60 research outputs found
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
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
On horizon structure of bimetric spacetimes
We discuss the structure of horizons in spacetimes with two metrics, with
applications to the Vainshtein mechanism and other examples. We show, without
using the field equations, that if the two metrics are static, spherically
symmetric, nonsingular, and diagonal in a common coordinate system, then a
Killing horizon for one must also be a Killing horizon for the other. We then
generalize this result to the axisymmetric case. We also show that the surface
gravities must agree if the bifurcation surface in one spacetime lies smoothly
in the interior of the spacetime of the other metric. These results imply for
example that the Vainshtein mechanism of nonlinear massive gravity theories
cannot work to recover black holes if the dynamical metric and the non
dynamical flat metric are both diagonal. They also explain the global structure
of some known solutions of bigravity theories with one diagonal and one
nondiagonal metric, in which the bifurcation surface of the Killing field lies
in the interior of one spacetime and on the conformal boundary of the other.Comment: 9 page
Classifying Galileon -form theories
We provide a complete classification of all abelian gauge invariant -form
theories with equations of motion depending only on the second derivative of
the field---the -form analogues of the Galileon scalar field theory. We
construct explicitly the nontrivial actions that exist for spacetime dimension
, but our methods are general enough and can be extended to arbitrary
. We uncover in particular a new -form Galileon cubic theory in
dimensions. As a by-product we give a simple proof of the fact that the
equations of motion depend on the -form gauge fields only through their
field strengths, and show this explicitly for the recently discovered -form
Galileon quartic theory.Comment: 17 pages; v2: references adde
Supernovae, CMB, and Gravitational Leakage into Extra Dimensions
We discuss observational constraints coming from CMB and type Ia supernovae,
for the model of accelerated universe produced by gravitational leakage into
extra dimensions. Our fits indicate that the model is currently in agreement
with the data. We also give the equations governing the evolution of
cosmological perturbations. Future observations will be able to severely
constrain the model.Comment: 20 pages, 6 figures, typos corrected and minor changes before
publicatio
Constraints on Shift-Symmetric Scalar-Tensor Theories with a Vainshtein Mechanism from Bounds on the Time Variation of G
We show that the current bounds on the time variation of the Newton constant
G can put severe constraints on many interesting scalar-tensor theories which
possess a shift symmetry and a nonminimal matter-scalar coupling. This
includes, in particular, Galileon-like models with a Vainshtein screening
mechanism. We underline that this mechanism, if efficient to hide the effects
of the scalar field at short distance and in the static approximation, can in
general not alter the cosmological time evolution of the scalar field. This
results in a locally measured time variation of G which is too large when the
matter-scalar coupling is of order one.Comment: RevTeX4 format; v.2: 5 pages, title changed, matches published
versio
A note on the wellposedness of scalar brane world cosmological perturbations
We discuss scalar brane world cosmological perturbations for a 3-brane world
in a maximally symmetric 5D bulk. We show that Mukoyama's master equations
leads, for adiabatic perturbations of a perfect fluid on the brane and for
scalar field matter on the brane, to a well posed problem despite the "non
local" aspect of the boundary condition on the brane. We discuss in relation to
the wellposedness the way to specify initial data in the bulk.Comment: 14 pages, one figure, v2 minor change
Non-conventional cosmology from a brane-universe
We consider ``brane-universes'', where matter is confined to four-dimensional
hypersurfaces (three-branes) whereas one extra compact dimension is felt by
gravity only. We show that the cosmology of such branes is definitely different
from standard cosmology and identify the reasons behind this difference. We
give a new class of exact solutions with a constant five-dimensional radius and
cosmologically evolving brane. We discuss various consequences.Comment: 22 pages, Latex; minor addition
Pseudo-anomalous U(1) symmetry in the strong coupling limit of the heterotic string
We discuss, in the context of the strongly coupled E_8 \times E_8 heterotic
string proposed by Horava and Witten, the appearance of anomalous U(1)_X
symmetries of a nonperturbative origin, related to the presence, after
compactification, of five-branes in the five-dimensional bulk of the theory. We
compute the gauge anomalies and the induced Fayet-Iliopoulos terms on each
boundary, which we find to be lower than the universal one induced in the
weakly coupled case.Comment: 15 pages, 2 figures, late
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