arXiv subjects

Sang Pyo Kim

Publications and source records attributed to Sang Pyo Kim.

At least 127 records · Page 7Linked to original sources

arXiv2005

Nonlinear Evolutions and Non-Gaussianity in Generalized Gravity

We use the Hamilton-Jacobi theory to study the nonlinear evolutions of inhomogeneous spacetimes during inflation in generalized gravity. We find the exact solutions to the lowest order Hamilton-Jacobi equation for special scalar potentials and introduce an approximation method for general potentials. The conserved quantity invariant under a change of timelike hypersurfaces proves useful in dealing with gravitational perturbations. In the long-wavelength approximation, we find a conserved quantity related to the new canonical variable that makes the Hamiltonian density vanish, and calculate the non-Gaussianity in generalized gravity. The slow-roll inflation models with a single scalar field in generalized gravity predict too small non-Gaussianity to be detected by future CMB experiments.

astro-ph↗

arXiv2005

First Law of Thermodynamics and Friedmann Equations of Friedmann-Robertson-Walker Universe

Applying the first law of thermodynamics to the apparent horizon of a Friedmann-Robertson-Walker universe and assuming the geometric entropy given by a quarter of the apparent horizon area, we derive the Friedmann equations describing the dynamics of the universe with any spatial curvature. Using entropy formulae for the static spherically symmetric black hole horizons in Gauss-Bonnet gravity and in more general Lovelock gravity, where the entropy is not proportional to the horizon area, we are also able to obtain the Friedmann equations in each gravity theory. We also discuss some physical implications of our results.

hep-th↗

arXiv2004

Constraint on the Squeeze Parameter of Inflaton from Cosmological Constant

The inflaton is highly likely to settle in a squeezed vacuum state after inflation. The relic inflaton after inflation and reheating undergoes a damped oscillatory motion and contributes to the effective cosmological constant. We interpret the renormalized energy density from the squeezed vacuum state as an effective cosmological constant. Using the recent observational data on the cosmological constant, we find the constraint on the squeeze parameter of the inflaton in the early universe.

gr-qc↗

arXiv2004

Nonequilibrium Extension of the Landau-Lifshitz-Gilbert Equation for Magnetic Systems

Using the invariant operator method for an effective Hamiltonian including the radiation-spin interaction, we describe the quantum theory for magnetization dynamics when the spin system evolves nonadiabatically and out of equilibrium, $d \hatρ/dt \neq 0$. It is shown that the vector parameter of the invariant operator and the magnetization defined with respect to the density operator, both satisfying the quantum Liouville equation, still obey the Landau-Lifshitz-Gilbert equation.

cond-mat.other↗

arXiv2004

Quantum Cosmology for Tunneling Universes

In a quantum cosmological model consisting of a Euclidean region and a Lorentzian region, Hartle-Hawking's no-bounary wave function, and Linde's wave function and Vilenkin's tunneling wave function are briefly described and compared with each other. We put a particular emphasis on semiclassical gravity from quantum cosmology and compare it with the conventional quantum field theory in curved spacetimes. Finally, we discuss the recent debate on catastrophic particle production in the tunneling universe between Rubakov and Vilenkin within the semiclassical gravity.

gr-qc↗

arXiv2004

Magnetic Black Holes Are Also Unstable

Most black holes are known to be unstable to emitting Hawking radiation (in asymptotically flat spacetime). If the black holes are non-extreme, they have positive temperature and emit thermally. If they are extremal rotating black holes, they still spontaneously emit particles like gravitons and photons. If they are extremal electrically charged black holes, they are unstable to emitting electrons or positrons. The only exception would be extreme magnetically charged black holes if there do not exist any magnetic monopoles for them to emit. However, here we show that even in this case, vacuum polarization causes all magnetic black holes to be unstable to emitting smaller magnetic black holes.

gr-qc↗

arXiv2004

Gravitational Wave Spectrum in Inflation with Nonclassical States

The initial quantum state during inflation may evolve to a highly squeezed quantum state due to the amplification of the time-dependent parameter, $ω_{phys}(k/a)$, which may be the modified dispersion relation in trans-Planckian physics. This squeezed quantum state is a nonclassical state that has no counterpart in the classical theory. We have considered the nonclassical states such as squeezed, squeezed coherent, and squeezed thermal states, and calculated the power spectrum of the gravitational wave perturbation when the mode leaves the horizon.

gr-qc↗

arXiv2004

Dynamical Theory of Disoriented Chiral Condensates at QCD Phase Transition

We apply the canonical quantum field theory based on the Liouville-von Neumann equation to the nonequilibrium linear sigma model. Particular emphasis is put on the mechanism for domain growth of disoriented chiral condensates due to long wavelength modes and its scaling behavior. Scattering effects, decoherence and emergence of order parameter are also discussed beyond the Hartree approximation.

hep-ph↗

arXiv2004

Remarks on Schwinger Pair Production by Charged Black Holes

We introduce a canonical method for pair production by electromagnetic fields. The canonical method in the space-dependent gauge provides pair-production rate even for inhomogeneous fields. Further, the instanton action including all corrections leads to an accurate formula for the pair-production rate. We discuss various aspects of the canonical method and clarify terminology for pair production. We study pair production by charged black holes first by finding states of the field equation that describe pair production and then by applying the canonical method.

gr-qc↗

arXiv2004

Dynamical Theory of Phase Transitions and Topological Defect Formation in the Early Universe

We review the current issues of nonequilibrium phase transitions, in particular, in the early universe. Phase transitions cannot maintain thermal equilibrium and become nonequilibrium when the thermal relaxation time scale is greater than the dynamical time scale. Nonequilibrium phase transitions would have happened in certain evolution stages of the early universe because the rapid expansion quenched matter fields. We apply the recently introduced Liouville-von Neumann method, another canonical method, to nonequilibrium phase transitions in the Minkowski spacetime and find the scaling behavior of domain sizes. Topological defects are thus determined by the dynamical processes of nonequilibrium phase transitions. However, the expansion of the universe freezes domain growth in the comoving frame and thus leads to a scale-invariant domain size. We also discuss the physical implications of nonequilibrium phase transitions in the early universe.

hep-ph↗

arXiv2003

Time-dependent Displaced and Squeezed Number States

We generalize the wave functions of the displaced and squeezed number states, found by Nieto, to a time-dependent harmonic oscillator with variable mass and frequency. These time-dependent displaced and squeezed number states are obtained by first squeezing and then displacing the exact number states and are exact solutions of the Schrödinger equation. Further, these wave functions are the time-dependent squeezed harmonic-oscillator wave functions centered at classical trajectories.

quant-ph↗

arXiv2003

Squeezed States of the Generalized Minimum Uncertainty State for the Caldirola-Kanai Hamiltonian

We show that the ground state of the well-known pseudo-stationary states for the Caldirola-Kanai Hamiltonian is a generalized minimum uncertainty state, which has the minimum allowed uncertainty $Δq Δp = \hbar σ_0/2$, where $σ_0 (\geq 1)$ is a constant depending on the damping factor and natural frequency. The most general symmetric Gaussian states are obtained as the one-parameter squeezed states of the pseudo-stationary ground state. It is further shown that the coherent states of the pseudo-stationary ground state constitute another class of the generalized minimum uncertainty states.

quant-ph↗

arXiv2003

Nonequilibrium Evolution of Correlation Functions: A Canonical Approach

We study nonequilibrium evolution in a self-interacting quantum field theory invariant under space translation only by using a canonical approach based on the recently developed Liouville-von Neumann formalism. The method is first used to obtain the correlation functions both in and beyond the Hartree approximation, for the quantum mechanical analog of the $ϕ^{4}$ model. The technique involves representing the Hamiltonian in a Fock basis of annihilation and creation operators. By separating it into a solvable Gaussian part involving quadratic terms and a perturbation of quartic terms, it is possible to find the improved vacuum state to any desired order. The correlation functions for the field theory are then investigated in the Hartree approximation and those beyond the Hartree approximation are obtained by finding the improved vacuum state corrected up to ${\cal O}(λ^2)$. These correlation functions take into account next-to-leading and next-to-next-to-leading order effects in the coupling constant. We also use the Heisenberg formalism to obtain the time evolution equations for the equal-time, connected correlation functions beyond the leading order. These equations are derived by including the connected 4-point functions in the hierarchy. The resulting coupled set of equations form a part of infinite hierarchy of coupled equations relating the various connected n-point functions. The connection with other approaches based on the path integral formalism is established and the physical implications of the set of equations are discussed with particular emphasis on thermalization.

hep-ph↗

arXiv2003

Thermofield Dynamics of Time-Dependent Boson and Fermion Systems

We formulate the thermofield dynamics for time-dependent systems by combining the Liouville-von Neumann equation, its invariant operators, and the basic notions of thermofield dynamics. The new formulation is applied to time-dependent bosons and fermions by using the time-dependent annihilation and creation operators that satisfy the Liouville-von Neumann equation. It is shown that the thermal state is the time- and temperature-dependent vacuum state and a general formula is derived to calculate the thermal expectation value of operators.

quant-ph↗

arXiv2003

Invariant Operators vs Heisenberg Operators for Time-Dependent Generalized Oscillators

We investigate the relation between the invariant operators satisfying the quantum Liouville-von Neumann and the Heisenberg operators satisfying the Heisenberg equation. For time-dependent generalized oscillators we find the invariant operators, known as the Ermakov-Lewis invariants, in terms of a complex classical solution, from which the evolution operator is derived, and obtain the Heisenberg position and momentum operators. Physical quantities such as correlation functions are calculated using both the invariant operators and Heisenberg operators.

quant-ph↗

arXiv2002

Exact Quantum-Statistical Dynamics of Time-Dependent Generalized Oscillators

Using linear invariant operators in a constructive way we find the most general thermal density operator and Wigner function for time-dependent generalized oscillators. The general Wigner function has five free parameters and describes the thermal Wigner function about a classical trajectory in phase space. The contour of the Wigner function depicts an elliptical orbit with a constant area moving about the classical trajectory, whose eccentricity determines the squeezing of the initial vacuum.

quant-ph↗

arXiv2002

Schwinger Pair Production via Instantons in Strong Electric Fields

In the space-dependent gauge, each mode of the Klein-Gordon equation in a strong electric field takes the form of a time-independent Schrödinger equation with a potential barrier. We propose that the single- and multi-instantons of quantum tunneling may be related with the single- and multi-pair production of bosons and the relative probability for the no-pair production is determined by the total tunneling probability via instantons. In the case of a uniform electric field, the instanton interpretation recovers exactly the well-known pair production rate for bosons and when the Pauli blocking is taken into account, it gives the correct fermion production rate. The instanton is used to calculate the pair production rate even in an inhomogeneous electric field. Furthermore, the instanton interpretation confirms the fact that bosons and fermions can not be produced by a static magnetic field only.

hep-th↗

arXiv2002

Decoherence of Quantum Damped Oscillators

Quantum dissipation is studied within two model oscillators, the Caldirola-Kanai (CK) oscillator as an open system with one degree of freedom and the Bateman-Feshbach-Tikochinsky (BFT) oscillator as a closed system with two degrees of freedom. Though these oscillators describe the same classical damped motion, the CK oscillator retains the quantum coherence, whereas the damped subsystem of the BFT oscillator exhibits both quantum decoherence and classical correlation. Furthermore the amplified subsystem of the BFT oscillator shows the same degree of quantum decohernce and classical correlation.

quant-ph↗