974 research outputs found
Cosmological perturbations of brane-induced gravity and the vDVZ discontinuity on FLRW space-times
We investigate the cosmological perturbations of the brane-induced
(Dvali-Gabadadze-Porrati) model which exhibits a van Dam-Veltman-Zakharov
(vDVZ) discontinuity when linearized over a Minkowski background. We show that
the linear brane scalar cosmological perturbations over an arbitrary
Friedmann-Lemaitre-Robertson-Walker (FLRW) space-time have a well defined limit
when the radius of transition between 4D and 5D gravity is sent to infinity
with respect to the background Hubble radius. This radius of transition plays
for the brane-induced gravity model a role equivalent to the Compton wavelength
of the graviton in a Pauli-Fierz theory, as far as the vDVZ discontinuity is
concerned. This well defined limit is shown to obey the linearized 4D
Einstein's equations whenever the Hubble factor is non vanishing. This shows
the disappearance of the vDVZ discontinuity for general FLRW background, and
extends the previously know result for maximally-symmetric space-times of non
vanishing curvature. Our reasoning is valid for matter with simple equation of
state such as a scalar field, or a perfect fluid with adiabatic perturbations,
and involves to distinguish between space-times with a vanishing scalar
curvature and space-times with a non vanishing one. We also discuss the
validity of the linear perturbation theory, in particular for those FLRW
space-times where the Ricci scalar is vanishing only on a set of zero measure.
In those cases, we argue that the linear perturbation theory breaks down when
the Ricci scalar vanishes (and the radius of transition is sent to infinity),
in a way similar to what has been found to occur around sources on a Minkowski
background.Comment: 36 pages, v.2, typos correcte
Probing Gravity with Spacetime Sirens
A gravitational observatory such as LISA will detect coalescing pairs of
massive black holes, accurately measure their luminosity distance and help
identify a host galaxy or an electromagnetic counterpart. If dark energy is a
manifestation of modified gravity on large scales, gravitational waves from
cosmologically-distant spacetime sirens are direct probes of this new physics.
For example, a gravitational Hubble diagram based on black hole pair luminosity
distances and host galaxy redshifts could reveal a large distance
extra-dimensional leakage of gravity. Various additional signatures may be
expected in a gravitational signal propagated over cosmological scales.Comment: 11 pages, 1 figure, accepted for publication in ApJ Letter
Self-accelerating solutions in the cascading DGP braneworld
The self-accelerating branch of the Dvali-Gabadadze-Porrati (DGP)
five-dimensional braneworld has provided a compelling model for the current
cosmic acceleration. Recent observations, however, have not favored it so much.
We discuss the solutions which contain a de Sitter 3-brane in the cascading DGP
braneworld model, which is a kind of higher-dimensional generalizations of the
DGP model,where a -dimensional brane is placed on a -dimensional one
and the -brane action contains the -dimensional induced scalar
curvature term. In the simplest six-dimensional model, we derive the solutions.
Our solutions can be classified into two branches, which reduce to the
self-accelerating and normal solutions in the limit of the original
five-dimensional DGP model. In the presence of the six-dimensional bulk
gravity, the `normal' branch provides a new self-accelerating solution. The
expansion rate of this new branch is generically lower than that of the
original one, which may alleviate the fine-tuning problem.Comment: 4 pages, critical change
Reconstructing the Distortion Function for Nonlocal Cosmology
We consider the cosmology of modified gravity models in which Newton's
constant is distorted by a function of the inverse d'Alembertian acting on the
Ricci scalar. We derive a technique for choosing the distortion function so as
to fit an arbitrary expansion history. This technique is applied numerically to
the case of LambdaCDM cosmology, and the result agrees well with a simple
hyperbolic tangent.Comment: 17 pages, 1 figure, dedicated to Stanley Deser on the occasion of his
78th birthday, revised version for publication in JCA
Gauged Galileons From Branes
We show how the coupling of SO(N) gauge fields to galileons arises from a
probe brane construction. The galileons arise from the brane bending modes of a
brane probing a co-dimension N bulk, and the gauge fields arise by turning on
certain off-diagonal components in the zero mode of the bulk metric. By
construction, the equations of motion for both the galileons and gauge fields
remain second order. Covariant gauged galileons are derived as well.Comment: 6 pages. v2: minor changes, version appearing in PL
General Relativity With An Auxiliary Dimension
I consider an extension of General Relativity by an auxiliary non-dynamical
dimension that enables our space-time to acquire an extrinsic curvature.
Obtained gravitational equations, without or with a cosmological constant, have
a selfaccelerated solution that is independent of the value of the cosmological
constant, and can describe the cosmic speedup of the Universe as a geometric
effect. Background evolution of the selfaccelerated solution is identical to
that of ordinary de Sitter space. I show that linear perturbations on this
solution describe either a massless graviton, or a massive graviton and a
scalar, which are free of ghosts and tachyons for certain choices of boundary
conditions. The obtained linearized expressions suggest that nonlinear
interactions should, for certain boundary conditions, be strongly coupled,
although this issue is not studied here. The full nonlinear Hamiltonian of the
theory is shown to be positive for the selfaccelerated solution, while in
general, it reduces to surface terms in our and auxiliary dimensions.Comment: 1+14 pages; v2: comments and 2 refs added; minor corrections and
improvements mad
Short distance non-perturbative effects of large distance modified gravity
In a model of large distance modified gravity we compare the nonperturbative
Schwarzschild solution of hep-th/0407049 to approximate solutions obtained
previously. In the regions where there is a good qualitative agreement between
the two, the nonperturbative solution yields effects that could have
observational significance. These effects reduce, by a factor of a few, the
predictions for the additional precession of the orbits in the Solar system,
still rendering them in an observationally interesting range. The very same
effects lead to a mild anomalous scaling of the additional scale-invariant
precession rate found by Lue and Starkman.Comment: 13 pages, paragraph with comment added, PLB versio
On p-form theories with gauge invariant second order field equations
We explore field theories of a single p-form with equations of motions of
order strictly equal to two and gauge invariance. We give a general method for
the classification of such theories which are extensions to the p-forms of the
Galileon models for scalars. Our classification scheme allows to compute an
upper bound on the number of different such theories depending on p and on the
space-time dimension. We are also able to build a non trivial Galileon like
theory for a 3-form with gauge invariance and an action which is polynomial
into the derivatives of the form. This theory has gauge invariant field
equations but an action which is not, like a Chern-Simons theory. Hence the
recently discovered no-go theorem stating that there are no non trivial gauge
invariant vector Galileons (which we are also able here to confirm with our
method) does not extend to other odd p cases.Comment: 29 page
Gravitational Leakage into Extra Dimensions: Probing Dark Energy Using Local Gravity
The braneworld model of Dvali-Gabadadze-Porrati (DGP) is a theory where
gravity is modified at large distances by the arrested leakage of gravitons off
our four-dimensional universe. Cosmology in this model has been shown to
support both "conventional" and exotic explanations of the dark energy
responsible for today's cosmic acceleration. We present new results for the
gravitational field of a clustered matter source on the background of an
accelerating universe in DGP braneworld gravity, and articulate how these
results differ from those of general relativity. In particular, we show that
orbits nearby a mass source suffer a universal anomalous precession as large as
5 microarcseconds/year, dependent only on the graviton's effective linewidth
and the global geometry of the full, five-dimensional universe. Thus, this
theory offers a local gravity correction sensitive to factors that dictate
cosmological history.Comment: 18 pages, 1 figure, revtex. Reference updated. Footnote change
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