arXiv subjects

Issei Koga

Publications and source records attributed to Issei Koga.

6 recordsLinked to original sources

arXiv2024

Big-Bang Nucleosynthesis on a bubble universe nucleated in Kerr-AdS$_5$ Black Hole

We present the Big-Bang Nucleosynthesis (BBN) simulation with a bubble universe scenario around a rotating black hole (BH) in Kerr-AdS$_5$ spacetime to explain recently updated observations of light elements such as the primordial helium abundance. In this scenario, the geometry of the 4D-early Universe is described as a vacuum bubble that undergoes quasi-de Sitter expansion in Kerr-AdS$_5$ spacetime. We find that the BH mass and spin parameter, which show an anti-correlation against the total radiation, are important to resolve the ${}^4{\rm He}$ anomaly. The present results provide clues to finding a connection between the observed results of light-element nucleosynthesis and the scenario of the 4D-bubble universe in AdS$_5$ spacetimes.

astro-ph.CO↗

arXiv2022

dS${}_4$ universe emergent from Kerr-AdS${}_5$ spacetime: bubble nucleation catalyzed by a black hole

The emergence of a four-dimensional de Sitter (dS${}_4$) universe on an expanding bubble in the five-dimensional anti-de Sitter (AdS${}_5$) background has been suggested as a possible cosmological scenario. It is motivated by the difficulties in the realization of a stable de Sitter vacua in string theory. The bubble can be nucleated in a meta-stable pure AdS${}_5$ spacetime, but it is known that a pure AdS spacetime is non-perturbatively unstable. It means that the pure AdS${}_5$ background is an idealized situation, and in realistic situations, non-linear perturbations in AdS may lead to the formation of black holes due to the gravitational turbulent instability. To investigate how the proposed scenario works in a more realistic situation, we here study the nucleation process of a vacuum bubble in the Kerr-AdS${}_5$ spacetime. Especially we investigate conditions sufficient to ensure the nucleation of a vacuum bubble with a rotating black hole and how the black hole affects the transition rate. We find that even in the Kerr-AdS${}_5$ spacetime, a quasi-dS${}_4$ expansion can be realized on the nucleated vacuum bubble without contradicting the de Sitter swampland conjectures.

hep-th↗

arXiv2022

Global study of the scalar quasi-normal modes of Kerr-AdS${}_5$ black holes: Stability, thermality, and horizon area quantization

We numerically explore the structure of quasi-normal (QN) frequencies of the five-dimensional small and large Kerr-anti de Sitter (Kerr-AdS${}_5$) black hole with equal and unequal rotations. Our investigation also covers low and high Hawking temperatures. We then study the stability of the Kerr-AdS${}_5$ black hole and the structure of highly damped QN modes, which would reflect the thermodynamic property of the Kerr-AdS${}_5$ black hole. We find that the highly damped complex QN frequencies of a nearly maximally spinning Kerr-AdS${}_5$ black hole have the periodic separation of the surface gravity at the horizon in the imaginary part while the real part converges to the superradiant frequency, which may be relevant to the pole structure of the thermal Green's function in the corresponding conformal field theory on the Kerr-AdS${}_5$ boundary. Finally, we discuss a relation between the QN modes of the Kerr-AdS${}_5$ black hole and the Hod's conjecture on the horizon area quantization along with the analysis of the horizon topology of the Kerr-AdS${}_5$ black hole. We show that in general, an ultra-spinning Kerr-AdS${}_5$ black hole, whose spin parameter is infinitesimally close to the AdS curvature radius, has its non-compact horizon, and based on the Hod's conjecture, we argue that the horizon area may be continuous, that is, the unit area of the horizon vanishes in the ultra-spinning regime.

hep-th↗

arXiv2020

Catalytic Creation of Bubble Universe Induced by Quintessence in Five Dimensions

We investigate the bubble nucleation in five dimensional spacetime catalyzed by quintessence. We especially focus on decay of a metastable Minkowski vacuum to an anti-de Sitter vacuum and study dynamics of the bubble on which four dimensional expanding universe is realized. We also discuss the trans-Planckian censorship conjecture and impose a constraint on the parameter space of the catalysis. As an application of this model, we propose an inflation mechanism and an origin of the dark energy in the context of quintessence in five dimensions.

hep-th↗

arXiv2019

Instability of Higgs Vacuum via String Cloud

We study the instability of the Higgs vacuum caused by a cloud of strings. By catalysis, the decay rate of the vacuum is highly enhanced and, when the energy density of the cloud is larger than the critical value, a semi-classical vacuum decay occurs. We also discuss the relation between the string cloud and observational constraints on the cosmic strings from the viewpoint of the catalysis, which are converted into bounds on the parameters of the Higgs potential.

hep-th↗

arXiv2019

Catalytic Creation of Baby Bubble Universe with Small Positive Cosmological Constant

We investigate the decay of metastable de Sitter, Minkowski and anti-de Sitter vacua catalyzed by a black hole and a cloud of strings. We apply the method to the creation of the four dimensional bubble universe in the five dimensional anti-de Sitter spacetime recently proposed by Banerjee, Danielsson, Dibitetto, Giri and Schillo. We study the bounce action for the creation and find that the bubble with very small cosmological constant, of order $Λ^{(4)}/M^2_4 \sim 10^{-120}$, is favored by the catalysis by assuming appropriate mass scales of the black hole and the cloud of strings to reproduce the present energy densities of matter and radiation in the bubble universe.

hep-th↗