2,258 research outputs found
Hypothesis of path integral duality: Applications to QED
We use the modified propagator for quantum field based on a ``principle of
path integral duality" proposed earlier in a paper by Padmanabhan to
investigate several results in QED. This procedure modifies the Feynman
propagator by the introduction of a fundamental length scale. We use this
modified propagator for the Dirac particles to evaluate the first order
radiative corrections in QED. We find that the extra factor of the modified
propagator acts like a regulator at the Planck scales thereby removing the
divergences that otherwise appear in the conventional radiative correction
calculations of QED. We find that:(i) all the three renormalisation factors
, , and pick up finite corrections and (ii) the modified
propagator breaks the gauge invariance at a very small level of
. The implications of this result to generation of the
primordial seed magnetic fields are discussed.Comment: 15 pages, LaTeX2e (uses ijmpd.sty); To appear in IJMP-D; References
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Scalar Field Dark Energy Perturbations and their Scale Dependence
We estimate the amplitude of perturbation in dark energy at different length
scales for a quintessence model with an exponential potential. It is shown that
on length scales much smaller than hubble radius, perturbation in dark energy
is negligible in comparison to that in in dark matter. However, on scales
comparable to the hubble radius () the
perturbation in dark energy in general cannot be neglected. As compared to the
CDM model, large scale matter power spectrum is suppressed in a
generic quintessence dark energy model. We show that on scales , this suppression is primarily due to different background
evolution compared to CDM model. However, on much larger scales
perturbation in dark energy can effect matter power spectrum significantly.
Hence this analysis can act as a discriminator between CDM model and
other generic dark energy models with .Comment: 12 pages, 13 figures, added new section, accepted for publication in
Phys. Rev.
Evolving Newton's Constant, Extended Gravity Theories and SnIa Data Analysis
If Newton's constant G evolves on cosmological timescales as predicted by
extended gravity theories then Type Ia supernovae (SnIa) can not be treated as
standard candles. The magnitude-redshift datasets however can still be useful.
They can be used to simultaneously fit for both H(z) and G(z) (so that local
G(z) constraints are also satisfied) in the context of appropriate
parametrizations. Here we demonstrate how can this analysis be done by applying
it to the Gold SnIa dataset. We compare the derived effective equation of state
parameter w(z) at best fit with the corresponding result obtained by neglecting
the evolution G(z). We show that even though the results clearly differ from
each other, in both cases the best fit w(z) crosses the phantom divide w=-1. We
then attempt to reconstruct a scalar tensor theory that predicts the derived
best fit forms of H(z) and G(z). Since the best fit G(z) fixes the scalar
tensor potential evolution F(z), there is no ambiguity in the reconstruction
and the potential U(z) can be derived uniquely. The particular reconstructed
scalar tensor theory however, involves a change of sign of the kinetic term
as in the minimally coupled case.Comment: Minor changes. Accepted in Phys. Rev. D. 7 revtex pages, 5 figures.
The mathematica file with the numerical analysis of the paper is available at
http://leandros.physics.uoi.gr/snevol.ht
Polaronic state and nanometer-scale phase separation in colossal magnetoresistive manganites
High resolution topographic images obtained by scanning tunneling microscope
in the insulating state of Pr0.68Pb0.32MnO3 single crystals showed regular
stripe-like or zigzag patterns on a width scale of 0.4 - 0.5 nm confirming a
high temperature polaronic state. Spectroscopic studies revealed inhomogeneous
maps of zero-bias conductance with small patches of metallic clusters on length
scale of 2 - 3 nm only within a narrow temperature range close to the
metal-insulator transition. The results give a direct observation of polarons
in the insulating state, phase separation of nanometer-scale metallic clusters
in the paramagnetic metallic state, and a homogeneous ferromagnetic state
Thermodynamic Interpretation of Field Equations at Horizon of BTZ Black Hole
A spacetime horizon comprising with a black hole singularity acts like a
boundary of a thermal system associated with the notions of temperature and
entropy. In case of static metric of BTZ black hole, the field equations near
horizon boundary can be expressed as a thermal identity ,
where is the mass of BTZ black hole, is the change in the area of
the black hole horizon when the horizon is displaced infinitesimally small,
is the radial pressure provided by the source of Einstein equations,
is the entropy and is the Hawking temperature
associated with the horizon. This approach is studied further to generalize it
for non-static BTZ black hole and show that it is also possible to interpret
the field equation near horizon as a thermodynamic identity , where is the angular velocity and is the
angular momentum of BTZ black hole. These results indicate that the field
equations for BTZ black hole possess intrinsic thermodynamic properties near
horizon.Comment: 8 page
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
Concept of temperature in multi-horizon spacetimes: Analysis of Schwarzschild-De Sitter metric
In case of spacetimes with single horizon, there exist several
well-established procedures for relating the surface gravity of the horizon to
a thermodynamic temperature. Such procedures, however, cannot be extended in a
straightforward manner when a spacetime has multiple horizons. In particular,
it is not clear whether there exists a notion of global temperature
characterizing the multi-horizon spacetimes. We examine the conditions under
which a global temperature can exist for a spacetime with two horizons using
the example of Schwarzschild-De Sitter (SDS) spacetime. We systematically
extend different procedures (like the expectation value of stress tensor,
response of particle detectors, periodicity in the Euclidean time etc.) for
identifying a temperature in the case of spacetimes with single horizon to the
SDS spacetime. This analysis is facilitated by using a global coordinate chart
which covers the entire SDS manifold. We find that all the procedures lead to a
consistent picture characterized by the following features: (a) In general, SDS
spacetime behaves like a non-equilibrium system characterized by two
temperatures. (b) It is not possible to associate a global temperature with SDS
spacetime except when the ratio of the two surface gravities is rational (c)
Even when the ratio of the two surface gravities is rational, the thermal
nature depends on the coordinate chart used. There exists a global coordinate
chart in which there is global equilibrium temperature while there exist other
charts in which SDS behaves as though it has two different temperatures. The
coordinate dependence of the thermal nature is reminiscent of the flat
spacetime in Minkowski and Rindler coordinate charts. The implications are
discussed.Comment: 12 page
Complex Effective Path: A Semi-Classical Probe of Quantum Effects
We discuss the notion of an effective, average, quantum mechanical path which
is a solution of the dynamical equations obtained by extremizing the quantum
effective action. Since the effective action can, in general, be complex, the
effective path will also, in general, be complex. The imaginary part of the
effective action is known to be related to the probability of particle creation
by an external source and hence we expect the imaginary part of the effective
path also to contain information about particle creation. We try to identify
such features using simple examples including that of effective path through
the black hole horizon leading to thermal radiation. Implications of this
approach are discussed.Comment: 20 pages; no figures; to appear in Phys.Rev.
Evolution of perturbations in distinct classes of canonical scalar field models of dark energy
Dark energy must cluster in order to be consistent with the equivalence
principle. The background evolution can be effectively modelled by either a
scalar field or by a barotropic fluid.The fluid model can be used to emulate
perturbations in a scalar field model of dark energy, though this model breaks
down at large scales. In this paper we study evolution of dark energy
perturbations in canonical scalar field models: the classes of thawing and
freezing models.The dark energy equation of state evolves differently in these
classes.In freezing models, the equation of state deviates from that of a
cosmological constant at early times.For thawing models, the dark energy
equation of state remains near that of the cosmological constant at early times
and begins to deviate from it only at late times.Since the dark energy equation
of state evolves differently in these classes,the dark energy perturbations too
evolve differently. In freezing models, since the equation of state deviates
from that of a cosmological constant at early times, there is a significant
difference in evolution of matter perturbations from those in the cosmological
constant model.In comparison, matter perturbations in thawing models differ
from the cosmological constant only at late times. This difference provides an
additional handle to distinguish between these classes of models and this
difference should manifest itself in the ISW effect.Comment: 11 pages, 6 figures, accepted for publication in Phys. Rev.
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