Osamu Seto

A shorter sound horizon scale at the recombination epoch, arising from introducing extra energy components such as extra radiation or early dark energy (EDE), is a simple approach to resolving the so-called Hubble tension. We compare EDE models, an extra radiation model, and a model in which EDE and extra radiation coexist, paying attention to the fit to big bang nucleosynthesis (BBN). We find that the fit to BBN in EDE models is somewhat poorer than that in the $\mathrm{\ensuremath{\Lambda}}\ma

Motivated by recently reported anomalies in a decay of an excited state of beryllium by the Atomki Collaboration, we study a radiative seesaw model with gauged $B\ensuremath{-}L$ symmetry and a ${Z}_{2}$ parity. Assuming that the anomalies originate from the decay of the $B\ensuremath{-}L$ gauge boson followed by the nuclear decay, the mass of the lightest right-handed neutrino or the dark matter candidate can be determined below 10 GeV. We show that for this mass range, the model can explain th

We show that axinos, which are dominantly generated by the decay of the next-to-lightest supersymmetric particles produced from the leptonic $Q$-ball ($L$-ball), become warm dark matter suitable for the solution of the missing satellite problem and the cusp problem. In addition, ${\ensuremath{\Omega}}_{b}\ensuremath{-}{\ensuremath{\Omega}}_{\mathrm{DM}}$ coincidence is naturally explained in this scenario.

The spectrum of adiabatic density perturbation generated during inflation is studied in the case the time derivative of an inflation-driving scalar field (inflaton) vanishes at some time during inflation. It is shown that the nondecaying mode of perturbation has a finite value even in this case and that its amplitude is given by the standard formula with the time derivation of the scalar field replaced by the potential gradient using the slow-roll equation.

The influence of higher-order terms in the K\"ahler potential of the supergravity $D$-term inflation model on the density perturbation is studied. We show that these terms can make the inflaton potential flatter, which lowers the energy scale of inflation under the COBE/WMAP normalization. As a result, the mass per unit length of cosmic strings, which are produced at the end of inflation, can be reduced to a harmless but detectable level without introducing a tiny Yukawa coupling. Our scenario c

We study the fit of cosmological models with two additional free parameters ${N}_{\mathrm{eff}}$ and ${\ensuremath{\xi}}_{e}$ in addition to the parameters of $\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$. We introduce extra radiation components such as hot axions or sterile neutrinos in addition to the energy density of neutrinos with large neutrino degeneracy. Then, a larger ${N}_{\mathrm{eff}}$ is allowed without spoiling big bang nucleosynthesis (BBN), as positive neutrino degeneracy ${\ensure

Baryon acoustic oscillation (BAO) is one of the important standard rulers in cosmology. The results of the latest BAO measurements by Dark Energy Spectroscopic Instrument (DESI) survey have been reported. Cosmology with the varying electron mass model and the early dark energy models are regarded as interesting models to resolve the Hubble tension. We present constraints on the varying electron mass model and early dark energy models in new DESI data as well as cosmic microwave background by Pla

Affleck-Dine baryogenesis in models where the gravitino is both the lightest supersymmetric particle and the dark matter candidate is investigated. For a high enough reheating temperature to produce sufficient gravitinos by thermal processes, the observed baryon asymmetry can be explained by Affleck-Dine baryogenesis as well as thermal leptogenesis. On the other hand, if the reheating temperature is not high enough, most of the gravitinos must be produced by the decay of the next-to-lightest sup

We examine the possibility that direct dark matter detection experiments find decay products from sterile neutrino dark mater in U(1)B−L and U(1)R models. This is possible if the sterile neutrino interacts with a light gauge boson, and decays into a neutrino and the light gauge boson with a certain lifetime. This decay produces energetic neutrinos scattering off nuclei with a large enough recoil energy in direct dark matter detection experiments. We stress that direct dark matter detection exper

A cosmological model with a time-varying mass of electrons seems a promising solution for the so-called Hubble tension. We examine the big bang nucleosynthesis (BBN) constraints on the time-varying electron mass model because a larger electron mass gives rise to the smaller weak interaction rate for the proton and neutron conversion, which could affect the light element abundance. Additionally, different inferred cosmological parameters, primarily baryon asymmetry, keeping the cosmic microwave b

We study the behavior of gravitational waves and their back reaction on the background in cosmological solutions of the five-dimensional Ho\ifmmode \check{r}\else \v{r}\fi{}ava-Witten theory. As a dynamical background, we consider two cosmological solutions with spatially flat expanding FRW branes, called $(\ensuremath{\uparrow})$ and $(\ensuremath{\downarrow})$ solutions, in which the orbifold size increases and decreases in time, respectively. For these background solutions, the wave equation