83 research outputs found
Particle creation, classicality and related issues in quantum field theory: II. Examples from field theory
We adopt the general formalism, which was developed in Paper I
(arXiv:0708.1233) to analyze the evolution of a quantized time-dependent
oscillator, to address several questions in the context of quantum field theory
in time dependent external backgrounds. In particular, we study the question of
emergence of classicality in terms of the phase space evolution and its
relation to particle production, and clarify some conceptual issues. We
consider a quantized scalar field evolving in a constant electric field and in
FRW spacetimes which illustrate the two extreme cases of late time adiabatic
and highly non-adiabatic evolution. Using the time-dependent generalizations of
various quantities like particle number density, effective Lagrangian etc.
introduced in Paper I, we contrast the evolution in these two limits bringing
out key differences between the Schwinger effect and evolution in the de Sitter
background. Further, our examples suggest that the notion of classicality is
multifaceted and any one single criterion may not have universal applicability.
For example, the peaking of the phase space Wigner distribution on the
classical trajectory \emph{alone} does not imply transition to classical
behavior. An analysis of the behavior of the \emph{classicality parameter},
which was introduced in Paper I, leads to the conclusion that strong particle
production is necessary for the quantum state to become highly correlated in
phase space at late times.Comment: RevTeX 4; 27 pages; 18 figures; second of a series of two papers, the
first being arXiv:0708.1233 [gr-qc]; high resolution figures available from
the authors on reques
Particle creation, classicality and related issues in quantum field theory: I. Formalism and toy models
The quantum theory of a harmonic oscillator with a time dependent frequency
arises in several important physical problems, especially in the study of
quantum field theory in an external background. While the mathematics of this
system is straightforward, several conceptual issues arise in such a study. We
present a general formalism to address some of the conceptual issues like the
emergence of classicality, definition of particle content, back reaction etc.
In particular, we parametrize the wave function in terms of a complex number
(which we call excitation parameter) and express all physically relevant
quantities in terms it. Many of the notions -- like those of particle number
density, effective Lagrangian etc., which are usually defined using asymptotic
in-out states -- are generalized as time-dependent concepts and we show that
these generalized definitions lead to useful and reasonable results. Having
developed the general formalism we apply it to several examples. Exact analytic
expressions are found for a particular toy model and approximate analytic
solutions are obtained in the extreme cases of adiabatic and highly
non-adiabatic evolution. We then work out the exact results numerically for a
variety of models and compare them with the analytic results and
approximations. The formalism is useful in addressing the question of emergence
of classicality of the quantum state, its relation to particle production and
to clarify several conceptual issues related to this. In Paper II
(arXiv:0708.1237), which is a sequel to this, the formalism will be applied to
analyze the corresponding issues in the context of quantum field theory in
background cosmological models and electric fields.Comment: RevTeX 4; 32 pages; 28 figures; first of a series of two papers, the
second being arXiv:0708.1237 [gr-qc]; high resolution figures available from
the authors on reques
Dark Energy and Gravity
I review the problem of dark energy focusing on the cosmological constant as
the candidate and discuss its implications for the nature of gravity. Part 1
briefly overviews the currently popular `concordance cosmology' and summarises
the evidence for dark energy. It also provides the observational and
theoretical arguments in favour of the cosmological constant as the candidate
and emphasises why no other approach really solves the conceptual problems
usually attributed to the cosmological constant. Part 2 describes some of the
approaches to understand the nature of the cosmological constant and attempts
to extract the key ingredients which must be present in any viable solution. I
argue that (i)the cosmological constant problem cannot be satisfactorily solved
until gravitational action is made invariant under the shift of the matter
lagrangian by a constant and (ii) this cannot happen if the metric is the
dynamical variable. Hence the cosmological constant problem essentially has to
do with our (mis)understanding of the nature of gravity. Part 3 discusses an
alternative perspective on gravity in which the action is explicitly invariant
under the above transformation. Extremizing this action leads to an equation
determining the background geometry which gives Einstein's theory at the lowest
order with Lanczos-Lovelock type corrections. (Condensed abstract).Comment: Invited Review for a special Gen.Rel.Grav. issue on Dark Energy,
edited by G.F.R.Ellis, R.Maartens and H.Nicolai; revtex; 22 pages; 2 figure
The backward-bending commute times of married women with household responsibility
The purpose of this paper is to examine theoretically and empirically whether the commute times of married women follow a backward-bending pattern with respect to wage rates. The existing literature has shown that married women tend to choose short commutes because of their relatively low wages combined with comparatively heavy household responsibilities. However, a workleisure model, which includes the simultaneous decision wives take regarding commute times and wage rates, suggests that married women employed in highly paid positions also undertake short commutes, while married women with wage rates in the middle range choose long commutes. These results suggest that the commute times of married women display a backward-bending pattern. Applying an instrumental variable strategy that accounts for the endogeneity of wage rates, the empirical results for employed married women in Japan appear to support this nding. Moreover, one of our results suggests that highly paid married women can still secure greater leisure time with short commutes, despite retaining a heavy load of domestic responsibilities.Working Paper, No.234, 2008.9.1çhttp://hdl.handle.net/10110/254
Packet Forwarding Algorithms in a Line Network
Abstract. We initiate a competitive analysis of packet forwarding poli-cies for maximum and average flow in a line network. We show that the policies Earliest Arrival and Furthest-To-Go are scalable, but not con-stant competitive, for maximum flow. We show that there is no constant competitive algorithm for average flow.
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 Schr\"{o}dinger
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.Comment: RevTex 7 Pages, No figure; Formulae for the production rate in very
strong fields and references added; the final version accepted in Phys. Rev.
Unruh--DeWitt detectors in spherically symmetric dynamical space-times
In the present paper, Unruh--DeWitt detectors are used in order to
investigate the issue of temperature associated with a spherically symmetric
dynamical space-times. Firstly, we review the semi-classical tunneling method,
then we introduce the Unruh--DeWitt detector approach. We show that for the
generic static black hole case and the FRW de Sitter case, making use of
peculiar Kodama trajectories, semiclassical and quantum field theoretic
techniques give the same standard and well known thermal interpretation, with
an associated temperature, corrected by appropriate Tolman factors. For a FRW
space-time interpolating de Sitter space with the Einstein--de Sitter universe
(that is a more realistic situation in the frame of CDM cosmologies),
we show that the detector response splits into a de Sitter contribution plus a
fluctuating term containing no trace of Boltzmann-like factors, but rather
describing the way thermal equilibrium is reached in the late time limit. As a
consequence, and unlike the case of black holes, the identification of the
dynamical surface gravity of a cosmological trapping horizon as an effective
temperature parameter seems lost, at least for our co-moving simplified
detectors. The possibility remains that a detector performing a proper motion
along a Kodama trajectory may register something more, in which case the
horizon surface gravity would be associated more likely to vacuum correlations
than to particle creation.Comment: 19 pages, to appear on IJTP. arXiv admin note: substantial text
overlap with arXiv:1101.525
Fine-mapping of prostate cancer susceptibility loci in a large meta-analysis identifies candidate causal variants
Prostate cancer is a polygenic disease with a large heritable component. A number of common, low-penetrance prostate cancer risk loci have been identified through GWAS. Here we apply the Bayesian multivariate variable selection algorithm JAM to fine-map 84 prostate cancer susceptibility loci, using summary data from a large European ancestry meta-analysis. We observe evidence for multiple independent signals at 12 regions and 99 risk signals overall. Only 15 original GWAS tag SNPs remain among the catalogue of candidate variants identified; the remainder are replaced by more likely candidates. Biological annotation of our credible set of variants indicates significant enrichment within promoter and enhancer elements, and transcription factor-binding sites, including AR, ERG and FOXA1. In 40 regions at least one variant is colocalised with an eQTL in prostate cancer tissue. The refined set of candidate variants substantially increase the proportion of familial relative risk explained by these known susceptibility regions, which highlights the importance of fine-mapping studies and has implications for clinical risk profiling. Š 2018 The Author(s).Prostate cancer is a polygenic disease with a large heritable component. A number of common, low-penetrance prostate cancer risk loci have been identified through GWAS. Here we apply the Bayesian multivariate variable selection algorithm JAM to fine-map 84 prostate cancer susceptibility loci, using summary data from a large European ancestry meta-analysis. We observe evidence for multiple independent signals at 12 regions and 99 risk signals overall. Only 15 original GWAS tag SNPs remain among the catalogue of candidate variants identified; the remainder are replaced by more likely candidates. Biological annotation of our credible set of variants indicates significant enrichment within promoter and enhancer elements, and transcription factor-binding sites, including AR, ERG and FOXA1. In 40 regions at least one variant is colocalised with an eQTL in prostate cancer tissue. The refined set of candidate variants substantially increase the proportion of familial relative risk explained by these known susceptibility regions, which highlights the importance of fine-mapping studies and has implications for clinical risk profiling. Š 2018 The Author(s).Peer reviewe
Track D Social Science, Human Rights and Political Science
Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/138414/1/jia218442.pd
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