arXiv subjects

Phillial Oh

Publications and source records attributed to Phillial Oh.

At least 19 recordsLinked to original sources

arXiv2021

Aspects of $(d+D)$-dimensional Anisotropic Conformal Gravity

We discuss various aspects of anisotropic gravity in $(d+D)$-dimensional spacetime where $D$ dimensions are treated as extra dimensions. It is based on the foliation preserving diffeomorphism invariance and anisotropic conformal invariance. The anisotropy is embodied by introducing a factor $z$ which discriminates the scaling degree of the extra $D$ dimensions against the $d$-dimensional base spacetime and Weyl scalar field which mediates the anisotropic scaling symmetry. There is no intrinsic scale but a physical scale $M_*$ emerges as a consequence of spontaneous conformal symmetry breaking. Some vacuum solutions are obtained and we discuss an issue of `size separation' between the base spacetime and the extra dimensions. The size separation means large hierarchy between the scales appearing in the base spacetime and the extra dimensions respectively. We also discuss interesting theories obtained from our model. In the case of (4,1), we propose a resolution of hierarchy problem and discuss comparison with the results of the brane-world model. In a $(d,D)=(2,2)$ case, we suggest a UV-complete unitary quantum gravity which might become Einstein gravity in IR. In a certain (2,1) case, we obtain CGHS-model.

hep-th↗

arXiv2020

A Vanishingly Small Vector Mass from Anisotropy of Higher Dimensional Spacetime

We consider five-dimensional massive vector-gravity theory which is based on the foliation preserving diffeomorphism and anisotropic conformal invariance. It does not have an intrinsic scale and the only relevant parameter is the anisotropic factor $z$ which characterizes the degree of anisotropy between the four-dimensional spacetime and the extra dimension. We assume that physical scale $M_*$ emerges as a consequence of spontaneous conformal symmetry breaking of vacuum solution. It is demonstrated that a very small mass for the vector particle compared to $M_*$ can be achieved with a relatively mild adjustment of the parameter $z$. At the same time, it is also observed that the motion along the extra dimension can be highly suppressed and the five-dimensional theory can be effectively reduced to four-dimensional spacetime.

hep-th↗

arXiv2019

Canonical Quantization of Noncompact Spin System

We consider spin system defined on the coadjoint orbit with noncompact symmetry and investigate the quantization. Classical spin with noncompact SU(N,1) symmetry is first formulated as a dynamical system and the constraint analysis is performed to reduce the system from the group space to the coadjoint orbit which is a symplectic manifold with Kahler structure. We achieve this by solving the constraint directly. It is shown that the dynamical variables describing the noncompact spins can be written as functions of canonically conjugate variables and canonical quantization is possible on the reduced phase space. With the quantum mechanical Hamiltonian acting on the holomorphic coherent state in Hilbert space, we obtain the exact propagator by solving the time-dependent Schrodinger equation.

math-ph↗

arXiv2018

Hamiltonian Formalism of Topologically Massive Electrodynamics

We consider the four dimensional topologically massive electrodynamics in which a gauge field is interacting with 2nd rank antisymmetric tensor field through a topological interaction. The photon becomes massive by eating the 2nd rank tensor field, which is dual to the Higgs mechanism. We explicitly demonstrate the nature of the mechanism by performing a canonical analysis of the theory and discuss various aspects of it.

hep-th↗

arXiv2018

The Effect of Anisotropic Extra Dimension in Cosmology

We consider five dimensional conformal gravity theory which describes an anisotropic extra dimension. Reducing the theory to four dimensions yields Brans-Dicke theory with a potential and a hidden parameter $z$ which implements the anisotropy between the four dimensional spacetime and the extra dimension. We find that a range of value of the parameter $z$ can address the current dark energy density compared to the Planck energy density. Constraining the parameter $z$ and the other cosmological model parameters using the recent observational data consisting of the Hubble parameters, type Ia supernovae, and baryon acoustic oscillations, together with the Planck or WMAP 9-year data of the cosmic microwave background radiation, we find $z>-2.05$ for Planck data and $z>-2.09$ for WMAP 9-year data at 95\% confidence level. We also obtained constraints on the rate of change of the effective Newtonian constant~($G_{\rm eff}$) at present and the variation of $G_{\rm eff}$ since the epoch of recombination to be consistent with observation.

astro-ph.CO↗

arXiv2017

$z-$Weyl gravity in higher dimensions

We consider higher dimensional gravity in which the four dimensional spacetime and extra dimensions are not treated on an equal footing. The anisotropy is implemented in the ADM decomposition of higher dimensional metric by requiring the foliation preserving diffeomorphism invariance adapted to the extra dimensions, thus keeping the general covariance only for the four dimensional spacetime. The conformally invariant gravity can be constructed with an extra (Weyl) scalar field and a real parameter $z$ which describes the degree of anisotropy of conformal transformation between the spacetime and extra dimensional metrics. In the zero mode effective 4D action, it reduces to four-dimensional scalar-tensor theory coupled with nonlinear sigma model described by extra dimensional metrics. There are no restrictions on the value of $z$ at the classical level and possible applications to the cosmological constant problem with a specific choice of $z$ is discussed.

hep-th↗

arXiv2016

Spontaneous symmetry breaking in 5D conformally invariant gravity

We explore the possibility of the spontaneous symmetry breaking in 5D conformally invariant gravity, whose action consists of a scalar field nonminimally coupled to the curvature with its potential. Performing dimensional reduction via ADM decomposition, we find that the model allows an exact solution giving rise to the 4D Minkowski vacuum. Exploiting the conformal invariance with Gaussian warp factor, we show that it also admits a solution which implement the spontaneous breaking of conformal symmetry. We investigate its stability by performing the tensor perturbation and find the resulting system is described by the conformal quantum mechanics. Possible applications to the spontaneous symmetry breaking of time-translational symmetry along the dynamical fifth direction and the brane-world scenario are discussed.

gr-qc↗

arXiv2016

Massive Photon and Dark Energy

We investigate cosmology of massive electrodynamics and explore the possibility whether massive photon could provide an explanation of the dark energy. The action is given by the scalar-vector-tensor theory of gravity which is obtained by non-minimal coupling of the massive Stueckelberg QED with gravity and its cosmological consequences are studied by paying a particular attention to the role of photon mass. We find that the theory allows cosmological evolution where the radiation- and matter-dominated epochs are followed by a long period of virtually constant dark energy that closely mimics $Λ$CDM model and the main source of the current acceleration is provided by the nonvanishing photon mass governed by the relation $Λ\sim m^2$. A detailed numerical analysis shows that the nonvanishing photon mass of the order of $\sim 10^{-34}$ eV is consistent with the current observations. This magnitude is far less than the most stringent limit on the photon mass available so far, which is of the order of $m \leq 10^{-27}$eV.

astro-ph.CO↗

arXiv2015

Large Scale Suppression of Scalar Power on a Spatial Condensation

Suppression of the scalar power spectrum on large scales is one way to reconcile the tension between Planck and BICEP2 data. This suppression can occur by introducing a phase transition from the fast-roll phase to the slow-roll phase in a single field inflation model. In this paper we consider a deformed single field inflation model in terms of three SO(3) symmetric moduli fields. We find that spatially linear solutions for the moduli fields induces a phase transition during the early stage of the inflation and the suppression of scalar power spectrum at large scale perturbation modes.

gr-qc↗

arXiv2014

Cosmological Coincidence without Fine Tuning

We present a simple cosmological model in which a single, non-minimally coupled scalar field with a quartic potential is responsible for both inflation at early times and acceleration at late times. Little or no fine tuning is needed to explain why the present density of dark energy is comparable to that of pressureless matter. Dark energy is identified with the potential of the scalar field, which is sourced by the trace of the energy-momentum tensor. This becomes significant when matter has decoupled from radiation and become fully non-relativistic, so that $ϕ\proptoρ_m^{1/3}\proptoρ_{m,0}^{1/3}(a_0/a)\sim(10^{-120})^{1/3}(10^{10})\sim10^{-30}$ and $V\simϕ^4\sim10^{-120}$ in Planck units, as observed.

hep-th↗

arXiv2014

Dark Energy with Logarithmic Cosmological Fluid

We propose a dark energy model with a logarithmic cosmological fluid which can result in a very small current value of the dark energy density and avoid the coincidence problem without much fine-tuning. We construct a couple of dynamical models that could realize this dark energy at very low energy in terms of four scalar fields quintessence and discuss the current acceleration of the Universe. Numerical values can be made to be consistent with the accelerating Universe with adjustment of the two parameters of the theory. The potential can be given only in terms of the scale factor, but the explicit form at very low energy can be obtained in terms of the scalar field to yield of the form V(ϕ)=\exp(-2ϕ)(\frac{4 A}{3}ϕ+B). Some discussions and the physical implications of this approach are given.

gr-qc↗

arXiv2014

Dark aspects of massive spinor electrodynamics

We investigate the cosmology of massive spinor electrodynamics when torsion is non-vanishing. A non-minimal interaction is introduced between the torsion and the vector field and the coupling constant between them plays an important role in subsequential cosmology. It is shown that the mass of the vector field and torsion conspire to generate dark energy and pressureless dark matter, and for generic values of the coupling constant, the theory effectively provides an interacting model between them with an additional energy density of the form $\sim 1/a^6$. The evolution equations mimic $Λ$CDM behavior up to $1/a^3$ term and the additional term represents a deviation from $Λ$CDM. We show that the deviation is compatible with the observational data, if it is very small. We find that the non-minimal interaction is responsible for generating an effective cosmological constant which is directly proportional to the mass squared of the vector field and the mass of the photon within its current observational limit could be the source of the dark energy.

astro-ph.CO↗

arXiv2014

Modified Brans-Dicke theory with space-time anisotropic parameters

We consider the ADM formalism of the Brans-Dicke theory and propose an anisotropic extension of the theory by introducing five free parameters. We find that the resulting theory reveals many interesting aspects which are not present in the original BD theory. We first discuss the ghost instability and strong coupling problems which are present in the gravity theory without the full diffeomorphism symmetry and show that they can be avoided in a region of the parameter space. We also perform the post-Newtonian approximation and show that the constraint of the Brans-Dicke parameter $ω_{\rm BD}$ being large to be consistent with the solar system observations could be evaded in the extended theory. We also discuss that accelerating Universe can be achieved without the need of the potential for the Brans-Dicke scalar.

hep-th↗

arXiv2013

Cosmological Perturbations of a Quartet of Scalar Fields with a Spatially Constant Gradient

We consider the linear perturbations for the single scalar field inflation model interacting with an additional triad of scalar fields. The background solutions of the three additional scalar fields depend on spatial coordinates with a constant gradient $α$ and the ensuing evolution preserves the homogeneity of the cosmological principle. After we discuss the properties of background evolution including an exact solution for the exponential-type potential, we investigate the linear perturbations of the scalar and tensor modes in the background of the slow-roll inflation. In our model with small $α$, the comoving wavenumber has {\it a lower bound} $\sim αM_{\rm P}$ to have well-defined initial quantum states. We find that cosmological quantities, for instance, the power spectrums and spectral indices of the comoving curvature and isocurvature perturbations, and the running of the spectral indices have small corrections depending on {\it the lower bound}. Similar behaviors happen for the tensor perturbation with the same lower bound.

gr-qc↗

arXiv2012

Dark spinor model with torsion and cosmology

We study cosmological consequences of the dark spinor model when torsion is included. Only some components of the torsion are allowed to be non-vanishing in homogeneous and isotropic cosmology, but there exist freedoms in the choice of these components which is consistent with the evolution equations. We exploit this and discuss several cases which can result in interesting cosmological consequences. Especially, we show that there exist exact cosmological solutions in which the Universe began its acceleration only recently and this solution is an attractor. This corresponds to a specific form of the torsion with a mild fine-tuning which can address the coincidence problem.

gr-qc↗

arXiv2012

Conformally-coupled dark spinor and FRW universe

We study conformal coupling of dark spinor fields to gravity and calculate the energy density and the pressure of the spinor in FRW spacetime. We consider the renormalizable potential of the spinor field. In the cases where the field is proportional to some power of the cosmic scale factor $a(t)$, we determine the Hubble parameter as a function of the scale factor and find analytic solutions for $a(t)$ when the spinor field matter dilutes as the universe expands. We discuss the possibility that both matter- and dark energy-dominated eras of our universe can be described by the dark spinor.

gr-qc↗

arXiv2012

Exact solution and exotic fluid in cosmology

We investigate cosmological consequences of nonlinear sigma model coupled with a cosmological fluid which satisfies the continuity equation. The target space action is of de Sitter type and is composed of four scalar fields. The potential which is a function of only one of the scalar fields is also introduced. We perform a general analysis of the ensuing cosmological equations and give various critical points and their properties. Then, we show that the model exhibits exact cosmological solution which yields a transition from matter domination into dark energy and compare it with the $Λ$CDM behavior. Especially, we calculate the age of the Universe and show that it is consistent with the observational value if the equation of the state $ω_f$ for the cosmological fluid is within the range of $0.13 < ω_f < 0.22.$ Some implication of this result is also discussed.

gr-qc↗

arXiv2012

Asymptotically Static Universe

We consider a cosmology in which the final stage of the Universe is neither accelerating nor decelerating, but approaches an asymptotic state where the scale factor becomes a constant value. In order to achieve this, we first bring in a scale factor with the desired property and then determine the details of the energy contents as a result of the cosmological evolution equations. We show that such a scenario can be realized if we introduce a generalized quintom model which consists of a scalar field and a phantom with a {\it negative} cosmological constant term. The standard cold dark matter with $w_m=0$ is also introduced. This is possible basically due to the balance between the matter and the {\it negative} cosmological constant which tend to attract and scalar field and phantom which repel in the asymptotic region. The stability analysis shows that this asymptotic solution is classically stable.

gr-qc↗