arXiv subjects

Mai Yashiki

Publications and source records attributed to Mai Yashiki.

5 recordsLinked to original sources

arXiv2025

DBI scalar field cosmology in $n$-DBI gravity

We study inflationary characterictics of the universe in $n$-DBI gravity, driven by DBI-deformed scalar fields. In this paper, we consider the evolution of the classical universe for a scalar potential whose equations of motion are expressed by a BPS-like system of first-order differential equations.The advantage of a BPS-like system is that it allows the initial condition for the wave function of the Universe in minisuperspace quantum cosmology to be automatically determined by the Hamiltonian constraint. By appropriately choosing the prepotential underlying the potential, we can construct single-scalar-field models that have the phases of exponential and power-law expansions. We show that the slow-roll parameters for our models can be expressed in terms of the prepotential with simple settings.

gr-qc↗

arXiv2025

Toward a simultaneous resolution of the $H_0$ and $S_8$ tensions: early dark energy and an interacting dark sector model

The tension between the Hubble constant ($H_0$) inferred from the cosmic microwave background (CMB) and that measured from late-time observations, such as the local distance ladder, is a major challenge in modern cosmology. Early dark energy (EDE) has been proposed as a possible resolution to the $H_0$ tension, but it typically worsens the $S_8$ tension by enhancing the small-scale matter power spectrum due to an increased cold dark matter density. To address this issue, we propose a model that combines EDE with an interacting dark energy-dark matter (iDEDM) scenario, and investigate whether this mixed model can simultaneously resolve both tensions. We find that the DE-DM interaction suppress the growth of structure and reduce $S_8$, while EDE contributes to increase $H_0$, although less effectively than in the EDE-only case. Our MCMC analysis using Planck 2018, DESI BAO, DES, Pantheon+, and SH0ES data shows that the mixed model provides modest improvements in both tensions, although it does not fully resolve either. This limitation appears to stem from the fact that both EDE and iDEDM independently favor a higher present-day matter density, which reduces the angular diameter distance and limits the degree to which EDE can lower the sound horizon.

astro-ph.CO↗

arXiv2024

Quantum BPS Cosmology

There has been much discussion about the initial conditions of the early Universe in the context of quantum theory. In this paper, we construct the wave function and probability distribution by adopting the quantum version of the BPS equation instead of the usual Wheeler-DeWitt equation in a minisuperspace quantum cosmology with spatially uniform scalar fields. Although the model treated in this study is technically valid for a limited form of scalar potential, we show that it is possible to construct a conserved probability current in our model. We also examine classical and quantum aspects of models with the Dirac-Born-Infeld type scalar fields.

gr-qc↗

arXiv2020

Local gravity test of unified models of inflation and dark energy in $f(R)$ gravity

We consider $f(R)$ gravity theories which unify $R^n$ inflation and dark energy models. First, from the final Planck data of the cosmic microwave background, we obtain a condition, $1.977 < n < 2.003$. Next, under this constraint, we investigate local-gravity tests for three models. We find that the $R^n$ term can dominate over the dark energy term even at the Earth's curvature scale, contrary to intuition; however, the $R^n$ term does not relax or tighten the constraints on the three models.

astro-ph.CO↗

arXiv2019

Classical and quantum cosmology of $K$-essentially modified $R^2$ and pure $R^p$ gravity

We present a gravitational action with a modified higher order term of a combination of scalar curvature and Lagrangian density of a scalar field. This type of models has been considered first by Cruz-Dombriz et al. The classical and quantum cosmologies governed by the modified action are studied. Models described by a positive-definite action and a pure arbitrary-powered scalar curvature action without the standard Einstein-Hilbert term are also investigated. We show some particular cases in which exact solutions can be obtained.

gr-qc↗