17 research outputs found
Causality Constraints on Massive Gravity
The de Rham-Gabadadze-Tolley massive gravity admits pp-wave backgrounds on
which linear fluctuations are shown to undergo time advances for all values of
the parameters. The perturbations may propagate in closed time-like curves
unless the parameter space is constrained to a line. These classical phenomena
take place well within the theory's validity regime.Comment: 6 pages, 4 figures; to appear in PR
Pure Lovelock Kasner metrics
We study pure Lovelock vacuum and perfect fluid equations for Kasner-type metrics. These equations correspond to a single Nth order Lovelock term in the action in dimensions, and they capture the relevant gravitational dynamics when aproaching the big-bang singularity within the Lovelock family of theories. Pure Lovelock gravity also bears out the general feature that vacuum in the critical odd dimension, , is kinematic, i.e. we may define an analogue Lovelock-Riemann tensor that vanishes in vacuum for , yet the Riemann curvature is non-zero. We completely classify isotropic and vacuum Kasner metrics for this class of theories in several isotropy types. The different families can be characterized by means of certain higher order 4th rank tensors. We also analyze in detail the space of vacuum solutions for five- and six dimensional pure Gauss-Bonnet theory. It possesses an interesting and illuminating geometric structure and symmetries that carry over to the general case. We also comment on a closely related family of exponential solutions and on the possibility of solutions with complex Kasner exponents. We show that the latter imply the existence of closed timelike curves in the geometry
New type of phase transition in gravitational theories
We set forth a new type of phase transition that might take place in
gravitational theories whenever higher-curvature corrections are considered. It
can be regarded as a sophisticated version of the Hawking-Page transition,
mediated by the nucleation of a bubble in anti-de Sitter (AdS) space. The
bubble hosts a black hole in its interior, and separates two spacetime regions
with different effective cosmological constants. We compute the free energy of
this configuration and compare it with that of thermal AdS. The result suggests
that a phase transition actually occurs above certain critical temperature,
ultimately changing the value of the cosmological constant. We discuss the
consistency of the thermodynamic picture and its possible relevance in the
context of AdS/CFT.Comment: 6 pages, 2 figures; v2: minor amendments; v3: some additions and
slight title modification requested by journa
A Lovelock black hole bestiary
We revisit the study of (A)dS black holes in Lovelock theories. We present a
new tool that allows to attack this problem in full generality. In analyzing
maximally symmetric Lovelock black holes with non-planar horizon topologies
many distinctive and interesting features are observed. Among them, the
existence of maximally symmetric vacua do not supporting black holes in vast
regions of the space of gravitational couplings, multi-horizon black holes, and
branches of solutions that suggest the existence of a rich diagram of phase
transitions. The appearance of naked singularities seems unavoidable in some
cases, raising the question about the fate of the cosmic censorship conjecture
in these theories. There is a preferred branch of solutions for planar black
holes, as well as non-planar black holes with high enough mass or temperature.
Our study clarifies the role of all branches of solutions, including
asymptotically dS black holes, and whether they should be considered when
studying these theories in the context of AdS/CFT.Comment: 40 pages, 16 figures; v2: references added and minor amendments; v3:
title changed to improve its accuracy and general reorganization of the
results to ameliorate their presentatio
Causality in AdS/CFT and Lovelock theory
We explore the constraints imposed on higher curvature corrections of the
Lovelock type due to causality restrictions in the boundary of asymptotically
AdS space-time. In the framework of AdS/CFT, this is related to positivity of
the energy constraints that arise in conformal collider physics. We present
explicit analytic results that fully address these issues for cubic Lovelock
gravity in arbitrary dimensions and give the formal analytic results that
comprehend general Lovelock theory. The computations can be performed in two
ways, both by considering a thermal setup in a black hole background and by
studying the scattering of gravitons with a shock wave in AdS. We show that
both computations coincide in Lovelock theory. The different helicities, as
expected, provide the boundaries defining the region of allowed couplings. We
generalize these results to arbitrary higher dimensions and discuss their
consequences on the shear viscosity to energy density ratio of CFT plasmas, the
possible existence of Boulware-Deser instabilities in Lovelock theory and the
extent to which the AdS/CFT correspondence might be valid for arbitrary
dimensions.Comment: 35 pages, 20 figures; v2: minor amendments and clarifications
include
Lovelock theories, holography and the fate of the viscosity bound
We consider Lovelock theories of gravity in the context of AdS/CFT. We show
that, for these theories, causality violation on a black hole background can
occur well in the interior of the geometry, thus posing more stringent
constraints than were previously found in the literature. Also, we find that
instabilities of the geometry can appear for certain parameter values at any
point in the geometry, as well in the bulk as close to the horizon. These new
sources of causality violation and instability should be related to CFT
features that do not depend on the UV behavior. They solve a puzzle found
previously concerning unphysical negative values for the shear viscosity that
are not ruled out solely by causality restrictions. We find that, contrary to
previous expectations, causality violation is not always related to positivity
of energy. Furthermore, we compute the bound for the shear viscosity to entropy
density ratio of supersymmetric conformal field theories from d=4 till d=10 -
i.e., up to quartic Lovelock theory -, and find that it behaves smoothly as a
function of d. We propose an approximate formula that nicely fits these values
and has a nice asymptotic behavior when d goes to infinity for any Lovelock
gravity. We discuss in some detail the latter limit. We finally argue that it
is possible to obtain increasingly lower values for the shear viscosity to
entropy density ratio by the inclusion of more Lovelock terms.Comment: 42 pages, 17 figures, JHEP3.cls. v2: reference adde
Causality constraints in AdS/CFT from conformal collider physics and Gauss-Bonnet gravity
We explore the relation between positivity of the energy constraints in
conformal field theories and causality in their dual gravity description. Our
discussion involves CFTs with different central charges whose description, in
the gravity side, requires the inclusion of quadratic curvature corrections. It
is enough, indeed, to consider the Gauss-Bonnet term. We find that both sides
of the AdS/CFT correspondence impose a restriction on the Gauss-Bonnet
coupling. In the case of 6d supersymmetric CFTs, we show the full matching of
these restrictions. We perform this computation in two ways. First by
considering a thermal setup in a black hole background. Second by scrutinizing
the scattering of gravitons with a shock wave in AdS. The different helicities
provide the corresponding lower and upper bounds. We generalize these results
to arbitrary higher dimensions and comment on some hints and puzzles they
prompt regarding the possible existence of higher dimensional CFTs and the
extent to which the AdS/CFT correspondence would be valid for them.Comment: 31 pages, 5 figures; v2: typos fixed, cosmetic amendments and
references adde