arXiv subjects

Koki Tokeshi

Publications and source records attributed to Koki Tokeshi.

12 recordsLinked to original sources

arXiv2026

Time-reversed stochastic inflation in the quantum well

Time-reversed stochastic inflation solves the stochastic evolution of the inflationary universe backward in time, by counting the number of e-folds from the end of quantum diffusion towards some initial state. The point of view of observers attached to the end-of-inflation hypersurface is thus enforced. In this work, we exactly solve time-reversed stochastic inflation in a flat and bounded potential, the so-called quantum well. At given lifetime, the field behaviour is found to be either indistinguishable from the one obtained in a semi-infinite flat potential, or, subject to enhanced stochasticity where any memory of the initial state is erased. The derived distribution of curvature perturbations reduces to the semi-infinite result for small fluctuations while it develops exponential tails for the large ones. Such tails arise for both positive and negative values, and decay twice as fast as the one obtained in the standard forward stochastic inflation. These differences may have important consequences for tail-sensitive phenomena, such as primordial black hole formation.

astro-ph.CO↗

arXiv2026

Primordial black holes in excursion set theory: Formation probabilities, mass functions, and window functions

We study the mass function of primordial black holes (PBHs) within the excursion-set theory, in which the response of the stochastic density contrast to the variation of the coarse-graining scale is described by colored noises. For several window functions often used in the literature, we investigate how this choice affects the formation probability as well as the resultant mass function of PBHs. It is found that the low-mass tail of the mass function differs from the one predicted from Carr's formula. The difference comes from the prevalence of correlated noises, by which degeneracy of the formation probabilities ceases to exist. Nevertheless, Carr's formula still provides a practical estimation in the vicinity of the characteristic mass scale, as long as a smooth window function in Fourier space is used.

astro-ph.CO↗

arXiv2026

Multifield stochastic inflation: Relevance of number of fields in statistical moments

In multifield inflation driven by $d$ scalar fields, $O (d)$ symmetry renders the number of fields irrelevant at classical level. This ceases to be the case once stochastic effects are accommodated. The statistical quantities such as the mean number and the variance of $e$-folds as well as the primordial power spectrum and its scale dependence are perturbatively calculated in a small-noise regime. In particular, a general formula is derived for arbitrary higher-order statistical moments of the stochastic number of $e$-folds at all perturbative orders, keeping the dependence on the number of fields fully analytical. It is also discussed that the requirement for inflation to be successfully terminated puts a theoretical bound on the number of fields from above. Those general results are demonstrated for several $O (d)$-symmetric models.

astro-ph.CO↗

arXiv2025

Primordial black holes and smooth coarse-graining in excursion set theory

The excursion-set formalism enables us to infer the mass distribution of collapsed objects, such as primordial black holes (PBHs), by the language of stochastic processes. Within the framework, this article investigates how a smooth coarse-graining procedure affects the resulting PBH mass function. As a demonstrative example, we employ a Gaussian window function, for which the stochastic noise becomes fully correlated across scales. It is found that these correlated noises result in a mass function of PBHs, whose maximum and its neighbourhood are predominantly determined by the probability that the density contrast exceeds a given threshold at each mass scale. Our results clarify the role of noise correlations induced by smooth coarse-graining and highlight their importance in predicting the abundance of PBHs.

astro-ph.CO↗

arXiv2025

Scalar field stochastic dynamics in de Sitter spacetime from exact solutions of quantum deficient oscillators

The stochastic dynamics of a scalar field in de Sitter spacetime can be regarded as a non-perturbative diffusion process, to which exact distribution and correlation functions are constructed by utilising the correspondence between diffusion and Schr\"{o}dinger equations. The Krein--Adler transformation of the quantum harmonic oscillator deletes several pairs of the energy levels to define anharmonic oscillators that we dub quantum deficient oscillators, based on which this article constructs a new class of exact solutions in stochastic inflation. In addition to the simplest single-well model, an exactly solvable double-well model is also presented. The results are further extended to exactly solvable models with multiple wells, allowing analytical studies on various cosmological phenomenologies.

hep-th↗

arXiv2025

More fields are different: Stochastic view of multi-field inflationary scenario

High-energy physics often motivates multi-field inflationary scenarios where stochastic effects play a crucial role. Peculiar to multi-field models, the noise-induced centrifugal force results in a longer duration of inflation depending on the number of fields, even when the stochastic noises themselves are small. We show that, in such small-noise regimes, the number of fields generically discriminates whether inflation successfully terminates or lasts forever. Our results indicate that inflation with an extremely large number of fields may fail to realise our observable Universe.

astro-ph.CO↗

arXiv2024

Curvaton distribution from stochastic inflation

The curvaton paradigm can realise a part of or all the observed curvature perturbation. Based on the stochastic formalism of inflation and closed-form exact distributions therein, the distribution of the curvature perturbation is presented in an analytical manner by identifying a test field with a curvaton. The parameter space consisting of the decay rate and the mass of the curvaton is studied to discuss the probability that the curvaton can contribute to the curvature perturbation in a non-negligible way.

astro-ph.CO↗

arXiv2024

Exactly solvable stochastic spectator

The stochastic formalism of inflation allows us to describe the scalar-field dynamics in a non-perturbative way. The correspondence between the diffusion and Schr\"{o}dinger equations makes it possible to exhaustively construct analytical solutions in stochastic inflation. Those exact statistical quantities such as distribution and correlation functions have one-to-one correspondence to the exactly solvable solutions in non-relativistic quantum mechanics in terms of classical orthogonal polynomials. A class of such solutions is presented by means of isospectral Hamiltonians with an underlying symmetry called shape invariance.

astro-ph.CO↗

arXiv2024

Why does inflation look single field to us?

Most high-energy constructions that realise a phase of cosmic inflation contain many degrees of freedom. Yet, cosmological observations are all consistent with single-field embeddings. We show how volume selection effects explain this apparent paradox. Because of quantum diffusion, different regions of space inflate by different amounts. In regions that inflate most, and eventually dominate the volume of the universe, a generic mechanism is unveiled that diverts the inflationary dynamics towards single-field attractors. The formalism of constrained stochastic inflation is developed to this end.

astro-ph.CO↗

arXiv2023

Borel resummation of secular divergences in stochastic inflation

We make use of Borel resummation to extract the exact time dependence from the divergent series found in the context of stochastic inflation. Correlation functions of self-interacting scalar fields in de Sitter spacetime are known to develop secular IR divergences via loops, and the first terms of the divergent series have been consistently computed both with standard techniques for curved spacetime quantum field theory and within the framework of stochastic inflation. We show that Borel resummation can be used to interpret the divergent series and to correctly infer the time evolution of the correlation functions. In practice, we adopt a method called Borel--Padé resummation where we approximate the Borel transformation by a Padé approximant. We also discuss the singularity structures of Borel transformations and mention possible applications to cosmology.

hep-th↗

arXiv2020

Superconducting Niobium Calorimeter for Studies of Adsorbed Helium Monolayers

We developed a calorimeter with a vacuum container made of superconducting niobium (Nb) to study monolayers of helium adsorbed on graphite which are prototypical two-dimensional quantum matters below 1 K. Nb was chosen because of its small specific heat in the superconducting state. It is crucially important to reduce the addendum heat capacity ($C_{\rm{ad}}$) when the specific surface area of substrate is small. Here we show details of design, construction and results of $C_{\rm{ad}}$ measurements of the Nb calorimeter down to 40 mK. The measured $C_{\rm{ad}}$ was sufficiently small so that we can use it for heat capacity measurements on helium monolayers in a wide temperature range below 1 K. We found a relatively large excess heat capacity in $C_{\rm{ad}}$, which was successfully attributed to atomic tunneling of hydrogen (H) and deuterium (D) between trap centers near oxygen or nitrogen impurities in Nb. The tunnel frequencies of H and D deduced by fitting the data to the tunneling model are consistent with the previous experiments on Nb doped with H or D.

cond-mat.other↗

arXiv2020

Window function dependence of the novel mass function of primordial black holes

We investigate the ambiguity of the novel mass function of primordial black holes, which has succeeded in identifying the black hole mass in a given configuration of fluctuations, due to the choice of window function of smoothed density fluctuations. We find that while the window function dependence of the exponential factor in the novel mass function is the same as the one in the conventional mass function around the top-hat scale, the dependences are different on other scales, which leads to the narrower mass function in the novel formulation for some window functions.

astro-ph.CO↗