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Da-Shin Lee

Publications and source records attributed to Da-Shin Lee.

At least 55 records · Page 3Linked to original sources
arXiv2018

Vacuum fluctuations in an ancestor vacuum: A possible dark energy candidate

We consider an open universe created by bubble nucleation, and study possible effects of an "ancestor vacuum" (de Sitter space in which bubble nucleation occurred) on the present universe. We compute vacuum expectation values of energy-momentum tensor for a minimally coupled scalar field, carefully taking into account the effect of the ancestor vacuum by the Euclidean prescription. In the study of the time evolution, an important role is played by the so-called supercurvature mode, which is non-normalizable on a spatial slice of open universe and decays in time most slowly. We point out that vacuum energy of a quantum field can be regarded as dark energy if mass of the field is of order the present Hubble parameter or smaller. We obtain preliminary results for the dark energy equation of state w(z) as a function of the redshift.

hep-th↗
arXiv2017

Geodesic Motion of Neutral Particles around a Kerr-Newman Black Hole

We examine the dynamics of a neutral particle around a Kerr-Newman black hole, and in particular focus on the effects of the charge of the spinning black hole on the motion of the particle. We first consider the innermost stable circular orbits (ISCO) on the equatorial plane. It is found that the presence of the charge of the black hole leads to the effective potential of the particle with stronger repulsive effects as compared with the Kerr black hole. As a result, the radius of ISCO decreases as charge $Q$ of the black hole increases for a fixed value of black hole's angular momentum $a$. We then consider a kick on the particle from its initial orbit out of the equatorial motion. The perturbed motion of the particle will eventually be bounded, or unbounded so that it escapes to spatial infinity. Even more, the particle will likely be captured by the black hole. Thus we analytically and numerically determine the parameter regions of the corresponding motions, in terms of the initial radius of the orbital motion and the strength of the kick. The comparison will be made with the motion of a neutral particle in the Kerr black hole.

gr-qc↗
arXiv2016

A Holographic Description of Negative Energy States

Using the AdS/CFT duality, we study the expectation value of stress tensor in $2+1$-dimensional quantum critical theories with a general dynamical scaling $z$, and explore various constrains on negative energy density for strongly coupled field theories. The holographic dual theory is the theory of gravity in 3+1-dimensional Lifshitz backgrounds. We adopt a consistent approach to obtain the boundary stress tensor from bulk construction, which satisfies the trace Ward identity associated with Lifshitz scaling symmetry. In particular, the boundary stress tensor, constructed from the gravitational wave deformed Lifshitz geometry, is found up to second order in gravitational wave perturbations. {The result} is compared to its counterpart in free {scalar} field theory at the same order in an expansion of small squeezing parameters. This allows us to relate the boundary values of gravitational waves to the squeezing parameters of squeezed vacuum states. We find that, in both cases with $z=1$, the stress tensor satisfies the averaged null energy condition, and is consistent with the quantum interest conjecture. Moreover, the negative lower bound on null-contracted stress tensor, which is averaged over time-like trajectories along nearly null directions, is obtained. We find a weaker constraint on the magnitude and duration of negative null energy density in strongly coupled field theory as compared with the constraint in free relativistic field theory. The implications are discussed.

hep-th↗
arXiv2016

Subvacuum effects in Quantum Critical Theories from Holographic Approach

Subvacuum phenomena on a massive particle induced by a squeezed vacuum state of strongly coupled critical fields with a dynamical scaling $z$ are studied by employing the holographic approach. The corresponding dual description is the string moving in the 4+1-dimensional Lifshitz geometry. The squeezed vacuum state is constructed from the Bogoliubov transformations of the creation and annihilation operators of the pure vacuum state as a result from the perturbed geometry. Then the influence on particle's velocity dispersion from the squeezed vacuum is studied. With appropriate choices of squeezing parameters, the velocity dispersion is found smaller than the value caused by the normal vacuum fluctuations. This leads to a subvacuum effect. We find that the reduction in the velocity dispersion is suppressed by a large coupling constant of quantum critical fields, but is in principle observable. We then investigate the effect of the squeezed vacuum to the decoherence dynamics of a quantum particle. It is found possible for this decoherence to be below the level from the pure vacuum, rendering another subvacuum phenomenon of recoherence. We make some estimates of the degree of recoherence, and show that, in contrary to the velocity dispersion, the recoherence effect is proportional to the large coupling constant, and can potentially be observed. Finally we make a comparison with the effect on the particle influenced by a relativistic free field with the dynamical scaling $z=1$.

hep-th↗
arXiv2015

Quantum stochastic behaviour in cold Fermi gases: Phonon propagation

We examine the effect of quantum fluctuations in a tunable cold Fermi gas on the propagation of phonons. We show that these fluctuations can be interpreted as inducing a stochastic (acoustic) space-time. The variation in times of flight induced by this stochastic behaviour can be significant in the transition region between BEC and BCS regimes.

cond-mat.quant-gas↗
arXiv2014

Holographic influence functional and its application to decoherence induced by quantum critical theories

The dynamics of a particle influenced by strongly-coupled quantum critical theories is studied by the holographic approach. A real-time prescription for the AdS/CFT correspondence in the context of nonequilibrium physics is proposed from mainly the field theoretic consideration, and the associated holographic influence functional is obtained. We then study the decoherence dynamics of a particle induced by the quantum critical theories with a dynamical exponent $z$. We find that as $z$ increases, the decoherence effect becomes less significant. The similar behavior is found as we heat up the environment. However, in this case the decoherence effect is enhanced not only by its strong coupling constant but also from the finite temperature effect. Finally a comparison is made with the result of a particle influenced by the free field.

hep-th↗
arXiv2014

Holographic Approach to Nonequilibrium Dynamics of Moving Mirrors Coupled to Quantum Critical Theories

We employ the holographic method to study fluctuations and dissipation of an $n$-dimensional moving mirror coupled to quantum critical theories in $d$ spacetime dimensions. The bulk counterpart of the mirror with perfect reflection is a D$(n+1)$ brane in the Lifshitz geometry of $d+1$ dimensions. The motion of the mirror can be realized from the dynamics of the brane at the boundary of the bulk. The excited modes of the brane in the bulk render the mirror undergoing Brownian motion. For small displacement of the mirror, we derive the analytical results of the correlation functions and response functions. The dynamics of the mirror due to small fluctuations around the brane vacuum state in the bulk is found supraohmic so that after initial growth, the velocity fluctuations approach a saturated value at late time with a power-law behavior. On the contrary, in the Lifshitz black hole background, the mirror in thermal fluctuations shows that its relaxation dynamics becomes ohmic, and the saturation of velocity fluctuations is reached exponentially in time. Finally a comparison is made with the result of a moving mirror driven by free fields.

hep-th↗
arXiv2013

The role of Causality in Tunable Fermi Gas Condensates

We develop a new formalism for the description of the condensates of cold Fermi atoms whose speed of sound can be tuned with the aid of a narrow Feshbach resonance. We use this to look for spontaneous phonon creation that mimics spontaneous particle creation in curved space-time in Friedmann-Robertson-Walker and other model universes.

gr-qc↗
arXiv2013

Macroscopic Quantum Mechanics in a Classical Spacetime

We apply the many-particle Schrödinger-Newton equation, which describes the co-evolution of an many-particle quantum wave function and a classical space-time geometry, to macroscopic mechanical objects. By averaging over motions of the objects' internal degrees of freedom, we obtain an effective Schrödinger-Newton equation for their centers of mass, which are degrees of freedom that can be monitored and manipulated at the quantum mechanical levels by state-of-the-art optoemchanics experiments. For a single macroscopic object moving quantum mechanically within a harmonic potential well, we found that its quantum uncertainty evolves in a different frequency from its classical eigenfrequency --- with a difference that depends on the internal structure of the object, and can be observable using current technology. For several objects, the Schrödinger-Newton equation predicts semiclassical motions just like Newtonian physics, yet they do not allow quantum uncertainty to be transferred from one object to another through gravity.

gr-qc↗
arXiv2012

Nonequilibrium Damping of Collective Motion of Homogeneous Cold Fermi Condensates with Feshbach Resonances

Collisionless damping of a condensate of cold Fermi atoms, whose scattering is controlled by a Feshbach resonance, is explored throughout the BCS and BEC regimes when small perturbations on its phase and amplitude modes are turned on to drive the system slightly out of equilibrium. Using a one-loop effective action, we first recreate the known result that for a broad resonance the amplitude of the condensate decays as $t^{-1/2}$ at late times in the BCS regime whereas it decays as $t^{-3/2}$ in the BEC regime. We then examine the case of an idealized narrow resonance, and find that this collective mode decays as $t^{-3/2}$ throughout both the BCS and BEC regimes. Although this seems to contradict earlier results that damping is identical for both broad and narrow resonances, the breakdown of the narrow resonance limit restores this universal behaviour. More measureably, the phase perturbation may give a shift on the saturated value to which the collective amplitude mode decays, which vanishes only in the deep BCS regime when the phase and amplitude modes are decoupled.

cond-mat.quant-gas↗
arXiv2011

Quantum noise in the mirror-field system: A field theoretic approach

We employ the field theoretic approach to study the quantum noise problem in the mirror-field system, where a perfectly reflecting mirror is illuminated by a single-mode coherent state of the massless scalar field. The associated radiation pressure is described by a surface integral of the stress-tensor of the field. The read-out field is measured by a monople detector, form which the effective distance between the detector and mirror can be obtained. In the slow-motion limit of the mirror, we are able to identify various sources of quantum noise that lead to uncertainty of the read-out measurement. Since the mirror is driven by radiation pressure, the sources of noise, other than the shot nose given by the intrinsic fluctuations of the incident state, may also result from random motion of mirror due to radiation pressure fluctuations and from modified field fluctuations induced by the displacement of the mirror. Correlation between different sources of noise can be established as the consequence of interference between the incident field and the reflected field out of the mirror in the read-out measurement. The overall uncertainty is found to decrease (increase) due to the negative (positive) correlation. In the case of negative correlation, the uncertainty can be lowered than the value predicted by the standard quantum limit. We also examine the validity of the particle number approach, which is often used in quantum optics, and compared its results with those given by the field theoretical approach. Finally we discuss the backreaction effects, induced by the radiation pressure, that alter the dynamics of the mean displacement of the mirror, and we argue this backreaction can be ignored for a slowly moving mirror.

quant-ph↗
arXiv2011

Subvacuum effects of the quantum field on the dynamics of a test particle

We study the effects of the electromagnetic subvacuum fluctuations on the dynamics of a nonrelativistic charged particle in a wavepacket. The influence from the quantum field is expected to give an additional effect to the velocity uncertainty of the particle. In the case of a static wavepacket, the observed velocity dispersion is smaller in the electromagnetic squeezed vacuum background than in the normal vacuum background. This leads to the subvacuum effect. The extent of reduction in velocity dispersion associated with this subvacuum effect is further studied by introducing a switching function. It is shown that the slow switching process may make this subvacuum effect insignificant. We also point out that when the center of the wavepacket undergoes non-inertial motion, reduction in the velocity dispersion becomes less effective with its evolution, no matter how we manipulate the nonstationary quantum noise via the choice of the squeeze parameters. The role of the underlying fluctuation-dissipation relation is discussed.

quant-ph↗
arXiv2011

Photon production from non-equilibrium disoriented chiral condensates in a longitudinal expansion : A theoretic framework

A theoretical framework is developed for treating the quantization of the photons in a spacetime with a longitudinal expansion. This can be used to study the production of the photons through the non-equilibrium relaxation of a disoriented chiral condensate presumably formed in the expanding hot central region in ultra-relativistic heavy-ion collisions. These photons can be a signature of the formation of disoriented chiral condensates in the direct photon measurements of heavy-ion collisions.

hep-ph↗
arXiv2010

Quantum Modifications to Gravity Waves in de Sitter Spacetime

We treat a model in which tensor perturbations of de~Sitter spacetime, represented as a spatially flat model, are modified by the effects of the vacuum fluctuations of a massless conformally invariant field, such as the electromagnetic field. We use the semiclassical theory of gravity with the expectation value of the conformal field stress tensor as a source. We first study the stability of de~Sitter spacetime by searching for growing, spatially homogeneous modes, and conclude that it is stable within the limits of validity of the semiclassical theory. We next examine the modification of linearized plane gravity waves by the effects of the quantum stress tensor. We find a correction term which is of the same form as the original wave, but displaced in phase by -π/2, and with an amplitude which depends upon the duration of inflation. The magnitude of this effect is proportional to the change in scale factor during inflation. So long as the energy scale of inflation and the proper frequency of the mode at the beginning of inflation are well below the Planck scale, the fractional correction is small. However, modes which are transplanckian at the onset of inflation can undergo a significant correction. The increase in amplitude can potentially have observable consequences through a modification of the power spectrum of tensor perturbations in inflationary cosmology. This enhancement of the power spectrum depends upon the duration of inflation and is greater for shorter wavelengths.

gr-qc↗
arXiv2009

Brownian motion of a charged particle in electromagnetic fluctuations at finite temperature

The fluctuation-dissipation theorem is a central theorem in nonequilibrium statistical mechanics by which the evolution of velocity fluctuations of the Brownian particle under a fluctuating environment is intimately related to its dissipative behavior. This can be illuminated in particular by an example of Brownian motion in an ohmic environment where the dissipative effect can be accounted for by the first-order time derivative of the position. Here we explore the dynamics of the Brownian particle coupled to a supraohmic environment by considering the motion of a charged particle interacting with the electromagnetic fluctuations at finite temperature. We also derive particle's equation of motion, the Langevin equation, by minimizing the corresponding stochastic effective action, which is obtained with the method of Feynman-Vernon influence functional. The fluctuation-dissipation theorem is established from first principles. The backreaction on the charge is known in terms of electromagnetic self-force given by a third-order time derivative of the position, leading to the supraohmic dynamics. This self-force can be argued to be insignificant throughout the evolution when the charge barely moves. The stochastic force arising from the supraohmic environment is found to have both positive and negative correlations, and it drives the charge into a fluctuating motion. Although positive force correlations give rise to the growth of the velocity dispersion initially, its growth slows down when correlation turns negative, and finally halts, thus leading to the saturation of the velocity dispersion. The saturation mechanism in a suparohmic environment is found to be distinctly different from that in an ohmic environment. The comparison is discussed.

cond-mat.stat-mech↗
arXiv2009

The Non-universal behaviour of Cold Fermi Condensates with Narrow Feshbach Resonances

In this paper we construct an effective field theory for a condensate of cold Fermi atoms whose scattering is controlled by a narrow Feshbach resonance. We show how, from first principles, it permits a hydrodynamic description of the BEC-BCS crossover from which the equation of state, intimately related to the speed of sound, can be derived. Specifically, we stress the non-universal behaviour of the equation of state at the unitary limit of infinite scattering length that arises when either, or both, of the range of the inter-atomic force and the scale of the molecular field become large.

cond-mat.supr-con↗
arXiv2009

Affleck-Dine Baryogenesis, Split Supersymmetry, and Inflation

It is shown that, in the context of split supersymmetry, a simple model with a single complex scalar field can produce chaotic inflation and generate the observed amount of baryon asymmetry via the Affleck-Dine mechanism. While the inflaton quantum fluctuations give rise to curvature perturbation, we show that quantum fluctuations of the phase of the scalar field can produce baryonic isocurvature perturbation. Combining with constraints from WMAP data, all parameters in the model can be determined to within a narrow range.

hep-ph↗