544 research outputs found
Chronology Protection in Galileon Models and Massive Gravity
Galileon models are a class of effective field theories that have recently
received much attention. They arise in the decoupling limit of theories of
massive gravity, and in some cases they have been treated in their own right as
scalar field theories with a specific nonlinearly realized global symmetry
(Galilean transformation). It is well known that in the presence of a source,
these Galileon theories admit superluminal propagating solutions, implying that
as quantum field theories they must admit a different notion of causality than
standard local Lorentz invariant theories. We show that in these theories it is
easy to construct closed timelike curves (CTCs) within the {\it naive} regime
of validity of the effective field theory. However, on closer inspection we see
that the CTCs could never arise since the Galileon inevitably becomes
infinitely strongly coupled at the onset of the formation of a CTC. This
implies an infinite amount of backreaction, first on the background for the
Galileon field, signaling the break down of the effective field theory, and
subsequently on the spacetime geometry, forbidding the formation of the CTC.
Furthermore the background solution required to create CTCs becomes unstable
with an arbitrarily fast decay time. Thus Galileon theories satisfy a direct
analogue of Hawking's chronology protection conjecture.Comment: 34 pages, no figure
The Self-Accelerating Universe with Vectors in Massive Gravity
We explore the possibility of realising self-accelerated expansion of the
Universe taking into account the vector components of a massive graviton. The
effective action in the decoupling limit contains an infinite number of terms,
once the vector degrees of freedom are included. These can be re-summed in
physically interesting situations, which result in non-polynomial couplings
between the scalar and vector modes. We show there are self-accelerating
background solutions for this effective action, with the possibility of having
a non-trivial profile for the vector fields. We then study fluctuations around
these solutions and show that there is always a ghost, if a background vector
field is present. When the background vector field is switched off, the ghost
can be avoided, at the price of entering into a strong coupling regime, in
which the vector fluctuations have vanishing kinetic terms. Finally we show
that the inclusion of a bare cosmological constant does not change the previous
conclusions and it does not lead to a ghost mode in the absence of a background
vector field.Comment: 23 pages, 2 figure
Exact Wave Solutions to 6D Gauged Chiral Supergravity
We describe a broad class of time-dependent exact wave solutions to 6D gauged
chiral supergravity with two compact dimensions. These 6D solutions are
nontrivial warped generalizations of 4D pp-waves and Kundt class solutions and
describe how a broad class of previously-static compactifications from 6D to 4D
(sourced by two 3-branes) respond to waves moving along one of the
uncompactified directions. Because our methods are generally applicable to any
higher dimensional supergravity they are likely to be of use for finding the
supergravity limit of time-dependent solutions in string theory. The 6D
solutions are interesting in their own right, describing 6D shock waves induced
by high energy particles on the branes, and as descriptions of the near-brane
limit of the transient wavefront arising from a local bubble-nucleation event
on one of the branes, such as might occur if a tension-changing phase
transition were to occur.Comment: 22 pages, 1 figure. Minor clarifications added. Accepted in JHE
A note on "symmetric" vielbeins in bimetric, massive, perturbative and non perturbative gravities
We consider a manifold endowed with two different vielbeins
and corresponding to two different metrics and
. Such a situation arises generically in bimetric or massive
gravity (including the recently discussed version of de Rham, Gabadadze and
Tolley), as well as in perturbative quantum gravity where one vielbein
parametrizes the background space-time and the other the dynamical degrees of
freedom. We determine the conditions under which the relation can be
imposed (or the "Deser-van Nieuwenhuizen" gauge chosen). We clarify and correct
various statements which have been made about this issue.Comment: 20 pages. Section 7, prop. 6 and 7. added. Some results made more
precis
Interacting spin-2 fields in three dimensions
Using the frame formulation of multi-gravity in three dimensions, we show
that demanding the presence of secondary constraints which remove the
Boulware-Deser ghosts restricts the possible interaction terms of the theory
and identifies invertible frame field combinations whose effective metric may
consistently couple to matter. The resulting ghost-free theories can be
represented by theory graphs which are trees. In the case of three frame
fields, we explicitly show that the requirement of positive masses and energies
for the bulk spin-2 modes in AdS is consistent with a positive central
charge for the putative dual CFT.Comment: 26 pages, 3 figures, v2: minor changes, matches published versio
On Black Holes in Massive Gravity
In massive gravity the so-far-found black hole solutions on Minkowski space
happen to convert horizons into a certain type of singularities. Here we
explore whether these singularities can be avoided if space-time is not
asymptotically Minkowskian. We find an exact analytic black hole (BH) solution
which evades the above problem by a transition at large scales to self-induced
de Sitter (dS) space-time, with the curvature scale set by the graviton mass.
This solution is similar to the ones discovered by Koyama, Niz and Tasinato,
and by Nieuwenhuizen, but differs in detail. The solution demonstrates that in
massive GR, in the Schwarzschild coordinate system, a BH metric has to be
accompanied by the St\"uckelberg fields with nontrivial backgrounds to prevent
the horizons to convert into the singularities. We also find an analogous
solution for a Reissner-Nordstr\"om BH on dS space. A limitation of our
approach, is that we find the solutions only for specific values of the two
free parameters of the theory, for which both the vector and scalar
fluctuations loose their kinetic terms, however, we hope our solutions
represent a broader class with better behaved perturbations.Comment: 17 LateX page
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