20 research outputs found
Black Holes in Six-dimensional Conformal Gravity
We study conformally-invariant theories of gravity in six dimensions. In four
dimensions, there is a unique such theory that is polynomial in the curvature
and its derivatives, namely Weyl-squared, and furthermore all solutions of
Einstein gravity are also solutions of the conformal theory. By contrast, in
six dimensions there are three independent conformally-invariant polynomial
terms one could consider. There is a unique linear combination (up to overall
scale) for which Einstein metrics are also solutions, and this specific theory
forms the focus of our attention in this paper. We reduce the equations of
motion for the most general spherically-symmetric black hole to a single
5th-order differential equation. We obtain the general solution in the form of
an infinite series, characterised by 5 independent parameters, and we show how
a finite 3-parameter truncation reduces to the already known Schwarzschild-AdS
metric and its conformal scaling. We derive general results for the
thermodynamics and the first law for the full 5-parameter solutions. We also
investigate solutions in extended theories coupled to conformally-invariant
matter, and in addition we derive some general results for conserved charges in
cubic-curvature theories in arbitrary dimensions.Comment: 28 pages. References adde
An Deformation of Gauged STU Supergravity
Four-dimensional gauged STU supergravity is a consistent
truncation of the standard gauged supergravity in which
just the four gauge fields in the Cartan subgroup of are
retained. One of these is the graviphoton in the supergravity
multiplet and the other three lie in three vector multiplets. In this paper we
carry out the analogous consistent truncation of the newly-discovered family of
-deformed gauged supergravities, thereby obtaining
a family of -deformed STU gauged supergravities. Unlike in some other
truncations of the deformed supergravity that have been
considered, here the scalar potential of the deformed STU theory is independent
of the parameter. However, it enters in the scalar couplings in the
gauge-field kinetic terms, and it is non-trivial because of the minimal
couplings of the fermion fields to the gauge potentials. We discuss the
supersymmetry transformation rules in the -deformed supergravities, and
present some examples of black hole solutions.Comment: 31 pages. Derivation of the range of \omega corrected; discussion of
supersymmetry of solutions extended, and a reference adde
AdS Dyonic Black Hole and its Thermodynamics
We obtain spherically-symmetric and -symmetric dyonic black holes that
are asymptotic to anti-de Sitter space-time (AdS), which are solutions in
maximal gauged four-dimensional supergravity, with just one of the U(1) fields
carrying both the electric and magnetic charges . We study the
thermodynamics, and find that the usually-expected first law does not hold
unless P=0, Q=0 or P=Q. For general values of the charges, we find that the
first law requires a modification with a new pair of thermodynamic conjugate
variables. We show that they describe the scalar hair that breaks some of the
asymptotic AdS symmetries.Comment: 21 pages, typos corrected, discussion of Euclidean action adde
A Complete Classification of Higher Derivative Gravity in 3D and Criticality in 4D
We study the condition that the theory is unitary and stable in
three-dimensional gravity with most general quadratic curvature,
Lorentz-Chern-Simons and cosmological terms. We provide the complete
classification of the unitary theories around flat Minkowski and (anti-)de
Sitter spacetimes. The analysis is performed by examining the quadratic
fluctuations around these classical vacua. We also discuss how to understand
critical condition for four-dimensional theories at the Lagrangian level.Comment: 20 pages, v2: minor corrections, refs. added, v3: logic modified, v4:
typos correcte
Torsion and accelerating expansion of the universe in quadratic gravitation
Several exact cosmological solutions of a metric-affine theory of gravity
with two torsion functions are presented. These solutions give a essentially
different explanation from the one in most of previous works to the cause of
the accelerating cosmological expansion and the origin of the torsion of the
spacetime. These solutions can be divided into two classes. The solutions in
the first class define the critical points of a dynamical system representing
an asymptotically stable de Sitter spacetime. The solutions in the second class
have exact analytic expressions which have never been found in the literature.
The acceleration equation of the universe in general relativity is only a
special case of them. These solutions indicate that even in vacuum the
spacetime can be endowed with torsion, which means that the torsion of the
spacetime has an intrinsic nature and a geometric origin. In these solutions
the acceleration of the cosmological expansion is due to either the scalar
torsion or the pseudoscalar torsion function. Neither a cosmological constant
nor dark energy is needed. It is the torsion of the spacetime that causes the
accelerating expansion of the universe in vacuum. All the effects of the
inflation, the acceleration and the phase transformation from deceleration to
acceleration can be explained by these solutions. Furthermore, the energy and
pressure of the matter without spin can produce the torsion of the spacetime
and make the expansion of the universe decelerate as well as accelerate.Comment: 20 pages. arXiv admin note: text overlap with gr-qc/0604006,
arXiv:1110.344
Perturbative instabilities in Horava gravity
We investigate the scalar and tensor perturbations in Horava gravity, with
and without detailed balance, around a flat background. Once both types of
perturbations are taken into account, it is revealed that the theory is plagued
by ghost-like scalar instabilities in the range of parameters which would
render it power-counting renormalizable, that cannot be overcome by simple
tricks such as analytic continuation. Implementing a consistent flow between
the UV and IR limits seems thus more challenging than initially presumed,
regardless of whether the theory approaches General Relativity at low energies
or not. Even in the phenomenologically viable parameter space, the tensor
sector leads to additional potential problems, such as fine-tunings and
super-luminal propagation.Comment: 21 pages, version published at Class. Quant. Gra