2,465 research outputs found
Hawking radiation in different coordinate settings: Complex paths approach
We apply the technique of complex paths to obtain Hawking radiation in
different coordinate representations of the Schwarzschild space-time. The
coordinate representations we consider do not possess a singularity at the
horizon unlike the standard Schwarzschild coordinate. However, the event
horizon manifests itself as a singularity in the expression for the
semiclassical action. This singularity is regularized by using the method of
complex paths and we find that Hawking radiation is recovered in these
coordinates indicating the covariance of Hawking radiation as far as these
coordinates are concerned.Comment: 18 pages, 2 figures, Uses IOP style file; final version; accepted in
Class. Quant. Gra
Response of finite-time particle detectors in non-inertial frames and curved spacetime
The response of the Unruh-DeWitt type monopole detectors which were coupled
to the quantum field only for a finite proper time interval is studied for
inertial and accelerated trajectories, in the Minkowski vacuum in (3+1)
dimensions. Such a detector will respond even while on an inertial trajctory
due to the transient effects. Further the response will also depend on the
manner in which the detector is switched on and off. We consider the response
in the case of smooth as well as abrupt switching of the detector. The former
case is achieved with the aid of smooth window functions whose width, ,
determines the effective time scale for which the detector is coupled to the
field. We obtain a general formula for the response of the detector when a
window function is specified, and work out the response in detail for the case
of gaussian and exponential window functions. A detailed discussion of both and limits are given and several
subtlities in the limiting procedure are clarified. The analysis is extended
for detector responses in Schwarzschild and de-Sitter spacetimes in (1+1)
dimensions.Comment: 29 pages, normal TeX, figures appended as postscript file, IUCAA
Preprint # 23/9
Observational constraints on low redshift evolution of dark energy: How consistent are different observations?
The dark energy component of the universe is often interpreted either in
terms of a cosmological constant or as a scalar field. A generic feature of the
scalar field models is that the equation of state parameter w= P/rho for the
dark energy need not satisfy w=-1 and, in general, it can be a function of
time. Using the Markov chain Monte Carlo method we perform a critical analysis
of the cosmological parameter space, allowing for a varying w. We use
constraints on w(z) from the observations of high redshift supernovae (SN), the
WMAP observations of CMB anisotropies and abundance of rich clusters of
galaxies. For models with a constant w, the LCDM model is allowed with a
probability of about 6% by the SN observations while it is allowed with a
probability of 98.9% by WMAP observations. The LCDM model is allowed even
within the context of models with variable w: WMAP observations allow it with a
probability of 99.1% whereas SN data allows it with 23% probability. The SN
data, on its own, favors phantom like equation of state (w<-1) and high values
for Omega_NR. It does not distinguish between constant w (with w<-1) models and
those with varying w(z) in a statistically significant manner. The SN data
allows a very wide range for variation of dark energy density, e.g., a
variation by factor ten in the dark energy density between z=0 and z=1 is
allowed at 95% confidence level. WMAP observations provide a better constraint
and the corresponding allowed variation is less than a factor of three.
Allowing for variation in w has an impact on the values for other cosmological
parameters in that the allowed range often becomes larger. (Abridged)Comment: 21 pages, PRD format (Revtex 4), postscript figures. minor
corrections to improve clarity; references, acknowledgement adde
The hypothesis of path integral duality II: corrections to quantum field theoretic results
In the path integral expression for a Feynman propagator of a spinless
particle of mass , the path integral amplitude for a path of proper length
connecting events and in a spacetime
described by the metric tensor is . In a recent paper, assuming the path integral amplitude to be
invariant under the duality transformation ,
Padmanabhan has evaluated the modified Feynman propagator in an arbitrary
curved spacetime. He finds that the essential feature of this `principle of
path integral duality' is that the Euclidean proper distance
between two infinitesimally separated spacetime events is replaced by . In other words, under the duality principle the spacetime
behaves as though it has a `zero-point length' , a feature that is
expected to arise in a quantum theory of gravity. In the Schwinger's proper
time description of the Feynman propagator, the weightage factor for a path
with a proper time is . Invoking Padmanabhan's `principle of
path integral duality' corresponds to modifying the weightage factor
to . In this paper, we use this modified
weightage factor in Schwinger's proper time formalism to evaluate the quantum
gravitational corrections to some of the standard quantum field theoretic
results in flat and curved spacetimes. We find that the extra factor
acts as a regulator at the Planck scale thereby `removing' the
divergences that otherwise appear in the theory. Finally, we discuss the wider
implications of our analysis.Comment: 26 pages, Revte
Valley polarization and susceptibility of composite fermions around nu=3/2
We report magnetotransport measurements of fractional quantum Hall states in
an AlAs quantum well around Landau level filling factor nu = 3/2, demonstrating
that the quasiparticles are composite Fermions (CFs) with a valley degree of
freedom. By monitoring the valley level crossings for these states as a
function of applied symmetry-breaking strain, we determine the CF valley
susceptibility and polarization. The data can be explained well by a simple
Landau level fan diagram for CFs, and are in nearly quantitative agreement with
the results reported for CF spin polarization.Comment: to appear in Phys. Rev. Let
The structure of dark matter halos in hierarchical clustering theories
During hierarchical clustering, smaller masses generally collapse earlier
than larger masses and so are denser on the average. The core of a small mass
halo could be dense enough to resist disruption and survive undigested, when it
is incorporated into a bigger object. We explore the possibility that a nested
sequence of undigested cores in the center of the halo, which have survived the
hierarchical, inhomogeneous collapse to form larger and larger objects,
determines the halo structure in the inner regions. For a flat universe with
, scaling arguments then suggest that the core density
profile is, with . But
whether such behaviour obtains depends on detailed dynamics. We first examine
the dynamics using a fluid approach to the self-similar collapse solutions for
the dark matter phase space density, including the effect of velocity
dispersions. We highlight the importance of tangential velocity dispersions to
obtain density profiles shallower than in the core regions. If
tangential velocity dispersions in the core are constrained to be less than the
radial dispersion, a cuspy core density profile shallower than 1/r cannot
obtain, in self-similar collapse. We then briefly look at the profiles of the
outer halos in low density cosmological models where the total halo mass is
convergent. Finally, we analyze a suite of dark halo density and velocity
dispersion profiles obtained in cosmological N-body simulations of models with
n= 0, -1 and -2. We find that the core-density profiles of dark halos, show
considerable scatter in their properties, but nevertheless do appear to reflect
a memory of the initial power spectrum, with steeper initial spectra producing
flatter core profiles. (Abridged)Comment: 31 pages, 7 figures, submitted to Ap
Method of complex paths and general covariance of Hawking radiation
We apply the technique of complex paths to obtain Hawking radiation in
different coordinate representations of the Schwarzschild space-time. The
coordinate representations we consider do not possess a singularity at the
horizon unlike the standard Schwarzschild coordinate. However, the event
horizon manifests itself as a singularity in the expression for the
semi-classical action. This singularity is regularized by using the method of
complex paths and we find that Hawking radiation is recovered in these
coordinates indicating the covariance of Hawking radiation. This also shows
that there is no correspondence between the particles detected by the model
detector and the particle spectrum obtained by the quantum field theoretic
analysis -- a result known in other contexts as well.Comment: 9 pages, uses MPLA Style file, Accepted for publication in Mod. Phys.
Letts.
Parallel Magnetic Field Tuning of Valley Splitting in AlAs Two-Dimensional Electrons
We demonstrate that, in a quasi-two-dimensional electron system confined to
an AlAs quantum well and occupying two conduction-band minima (valleys), a
parallel magnetic field can couple to the electrons' orbital motion and tune
the energies of the two valleys by different amounts. The measured density
imbalance between the two valleys, which is a measure of the valley
susceptibility with respect to parallel magnetic field, is enhanced compared to
the predictions of non-interacting calculations, reflecting the role of
electron-electron interaction.Comment: 4+ pages, 4 figures, accepted for publication in Phys. Rev.
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
Phase transition and scaling behavior of topological charged black holes in Horava-Lifshitz gravity
Gravity can be thought as an emergent phenomenon and it has a nice
"thermodynamic" structure. In this context, it is then possible to study the
thermodynamics without knowing the details of the underlying microscopic
degrees of freedom. Here, based on the ordinary thermodynamics, we investigate
the phase transition of the static, spherically symmetric charged black hole
solution with arbitrary scalar curvature in Ho\v{r}ava-Lifshitz gravity at
the Lifshitz point . The analysis is done using the canonical ensemble
frame work; i.e. the charge is kept fixed. We find (a) for both and
, there is no phase transition, (b) while case exhibits the second
order phase transition within the {\it physical region} of the black hole. The
critical point of second order phase transition is obtained by the divergence
of the heat capacity at constant charge. Near the critical point, we find the
various critical exponents. It is also observed that they satisfy the usual
thermodynamic scaling laws.Comment: Minor corrections, refs. added, to appear in Class. Quant. Grav.
arXiv admin note: text overlap with arXiv:1111.0973 by other author
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