3,984 research outputs found
A note on the thermodynamics of gravitational radiation
It is shown that linearized gravitational radiation confined in a cavity can
achieve thermal equilibrium if the mean density of the radiation and the size
of the cavity satisfy certain constraints.Comment: 8 pages; discussion expanded and some issues clarified; revised
version accepted for publication in Classical and Quantum Gravit
Combining general relativity and quantum theory: points of conflict and contact
The issues related to bringing together the principles of general relativity
and quantum theory are discussed. After briefly summarising the points of
conflict between the two formalisms I focus on four specific themes in which
some contact has been established in the past between GR and quantum field
theory: (i) The role of planck length in the microstructure of spacetime (ii)
The role of quantum effects in cosmology and origin of the universe (iii) The
thermodynamics of spacetimes with horizons and especially the concept of
entropy related to spacetime geometry (iv) The problem of the cosmological
constant.Comment: Invited Talk at "The Early Universe and Cosmological Observations: a
Critical Review", UCT, Cape Town, 23-25 July,2001; to appear in
Class.Quan.Gra
Strong CP Violation in External Magnetic Fields
We study the response of the QCD vacuum to an external magnetic field, in the
presence of strong CP violation. Using chiral perturbation theory and large N_c
expansion, we show that the external field would polarize quantum fluctuations
and induce an electric dipole moment of the vacuum, along the direction of the
magnetic field. We estimate the magnitude of this effect in different physical
scenarios. In particular, we find that the polarization induced by the magnetic
field of a magnetar could accelerate electric charges up to energies of the
order \theta 10^3 TeV. We also suggest a connection with the possible existence
of "hot-spots" on the surface of neutron stars.Comment: 4 pages, 1 figure. Major revision. Phenomenological analysis extende
Self-similar collapse and the structure of dark matter halos: A fluid approach
We explore the dynamical restrictions on the structure of dark matter halos
through a study of cosmological self-similar gravitational collapse solutions.
A fluid approach to the collisionless dynamics of dark matter is developed and
the resulting closed set of moment equations are solved numerically including
the effect of halo velocity dispersions (both radial and tangential), for a
range of spherically averaged initial density profiles. Our results highlight
the importance of tangential velocity dispersions to obtain density profiles
shallower than in the core regions, and for retaining a memory of the
initial density profile, in self-similar collapse. For an isotropic core
velocity dispersion only a partial memory of the initial density profile is
retained. If tangential velocity dispersions in the core are constrained to be
less than the radial dispersion, a cuspy core density profile shallower than
cannot obtain, in self-similar collapse.Comment: 25 pages, 7 figures, submitted to Ap
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
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
Non-Pauli Transitions From Spacetime Noncommutativity
There are good reasons to suspect that spacetime at Planck scales is
noncommutative. Typically this noncommutativity is controlled by fixed
"vectors" or "tensors" with numerical entries. For the Moyal spacetime, it is
the antisymmetric matrix . In approaches enforcing Poincar\'e
invariance, these deform or twist the method of (anti-)symmetrization of
identical particle state vectors. We argue that the earth's rotation and
movements in the cosmos are "sudden" events to Pauli-forbidden processes. They
induce (twisted) bosonic components in state vectors of identical spinorial
particles in the presence of a twist. These components induce non-Pauli
transitions. From known limits on such transitions, we infer that the energy
scale for noncommutativity is . This suggests a
new energy scale beyond Planck scale.Comment: 11 pages, 1 table, Slightly revised for clarity
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
Noncommutative gravity, a `no strings attached' quantum-classical duality, and the cosmological constant puzzle
There ought to exist a reformulation of quantum mechanics which does not
refer to an external classical spacetime manifold. Such a reformulation can be
achieved using the language of noncommutative differential geometry. A
consequence which follows is that the `weakly quantum, strongly gravitational'
dynamics of a relativistic particle whose mass is much greater than Planck mass
is dual to the `strongly quantum, weakly gravitational' dynamics of another
particle whose mass is much less than Planck mass. The masses of the two
particles are inversely related to each other, and the product of their masses
is equal to the square of Planck mass. This duality explains the observed value
of the cosmological constant, and also why this value is nonzero but extremely
small in Planck units.Comment: 7 pages. Second Prize in Gravity Research Foundation Essay
Competition, 2008. Two paragraphs added to original essay to enhance clarity.
To appear in Gen. Rel. Gra
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