6,691 research outputs found
Polycritical Gravities
We present higher-derivative gravities that propagate an arbitrary number of
gravitons of different mass on (A)dS backgrounds. These theories have multiple
critical points, at which the masses degenerate and the graviton energies are
non-negative. For six derivatives and higher there are critical points with
positive energy.Comment: Version to be publishe
Perturbations of Matter Fields in the Second-order Gauge-invariant Cosmological Perturbation Theory
Some formulae for the perturbations of the matter fields are summarized
within the framework of the second-order gauge-invariant cosmological
perturbation theory in a four dimensional homogeneous isotropic universe, which
is developed in the papers [K.Nakamura, Prog.Theor.Phys., 117 (2007), 17.]. We
derive the formulae for the perturbations of the energy momentum tensors and
equations of motion for a perfect fluid, an imperfect fluid, and a signle
scalar field, and show that all equations are derived in terms of
gauge-invariant variables without any gauge fixing.Comment: (v1) 76 pages, no figure; (v2) minor revision, typos are corrected,
references are added; (v3) Title is changed, Compactified into 55 pages,
Comment on the comparison with the other work is added; (v4)typos are
correcte
Quantization of the black hole area as quantization of the angular momentum component
In transforming from Schwarzschild to Euclidean Rindler coordinates the
Schwarzschild time transforms to a periodic angle. As is well-known, this
allows one to introduce the Hawking temperature and is an origin of black hole
thermodynamics. On the other hand, according to quantum mechanics this angle is
conjugate to the component of the angular momentum. From the commutation
relation and quantization condition for the angular momentum component it is
found that the area of the horizon of a Schwarzschild black hole is quantized
with the quantum . It is shown that this conclusion is
also valid for a generic Kerr-Newman black hole.Comment: 4 pages (revtex), no figures; a boundary condition for the
differential equation (15) added; the absent of the remnants in the approach
noted; a reference added; accepted by Physical Review D for publicatio
Matter instability in modified gravity
The Dolgov-Kawasaki instability discovered in the matter sector of the
modified gravity scenario incorporating a 1/R correction to Einstein gravity is
studied in general f(R) theories. A stability condition is found in the metric
version of these theories to help ruling out models that are unviable from the
theoretical point of view.Comment: 4 pages, revtex, to appear in Phys. Rev. D. In the revised version,
an error concerning the Palatini version of these theories has been corrected
and the references update
A Rigorous Derivation of Electromagnetic Self-force
During the past century, there has been considerable discussion and analysis
of the motion of a point charge, taking into account "self-force" effects due
to the particle's own electromagnetic field. We analyze the issue of "particle
motion" in classical electromagnetism in a rigorous and systematic way by
considering a one-parameter family of solutions to the coupled Maxwell and
matter equations corresponding to having a body whose charge-current density
and stress-energy tensor scale to zero size
in an asymptotically self-similar manner about a worldline as . In this limit, the charge, , and total mass, , of the body go to
zero, and goes to a well defined limit. The Maxwell field
is assumed to be the retarded solution associated with
plus a homogeneous solution (the "external field") that varies
smoothly with . We prove that the worldline must be a
solution to the Lorentz force equations of motion in the external field
. We then obtain self-force, dipole forces, and spin force
as first order perturbative corrections to the center of mass motion of the
body. We believe that this is the first rigorous derivation of the complete
first order correction to Lorentz force motion. We also address the issue of
obtaining a self-consistent perturbative equation of motion associated with our
perturbative result, and argue that the self-force equations of motion that
have previously been written down in conjunction with the "reduction of order"
procedure should provide accurate equations of motion for a sufficiently small
charged body with negligible dipole moments and spin. There is no corresponding
justification for the non-reduced-order equations.Comment: 52 pages, minor correction
A general maximum entropy principle for self-gravitating perfect fluid
We consider a self-gravitating system consisting of perfect fluid with
spherical symmetry. Using the general expression of entropy density, we
extremize the total entropy under the constraint that the total number of
particles is fixed. We show that extrema of coincides precisely with the
relativistic Tolman-Oppenheimer-Volkoff (TOV) equation of hydrostatic
equilibrium. Furthermore, we apply the maximum entropy principle to a charged
perfect fluid and derive the generalized TOV equation. Our work provides a
strong evidence for the fundamental relationship between general relativity and
ordinary thermodynamics.Comment: 13 pages, no figure. The arguments have been improved so that the
assumption p=p(\rho) is no longer neede
How often does the Unruh-DeWitt detector click beyond four dimensions?
We analyse the response of an arbitrarily-accelerated Unruh-DeWitt detector
coupled to a massless scalar field in Minkowski spacetimes of dimensions up to
six, working within first-order perturbation theory and assuming a smooth
switch-on and switch-off. We express the total transition probability as a
manifestly finite and regulator-free integral formula. In the sharp switching
limit, the transition probability diverges in dimensions greater than three but
the transition rate remains finite up to dimension five. In dimension six, the
transition rate remains finite in the sharp switching limit for trajectories of
constant scalar proper acceleration, including all stationary trajectories, but
it diverges for generic trajectories. The divergence of the transition rate in
six dimensions suggests that global embedding spacetime (GEMS) methods for
investigating detector response in curved spacetime may have limited validity
for generic trajectories when the embedding spacetime has dimension higher than
five.Comment: 30 pages. v3: presentational improvement. Published versio
Gravity-induced vacuum dominance
It has been widely believed that, except in very extreme situations, the
influence of gravity on quantum fields should amount to just small,
sub-dominant contributions. This view seemed to be endorsed by the seminal
results obtained over the last decades in the context of renormalization of
quantum fields in curved spacetimes. Here, however, we argue that this belief
is false by showing that there exist well-behaved spacetime evolutions where
the vacuum energy density of free quantum fields is forced, by the very same
background spacetime, to become dominant over any classical energy-density
component. This semiclassical gravity effect finds its roots in the infrared
behavior of fields on curved spacetimes. By estimating the time scale for the
vacuum energy density to become dominant, and therefore for backreaction on the
background spacetime to become important, we argue that this vacuum dominance
may bear unexpected astrophysical and cosmological implications.Comment: To appear in Phys. Rev. Lett
Integrable Cosmological Models From Higher Dimensional Einstein Equations
We consider the cosmological models for the higher dimensional spacetime
which includes the curvatures of our space as well as the curvatures of the
internal space. We find that the condition for the integrability of the
cosmological equations is that the total space-time dimensions are D=10 or D=11
which is exactly the conditions for superstrings or M-theory. We obtain
analytic solutions with generic initial conditions in the four dimensional
Einstein frame and study the accelerating universe when both our space and the
internal space have negative curvatures.Comment: 10 pages, 2 figures, added reference, corrected typos(v2),
explanation improved and references and acknowledgments added, accepted for
publication in PRD(v3
The Effects of Stress Tensor Fluctuations upon Focusing
We treat the gravitational effects of quantum stress tensor fluctuations. An
operational approach is adopted in which these fluctuations produce
fluctuations in the focusing of a bundle of geodesics. This can be calculated
explicitly using the Raychaudhuri equation as a Langevin equation. The physical
manifestation of these fluctuations are angular blurring and luminosity
fluctuations of the images of distant sources. We give explicit results for the
case of a scalar field on a flat background in a thermal state.Comment: 26 pages, 1 figure, new material added in Sect. III and in Appendices
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