arXiv subjects

Kazunori Kohri

Publications and source records attributed to Kazunori Kohri.

At least 37 records · Page 2Linked to original sources

arXiv2026

CMB signatures of gravity-mediated dark radiation in $\mathbf{ΔN_{\rm eff}}$

Measurement of $N_{\rm eff}$ in the CMB (Cosmic Microwave Background) observations, like Planck 2018 and BBN (Big Bang Nucleosynthesis) has already set stringent constraints on the interaction strength of light particles beyond the Standard Model (BSM). Despite such negligible couplings of such BSM particles to the visible sector, they are inevitably produced in the early universe through gravity-mediated processes. If a sizable density of light particles survives around CMB formation, they may act as dark radiation (DR) contributing to $N_{\rm eff}$ at CMB epoch. In this work, we study the production of such light BSM particles through the gravity-mediated scatterings in an effective field theory (EFT) setup assuming that all non-gravitational couplings of the BSM particle are negligible. Since the production is sensitive to the spin of the produced particle, we perform a concrete analysis for two representative cases: scalar dark Higgs DR and vector dark photons DR.Using the Planck 2018 observations, we find constraints on the reheating temperature ($T_{\rm RH}$) and background equation of state ($w_Φ$) during reheating in such scenarios featuring dark Higgs and dark photon. A comparative discussion involving gravity-mediated production of Dirac right-handed neutrinos ($ν_R$) and light axion-like particles (ALP) is also presented. Finally, for completeness, we also analyze the scenario where the production occurs through a generic spin-2 mediator characterized by an effective scale $Λ$ delineating the parameter space that is currently ruled out from Planck-2018 and can be probed by the future CMB experiments like LiteBird, Simon Observatory, CMB-S4, CMB-HD.

hep-ph↗

arXiv2026

Primordial black holes save $R^2$ inflation

In light of the latest Planck and Atacama Cosmology Telescope (P-ACT) joint results on the primordial scalar power spectrum, we show that the $R^2$ inflation model extended with a non-minimally coupled scalar field $χ$--namely the $χ$-extended $R^2$ inflation model--can naturally accommodate a larger spectral index $n_s$ and a small positive running $α_s$ at cosmic microwave background (CMB) scales, both of which are consistent with the latest P-ACT constraints. This is because the $χ$ field contributes a blue-tilted component to the primordial power spectrum, which both modifies the large-scale power and, as a result, significantly enhances power on small scales. The deviation of the $n_s$ and $α_s$ from the single field $R^2$ inflation is related to the non-minimal coupling constant $ξ$. The consequent enhancement in the primordial power spectrum can be large enough to lead to the formation of primordial black holes (PBHs) of mass $\lesssim 10^{20}\mathrm{g}$ as dark matter candidates. Furthermore, future observations of the small-scale power spectrum, CMB spectral distortions, and stochastic gravitational waves will provide decisive tests of this model and its predictions for PBHs. We stress its strong connection to the seesaw mechanism for the generation of the observed small masses.

astro-ph.CO↗

arXiv2026

Testing Seesaw and Leptogenesis via Gravitational Waves: Majorana versus Dirac

We investigate the B-L gauge extension of the Standard Model that the Dirac seesaw mechanism with thermal Leptogenesis can be tested using the stochastic gravitational background (SGWB) emanating from a network of cosmic strings when B-L symmetry is broken. With right-handed neutrino mass lighter than the typical scale of grand unification, the B-L symmetry protecting the right-handed neutrinos leads to constraints on the Yukawa couplings for both Dirac and Majorana scenarios. Estimating the predicted gravitational wave background we find that future space-borne missions could probe the range concerning thermal Dirac Leptogenesis. In a comparative analysis between such probes of gravitational wave sourced from cosmic strings in Dirac and Majorana Leptogenesis in the B-L extension, based on the energy scales of the Leptogenesis, for instance, GW detectors will be able to probe the scale of Dirac Leptogenesis upto $ 10^{9}$ GeV, while for Majorana Leptogenesis it would be upto $ 10^{12}$ GeV.

hep-ph↗

arXiv2026

Verifying the failing supernova constraint on dark photons with two-dimensional hydrodynamic simulations

Recent studies on the dark photon (DP) production in collapsing stars argue that the cooling effect induced by DPs can hinder supernova explosions and lead to a ``failing supernova" constraint on the photon-DP mixing parameter $ε$. In order to verify the idea, we perform two-dimensional neutrino-radiation hydrodynamic simulations coupled with the DP production with the masses of 0.3 and 0.45\,MeV. We find that the shock revival does not happen until the end of the simulations when $ε\gtrsim3\times10^{-9}$. The photon-DP mixing parameter above this value can be excluded by the failing supernova argument. Interestingly, our constraint roughly coincides with the one reported by the previous studies which adopted the post-processing framework. This result motivates one to investigate a wider parameter range of DPs with self-consistent simulations and evaluate uncertainties in the constraint.

astro-ph.HE↗

arXiv2025

Primordial Black Holes as Dark Matter and the Tachyonic Trap During Inflation

We show that resonant processes during multi-field inflation can generate a large curvature perturbation on small scales. This perturbation naturally leads to the formation of primordial black holes that may constitute dark matter, as well as to the production of stochastic induced gravitational waves in the deci-Hz band. Such waves are within reach of future space-based interferometers such as LISA, DECIGO and BBO. In addition, primordial black hole binaries formed at late times produce merger gravitational waves that can be probed by the resonant cavity experiments in addition to DECIGO and BBO.

astro-ph.CO↗

arXiv2025

Study of primordial non-Gaussianity $f_{\mathrm{NL}}$ and $g_{\mathrm{NL}}$ with the cross-correlations between the scalar-induced gravitational waves and the cosmic microwave background

The stochastic gravitational-wave background originating from cosmic sources contains vital information about the early universe. In this work, we comprehensively study the cross-correlations between the energy-density anisotropies in scalar-induced gravitational waves (SIGWs) and the temperature anisotropies and polarization in the cosmic microwave background (CMB). In our analysis of the angular power spectra for these cross-correlations, we consider all contributions of the local-type primordial non-Gaussianity $f_{\mathrm{NL}}$ and $g_{\mathrm{NL}}$ that can lead to large anisotropies. We show that the angular power spectra are highly sensitive to primordial non-Gaussianity. Furthermore, we project the sensitivity of future gravitational-wave detectors to detect such signals and, consequently, measure the primordial non-Gaussianity.

astro-ph.CO↗

arXiv2025

Inflation models selected by the swampland distance conjecture with the Lyth bound

We investigate the extent to which the Swampland Conjecture can be employed to constrain large-field inflationary models from the perspective of quantum gravity consistency. In particular, we focus on the Swampland Distance Conjecture, which imposes an upper bound on the amplitude of primordial gravitational waves predicted by large-field inflation scenarios. This provides a striking contrast with the well-known Lyth bound, which yields a lower bound on the tensor-to-scalar ratio in such models. The two bounds thus play complementary roles in assessing the viability of inflationary scenarios. We demonstrate that, for certain representative large-field inflation models, the Swampland Distance Conjecture alone can impose more stringent upper limits on the tensor-toscalar ratio than current observational constraints from the cosmic microwave background. These findings highlight the utility of Swampland criteria as a theoretical discriminator among competing inflationary models, independent of empirical data.

hep-th↗

arXiv2025

Super heavy dark matter origin of the PeV neutrino event: KM3-230213A

The recent observation of the ultrahigh-energy neutrino event KM3-230213A by the KM3NeT experiment offers a compelling avenue to explore physics beyond the Standard Model (SM). In this paper, we explore a simplest possibility that this event originates from the decay of a super-heavy dark matter (SHDM). We consider a minimal scenario where the SHDM decays to neutrino and SM Higgs. We derive constraints on the DM lifetime as a function of DM mass, ensuring consistency with IceCube, Auger upper limits, and the observed KM3-230213A event, along with the gamma-ray constraints. We find that KM3-230213A gives stringent constraint on the DM mass ranging from $1.5\times10^8$ GeV to $5.2\times10^9$ GeV with lifetime in the range: $1.42\times10^{30}$ s to $5.4\times10^{29}$ s. Remarkably, in our SHDM scenario, the apparent tension between the KM3NeT observation and the nonobservation of this event by IceCube and Auger can be reduced to below $1.2σ$. Our results are applicable to any neutrinophilic SHDM models while evading gamma-ray constraints.

hep-ph↗

arXiv2025

Primordial Black Hole Formation and Spin in Matter Domination Revisited

In this article, we calculate the mass distribution of primordial black holes (PBHs) formed in the matter-dominated (MD) era by the peak theory. We apply the Zel'dovich approximation to track the nonlinear evolution of overdensities and compute the PBH abundance and mass function by incorporating a PBH formation criterion based on the hoop conjecture. We find that the PBH abundance $β$ follows the scaling law $β\simeq A_γσ_h^{*5}$ for $σ_h^*\ll 1$. Here, $σ_h^*$ is the quantity that characterizes the variance of the density fluctuation at the horizon entry. We also find that, in contrast to the previous estimates, the PBH spin is very small for $σ_h^*\ll 1$ but could be larger for larger $σ_h^*$ and broader power spectra. Finally, specializing to a monochromatic power spectrum, we prove analytically that the PBH mass distribution becomes effectively monochromatic and reveal that the resultant PBH abundance is approximately 19 times the previous prediction.

gr-qc↗

arXiv2025

New bounds on Memory Burdened Primordial Black Holes from Big Bang Nucleosynthesis

Primordial black holes (PBHs) with masses below $10^9\,\rm{g}$ are typically assumed to have negligible cosmological impact due to their rapid evaporation via Hawking radiation. However, the 'memory burden' effect, which is a quantum suppression of PBH evaporation, can dramatically alter their decay dynamics. In this work, we revisit early-Universe constraints on ultralight PBHs in this mass range, demonstrating that memory burden significantly alters previous constraints. We compute new cosmological bounds from BBN that strongly limit the presence of ultralight PBHs in the early Universe. We report that the PBHs in the mass range $10^0$-$10^2\,\rm{g}$ for $k=2$ are unconstrained by observations.

astro-ph.CO↗

arXiv2025

Core-collapse supernova explosions hindered by eV-mass sterile neutrinos

Light sterile neutrinos, $ν_s$, are often introduced to explain an anomalous deficit in the electron antineutrino flux from nuclear reactors. If they exist, sterile neutrinos would also be produced in collapsing massive stars through the active-sterile neutrino oscillation. In order to investigate the impacts of sterile neutrinos on supernova dynamics, we perform two-dimensional neutrino-radiation hydrodynamic simulations of stellar core-collapse coupled with the active-sterile oscillation through the Mikheyev-Smirnov-Wolfenstein effect. As the initial condition of our simulations, we adopt a blue supergiant model that is tuned to reproduce observational features of the SN 1987A progenitor to compare our models with observations of the event. It is found that the active-sterile oscillation reduces the $ν_{e}$ and $\barν_e$ fluxes and decreases the explosion energy. We also find that, if the mixing angle $θ$ and the mass difference $δm_\mathrm{s}^2$ between $ν_e$ and $ν_s$ are large enough, the star fails to explode. This suggests that these mixing parameters relevant to sterile neutrinos could be constrained by supernova explodability, though other uncertainties in supernova theory need to be addressed to refine them. In addition, we predict neutrino signals from a nearby supernova event and find that the neutrino event number can significantly decrease because the $ν_e$ and $\barν_e$ fluxes are reduced. In particular, DUNE observations of $ν_e$ will be useful to search for a signature of sterile neutrinos with a tiny mixing angle because a smaller mixing angle leads to a larger effect on the $ν_e$ flux.

astro-ph.HE↗

arXiv2025

General relativistic effects on photon spectrum emitted from dark matter halos around primordial black holes

We investigate general relativistic effects on the photon spectrum emitted from decaying (or annihilating) particle dark matter in the halo surrounding a primordial black hole. The spectrum undergoes significant modification due to gravitational redshifts, which induces broadening as a result of the intense gravitational field near the black hole. This characteristic alteration in the photon spectrum presents a unique observational signature. Future observations of such spectral features may provide critical evidence for a mixed dark matter scenario, involving both primordial black holes and particle dark matter.

hep-ph↗

arXiv2025

Induced Gravitational Waves probing Primordial Black Hole Dark Matter with Memory Burden

Quantum evaporation of a black hole is conventionally studied semiclassically by assuming self-similarity of the black hole throughout the evaporation process. However, its validity was recently questioned, and the lifetime of a black hole is conjectured to be much extended by the memory burden effect. It gives rise to the possibility that the primordial black holes (PBHs) lighter than $10^{10}$ grams are the dark matter in the Universe. To probe such PBH dark matter, we study gravitational waves (GWs) induced by primordial curvature perturbations that produced the PBHs. We find $Ω_\text{GW}(f_\text{peak})h^2 = 7 \times 10^{-9}$ with the peak frequency $f_\text{peak} = 1\times 10^{3} \, (M_\text{PBH}/(10^{10}\,\mathrm{g}))^{-1/2}\, \mathrm{Hz}$, and the induced GWs associated with the PBH dark matter whose initial mass is greater than about $10^7$ grams can be tested by future observations such as Cosmic Explorer. Furthermore, the scenario can be in principle confirmed by detecting another GW signal from the mergers of PBHs, which leads to high-frequency GWs with $f_\text{peak} = 2 \times 10^{27}\, (M_\text{PBH, ini}/(10^{10}\, \mathrm{g}))^{-1} \, \mathrm{Hz} $. On the other hand, the induced GW signals stronger than expected would contradict the dark matter abundance and exclude the memory burden effect.

astro-ph.CO↗

arXiv2024

Thermalization and hotspot formation around small primordial black holes

We quantitatively analyze a basic question: what is the stationary solution of the background plasma temperature profile around a black hole (BH)? One may naively expect that the temperature profile continuously decreases from the Hawking temperature at the surface of the BH towards an outer region. We show analytically and numerically that this is not the case because local thermal equilibrium cannot be maintained near the surface of the BH and also because the high-energy particles emitted from Hawking radiation cannot be instantaneously thermalized into the background plasma. The temperature profile has a plateau within a finite distance from the BH, and even the overall amplitude of background temperature at a distance far away from the BH is significantly suppressed compared with the naive expectation. The main reason for these counterintuitive results comes from the fact that the size of the BH is too small that particles of Hawking radiation goes far away within the typical time scale of interactions.

hep-ph↗

arXiv2024

Breakdown of Hawking Evaporation opens new Mass Window for Primordial Black Holes as Dark Matter Candidate

The energy injection through Hawking evaporation has been used to put strong constraints on primordial black holes as a dark matter candidate at masses below $10^{17}\,\rm{g}$. However, Hawking's semiclassical approximation breaks down at latest after half-decay. Beyond this point, the evaporation could be significantly suppressed, as was shown in recent work. In this study, we review existing cosmological and astrophysical bounds on primordial black holes, taking this effect into account. We show that the constraints disappear completely for a reasonable range of parameters, which opens a new window below $10^{10}\,\rm{g}$ for light primordial black holes as a dark matter candidate.

astro-ph.CO↗

arXiv2024

Complete analysis of the background and anisotropies of scalar-induced gravitational waves: primordial non-Gaussianity $f_{\mathrm{NL}}$ and $g_{\mathrm{NL}}$ considered

Investigation of primordial non-Gaussianity holds immense importance in testing the inflation paradigm and shedding light on the physics of the early Universe. In this study, we conduct the complete analysis of scalar-induced gravitational waves (SIGWs) by incorporating the local-type non-Gaussianity $f_{\mathrm{NL}}$ and $g_{\mathrm{NL}}$. We develop Feynman-like diagrammatic technique and derive semi-analytic formulas for both the energy-density fraction spectrum and the angular power spectrum. For the energy-density fraction spectrum, we analyze all the relevant Feynman-like diagrams, determining their contributions to the spectrum in an order-by-order fashion. As for the angular power spectrum, our focus lies on the initial inhomogeneities, giving rise to anisotropies in SIGWs, that arise from the coupling between short- and long-wavelength modes due to primordial non-Gaussianity. Our analysis reveals that this spectrum exhibits a typical multipole dependence, characterized by $\tilde{C}_{\ell}\propto[\ell(\ell+1)]^{-1}$, which plays a crucial role in distinguishing between different sources of gravitational waves. Depending on model parameters, significant anisotropies can be achieved. We also show that the degeneracies in model parameters can be broken. The findings of our study underscore the angular power spectrum as a robust probe for investigating primordial non-Gaussianity and the physics of the early Universe. Moreover, our theoretical predictions can be tested using space-borne gravitational-wave detectors and pulsar timing arrays.

astro-ph.CO↗

arXiv2024

Angular bispectrum and trispectrum of scalar-induced gravitational waves: all contributions from primordial non-Gaussianity $f_\mathrm{NL}$ and $g_\mathrm{NL}$

Studying the primordial non-Gaussianity of inflationary perturbations is crucial for testing the inflation paradigm of the early universe. In this work, we conduct a comprehensive analysis of the angular bispectrum and trispectrum of scalar-induced gravitational waves (SIGWs) in the presence of local-type primordial non-Gaussianity parameterized by $f_\mathrm{NL}$ and $g_\mathrm{NL}$, deriving their semi-analytical formulae for the first time. Our findings indicate that it is the presence of primordial non-Gaussianity that leads to a non-Gaussian SIGW background, suggesting that the angular bispectrum and trispectrum of SIGWs could serve as probes of the primordial non-Gaussianity. Our numerical results further illustrate that $f_\mathrm{NL}$ and $g_\mathrm{NL}$ exert significant impacts on the spectral amplitudes, potentially reaching up to $10^{-5}$ for the former and $10^{-8}$ for the latter. In particular, we demonstrate that the angular bispectrum and trispectrum exhibit characteristic dependence on the angular multipoles and frequency bands. They hold potentials to be measured by gravitational-wave detectors that may advance our understanding of the origin of the universe.

astro-ph.CO↗

arXiv2024

The N2HDM, Entropy Production and Stochastic Gravitational Waves

This study undertakes a reconsideration of the potential for a first-order electroweak phase transition, focusing on the next-to-minimal two Higgs doublet model (N2HDM). Our exploration spans diverse parameter spaces associated with the phase transition, with a particular emphasis on examining the generation of stochastic Gravitational Waves (GW) resulting from this transition. The obtained results are meticulously compared against data from prominent gravitational wave observatories, and the possibility of their detection in the future GW observations have been established. In passing by we analyse the strength of the phase transition through the production of entropy during the electroweak phase transition.

hep-ph↗