357 research outputs found
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
From k-essence to generalised Galileons
We determine the most general scalar field theories which have an action that
depends on derivatives of order two or less, and have equations of motion that
stay second order and lower on flat space-time. We show that those theories can
all be obtained from linear combinations of Lagrangians made by multiplying a
particular form of the Galileon Lagrangian by an arbitrary scalar function of
the scalar field and its first derivatives. We also obtain curved space-time
extensions of those theories which have second order field equations for both
the metric and the scalar field. This provide the most general extension, under
the condition that field equations stay second order, of k-essence, Galileons,
k-Mouflage as well as of the kinetically braided scalars. It also gives the
most general action for a scalar classicalizer, which has second order field
equations. We discuss the relation between our construction and the Euler
hierachies of Fairlie et al, showing in particular that Euler hierachies allow
one to obtain the most general theory when the latter is shift symmetric. As a
simple application of our formalism, we give the covariantized version of the
conformal Galileon.Comment: 25 page
The radion in brane cosmology
We consider the homogeneous cosmological radion, which we define as the interbrane distance in a two brane and symmetrical configuration. In a coordinate system where one of the brane is at rest, the junction conditions for the second (moving) brane give directly the (non-linear) equations of motion for the radion. We analyse the radion fluctuations and solve the non-linear dynamics in some simple cases of interest
Bigravity and Lorentz-violating Massive Gravity
Bigravity is a natural arena where a non-linear theory of massive gravity can
be formulated. If the interaction between the metrics and is
non-derivative, spherically symmetric exact solutions can be found. At large
distances from the origin, these are generically Lorentz-breaking bi-flat
solutions (provided that the corresponding vacuum energies are adjusted
appropriately). The spectrum of linearized perturbations around such
backgrounds contains a massless as well as a massive graviton, with {\em two}
physical polarizations each. There are no propagating vectors or scalars, and
the theory is ghost free (as happens with certain massive gravities with
explicit breaking of Lorentz invariance). At the linearized level, corrections
to GR are proportional to the square of the graviton mass, and so there is no
vDVZ discontinuity. Surprisingly, the solution of linear theory for a static
spherically symmetric source does {\em not} agree with the linearization of any
of the known exact solutions. The latter coincide with the standard
Schwarzschild-(A)dS solutions of General Relativity, with no corrections at
all. Another interesting class of solutions is obtained where and are
proportional to each other. The case of bi-de Sitter solutions is analyzed in
some detail.Comment: 25 pages. v3 Typos corrected, references added. v4 Introduction
extende
Global Structure of Deffayet (Dvali-Gabadadze-Porrati) Cosmologies
We detail the global structure of the five-dimensional bulk for the
cosmological evolution of Dvali-Gabadadze-Porrati braneworlds. The picture
articulated here provides a framework and intuition for understanding how
metric perturbations leave (and possibly reenter) the brane universe. A bulk
observer sees the braneworld as a relativistically expanding bubble, viewed
either from the interior (in the case of the
Friedmann-Lemaitre-Robertson-Walker phase) or the exterior (the
self-accelerating phase). Shortcuts through the bulk in the first phase can
lead to an apparent brane causality violation and provide an opportunity for
the evasion of the horizon problem found in conventional four-dimensional
cosmologies. Features of the global geometry in the latter phase anticipate a
depletion of power for linear metric perturbations on large scales.Comment: 15 pages, 4 figures, RevTeX. References adde
The Vainshtein mechanism in the Decoupling Limit of massive gravity
We investigate static spherically symmetric solutions of nonlinear massive
gravities. We first identify, in an ansatz appropriate to the study of those
solutions, the analog of the decoupling limit (DL) that has been used in the
Goldstone picture description. We show that the system of equations left over
in the DL has regular solutions featuring a Vainshtein-like recovery of
solutions of General Relativity (GR). Hence, the singularities found to arise
integrating the full nonlinear system of equations are not present in the DL,
despite the fact those singularities are usually thought to be due to a
negative energy mode also seen in this limit. Moreover, we show that the
scaling conjectured by Vainshtein at small radius is only a limiting case in an
infinite family of non singular solutions each showing a Vainshtein recovery of
GR solutions below the Vainshtein radius but a different common scaling at
small distances. This new scaling is shown to be associated with a zero mode of
the nonlinearities left over in the DL. We also show that, in the DL, this
scaling allows for a recovery of GR solutions even for potentials where the
original Vainshtein mechanism is not working. Our results imply either that the
DL misses some important features of nonlinear massive gravities or that
important features of the solutions of the full nonlinear theory have been
overlooked. They could also have interesting outcomes for the DGP model and
related proposals.Comment: 52 pages, 7 figures; v3: minor typos corrected, discussion of the
validity of the Decoupling Limit extended; accepted for publication in JHE
The Accelerated Universe and the Moon
Cosmologically motivated theories that explain small acceleration rate of the
Universe via modification of gravity at very large, horizon or super-horizon
distances, can be tested by precision gravitational measurements at much
shorter scales, such as the Earth-Moon distance. Contrary to the naive
expectation the predicted corrections to the Einsteinian metric near
gravitating sources are so significant that fall within sensitivity of the
proposed Lunar Ranging experiments. The key reason for such corrections is the
van Dam-Veltman-Zakharov discontinuity present in linearized versions of all
such theories, and its subsequent absence at the non-linear level ala
Vainshtein
On Brane World Cosmological Perturbations
We discuss the scalar cosmological perturbations in a 3-brane world with a 5D
bulk. We first show explicitly how the effective perturbed Einstein's equations
on the brane (involving the Weyl fluid) are encoded into Mukohyama's master
equation. We give the relation between Mukohyama's master variable and the
perturbations of the Weyl fluid, we also discuss the relation between the
former and the perturbations of matter and induced metric on the brane. We show
that one can obtain a boundary condition on the brane for the master equation
solely expressible in term of the master variable, in the case of a perfect
fluid with adiabatic perturbations on a Randall-Sundrum (RS) or
Dvali-Gabadadze-Porrati (DGP) brane. This provides an easy way to solve
numerically for the evolution of the perturbations as well as should shed light
on the various approximations done in the literature to deal with the Weyl
degrees of freedom.Comment: 36 pages, 1 figur
Halo models in modified gravity theories with self-accelerated expansion
We investigate the structure of halos in the sDGP (self-accelerating branch
of the Dvali-Gavadadze-Porrati braneworld gravity) model and the galileon
modified gravity model on the basis of the static and spherically symmetric
solutions of the collisionless Boltzmann equation, which reduce to the singular
isothermal sphere model and the King model in the limit of Newtonian gravity.
The common feature of these halos is that the density of a halo in the outer
region is larger (smaller) in the sDGP (galileon) model, respectively, in
comparison with Newtonian gravity. This comes from the suppression
(enhancement) of the effective gravity at large distance in the sDGP (galileon)
model, respectively. However, the difference between these modified gravity
models and Newtonian gravity only appears outside the halo due to the
Vainshtein mechanism, which makes it difficult to distinguish between them. We
also discuss the case in which the halo density profile is fixed independently
of the gravity model for comparison between our results and previous work.Comment: 15pages, 6 figures, maches the version to be published in Int. J.
Mod. Phys. D, typos correcte
Born-Infeld-type phantom on the brane world
We study the evolution of Born-Infeld-type phantom in the second
Randall-Sundrum brane scenario, and find that there exists attractor solution
for the potential with a maximum, which implies a cosmological constant at the
late time. Especially, we discuss the BI model of constant potential without
and with dust matter. In the weak tension limit of the brane, we obtain an
exact solution for the BI phantom and scale factor and show that there is no
big rip during the evolution of the brane.Comment: 5 pages, 2 figures, Reference added, Phys. Rev. D in pres
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