{\ensuremath{\bigodot}}$, which was ignored originally but may weaken the constraints significantly. In this paper, facing this uncertainty, we investigate the possibility that P

We study gravitational waves induced from the primordial scalar perturbations at second order around the reheating of the Universe. We consider reheating scenarios in which a transition from an early matter-dominated era to the radiation-dominated era completes within a timescale much shorter than the Hubble time at that time. We find that an enhanced production of induced gravitational waves occurs just after the reheating transition because of fast oscillations of scalar modes well inside the

We revisit the effects of an early matter-dominated era on gravitational waves induced by scalar perturbations. We carefully take into account the evolution of the gravitational potential, the source of these induced gravitational waves, during a gradual transition from an early matter-dominated era to the radiation-dominated era, where the transition timescale is comparable to the Hubble time at that time. Realizations of such a gradual transition include the standard perturbative reheating wit

We study gauge (in)dependence of the gravitational waves (GWs) induced from curvature perturbations. For the GWs produced in a radiation-dominated era, we find that the observable (late-time) GWs in the transverse-traceless (synchronous) gauge and in the Newtonian gauge are the same in contrast to a claim in the literature. We also mention the interpretation of the gauge dependence of the tensor perturbations which appears in the context of the induced GWs.

Abstract Inflationary models predicting a scale-dependent large amplification of the density perturbations have recently attracted a lot of attention because the amplified perturbations can seed a sizable amount of primordial black holes (PBHs) and stochastic background of gravitational waves (GWs). While the power spectra in these models are computed based on the linear equation of motion, it is not obvious whether loop corrections are negligible when such a large amplification occurs during in

We discuss a possible connection between the recent NANOGrav results and the primordial black holes (PBHs) for the LIGO-Virgo events. In particular, we focus on the axionlike curvaton model, which provides a sizable amount of PBHs and gravitational waves (GWs) induced by scalar perturbations around the NANOGrav frequency range. The inevitable non-Gaussianity of this model suppresses the induced GWs associated with PBHs for the LIGO-Virgo events to be compatible with the NANOGrav results. We show

We revisit the constraints on the small scale density perturbations ($1{0}^{4}\text{ }\text{ }{\mathrm{Mpc}}^{\ensuremath{-}1}\ensuremath{\lesssim}k\ensuremath{\lesssim}1{0}^{5}\text{ }\text{ }{\mathrm{Mpc}}^{\ensuremath{-}1}$) from the modification of the freeze-out value of the neutron-proton ratio at the big-bang nucleosynthesis era. Around the freeze-out temperature $T\ensuremath{\sim}0.5\text{ }\text{ }\mathrm{MeV}$, the universe can be divided into several local patches that have different

Multiple pulsar timing array (PTA) collaborations recently announced the evidence of common-spectral processes caused by gravitational waves (GWs). These can be the stochastic GW background and its origin may be astrophysical and/or cosmological. We interpret it as the GWs induced by the primordial curvature perturbations and discuss their implications on primordial black holes (PBHs). We show that the newly released data suggest PBHs much lighter than the Sun [$\mathcal{O}({10}^{\ensuremath{-}4

Abstract The next generation of cosmic microwave background, gravitational wave, and large scale structure, experiments will provide an unprecedented opportunity to probe the primordial power spectrum on small scales. An exciting possibility for what lurks on small scales is a sharp rise in the primordial power spectrum: this can lead to the formation of primordial black holes, providing a dark matter candidate or the black holes observed by the LIGO-Virgo collaboration. In this work we develop

Primordial black holes (PBHs) have entered the forefront of theoretical cosmology, due their potential role in phenomena ranging from gravitational waves, to dark matter, to galaxy formation. While producing PBHs from inflationary fluctuations naively would seem to require a large deceleration of the inflaton from its velocity at the horizon exit of CMB scales, in this work we demonstrate that an acceleration from a relatively small downward step in the potential that is transited in much less t

Received 31 July 2023DOI:https://doi.org/10.1103/PhysRevD.108.049901© 2023 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasCosmologyEvolution of the UniverseGravitational wave sourcesGravitational wavesGravitation, Cosmology & Astrophysics