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Nahomi Kan

Publications and source records attributed to Nahomi Kan.

At least 19 recordsLinked to original sources
arXiv2025

Isothermal spheres from grand partition functions in nonextensive statistical mechanics

We analytically study isothermal spheres in the light of nonextensive statistical mechanics. The equations for the isothermal spheres are derived from the grand partition function of the gravitating particle system in the Tsallis statistical mechanics. The effect of nonextensive statistics appears in relatively dense state, which appears at the center of the isothermal sphere. The stability of the isothermal sphere in the general relativistic system is found to be sensitive to the parameter q in the Tsallis statistics.

gr-qc↗
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↗
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↗
arXiv2024

Light fermion masses in partially deconstructed models

Considering a theory space consisting of a large number of five-dimensional Dirac fermion field theories including background abelian gauge fields, we can construct a theory similar to a continuous six-dimensional theory compactified with two-dimensional manifolds with and without magnetic flux or orbifolds as extra dimensions. This method, called dimensional deconstruction, can be used to construct a model with one-dimensional discrete space, which represents general graph structures. In this paper, we propose the models with two extra dimensions, which resemble two-dimensional tori, cylinders, and rectangular regions, as continuum limits. We also try to build a model that mimics one with the two-dimensional orbifold compactification.

hep-th↗
arXiv2024

Electric and dilatonic fields of a charged massive particle at rest in the field of a charged dilaton black hole

We study linear-perturbation equations for the two-body system of a charged dilaton black hole, of which dilaton coupling constant is $α$, and a static particle with mass $m$, electric charge $q$, and dilatonic charge $βm$. We find that a consistent condition for the coupled equations corresponds to the equilibrium condition of the test particle. The expressions of classical fields are given in closed analytical formulas in the most interesting case with $β=α$. We examine the electrical field around a charged dilaton black hole especially in the limit of the maximum electric charge and we find the electric Meissner effect which has been found for the Reissner-Nordström black hole in the Einstein-Maxwell system.

gr-qc↗
arXiv2023

Spinorial Wheeler-DeWitt wave functions inside black hole horizons

We revisit the solutions of the Wheeler-DeWitt (WDW) equation inside the horizons of spherical black holes and planar topological black holes in arbitrary dimensions. For these systems, the solutions of the equations are found to have the same form. Therefore, Yeom's Annihilation-to-nothing interpretation can be applied to each case. We have introduced the Dirac-type WDW equations into quantum cosmology in a recent paper, so we also apply our formulation to the quantum theory of the interior of the black hole in order to obtain the solution of the spinorial wave function. The shape of the wave packet of the spinorial WDW wave function indicates that the variation of Yeom's interpretation holds in this scheme.

gr-qc↗
arXiv2023

Discrete time heat kernel and UV modified propagators with Dimensional Deconstruction

We revisit the dimensionally deconstructed scalar quantum electrodynamics and consider the (Euclidean) propagator of the scalar field in the model. Although we have previously investigated the one-loop effect in this model by obtaining the usual heat kernel trace, we adopt discrete proper-time heat kernels in this paper and aim to construct the modified propagator, which has improved behaviors in the ultraviolet region, by changing the range of sum of the discrete heat kernels.

hep-th↗
arXiv2023

Geometrical effective actions for a partially massless spin-2 field

We consider nonlinear effective actions for a spin-2 field, whose `decoupling' limit gives Fierz-Pauli action in D dimensional maximally symmetric spacetime. We find, especially, the effective action for a partially massless field can take a concise geometrical form. The exact solution for time evolution of the background metric in the model using the effective action is also studied.

hep-th↗
arXiv2022

Classical and quantum bicosmology with noncommutativity

Recently, Falomir, Gamboa, Mendez, Gondolo and Maldonado proposed a bicosmology scenario [1-4] for solving some cosmological problems related to inflation, dark matter, and thermal history of the universe. Their plan is to introduce noncommutativity into the momentum space of the two scale factors. In the present paper, we revisit their model and first consider exact classical solutions in the model with constant noncommutativity between dynamical variables and between canonical momenta. We also hypothesize that the noncommutativity appears when the scale factors are small, and show the behavior of the classical solution in that case with momentum-space noncommutativity. Finally, we write down the Wheeler-DeWitt equation in that case and examine the behavior of the solution.

gr-qc↗
arXiv2022

Third quantization for scalar and spinor wave functions of the Universe in an extended minisuperspace

We consider the third quantization in quantum cosmology of a minisuperspace extended by the Eisenhart-Duval lift. We study the third quantization based on both Klein-Gordon type and Dirac-type equations in the extended minisuperspace. Spontaneous creation of "universes" is investigated upon the quantization of a simple model. We find that the quantization of the Dirac-type wave function reveals that the number density of universes is expressed by the Fermi-Dirac distribution. We also calculate the entanglement entropy of the multi-universe system.

gr-qc↗
arXiv2022

Hosotani mechanism in the "color"-singlet plasma

By projecting the partition function on the "color"-singlet state, we investigate the Hosotani mechanism in the fermion-gauge boson plasma. The present toy-model analysis of the one-loop effective potential at finite temperature shows that the critical temperature of gauge symmetry breaking increases at higher temperature in the smaller volume.

hep-th↗
arXiv2022

Calculations in induced gravity from higher-derivative field theories

In this paper, we investigate Einstein's gravity induced from higher-derivative scalar field theories. We develop an approach utilizing an effective theory of multiple fields for the higher-derivative theory. The expressions for induced cosmological constant and the induced gravitational constant are obtained in the present scenario of induced gravity in D dimensions. We also show that finite values for the induced constants can be extracted in certain infinite-derivative theories.

gr-qc↗
arXiv2021

Eisenhart--Duval lift for minisuperspace quantum cosmology

We study covariant equations in quantum cosmology of an extended minisuperspace obtained by the Eisenhart--Duval lift. We find that a Dirac-type equation is naturally introduced in the extended minisuperspace. Explicit forms of the fundamental solutions are yielded for specific models. The possible further development in this direction is also discussed.

gr-qc↗
arXiv2021

Quantum fluctuation of stress tensor in a higher-derivative scalar field theory around a cosmic string

We calculate the vacuum fluctuation of the stress tensor of a higher-derivative theory around a thin cosmic string. To this end, we adopt the method to obtain the stress tensor from the effective action developed by Gibbons et al. By their method, the quantum stress tensor of higher-derivative scalar theories without self-interaction is expressed as a simple sum of quantum stress tensors of free massive scalar fields. Unlike the vacuum expectation value of the scalar field squared obtained in the similar model, there appears no reduction of the values near the conical singularity.

hep-th↗
arXiv2021

Discrete heat kernel, UV modified Green's function, and higher derivative theories

We perform the UV deformation of the Green's function in free scalar field theory using a discrete heat kernel method. It is found that the simplest UV deformation based on the discretized diffusion equation leads to the well-known Pauli-Villars effective Lagrangian. Furthermore, by extending assumptions on the discretized equation, we find that the general higher derivative theory is derived from the present UV deformation. In some specific cases, we also calculate the vacuum expectation values of the scalar field squared in nontrivial background spaces and examine their dependence on the UV cutoff constant.

hep-th↗
arXiv2021

Isothermal spheres in general relativity and Horava-type gravity

We construct a toy model for isothermal spheres in Horava gravity, which includes Einstein's gravity if a parameter is appropriately chosen. The equations for the isothermal spheres are derived from the partition function of the gravitating particle system. We confirm that the Newtonian limit of the system coincides with the model of the well-known isothermal sphere. The stability of the isothermal sphere is found to be sensitive to the energy density at the center of the sphere.

gr-qc↗