967 research outputs found
A New Class of Four-Dimensional N=1 Supergravity with Non-minimal Derivative Couplings
In the N=1 four-dimensional new-minimal supergravity framework, we
supersymmetrise the coupling of the scalar kinetic term to the Einstein tensor.
This coupling, although introduces a non-minimal derivative interaction of
curvature to matter, it does not introduce harmful higher-derivatives. For this
construction, we employ off-shell chiral and real linear multiplets. Physical
scalars are accommodated in the chiral multiplet whereas curvature resides in a
linear one.Comment: 18 pages, version published at JHE
On the Onset of Inflation in Loop Quantum Cosmology
Using a Liouville measure, similar to the one proposed recently by Gibbons
and Turok, we investigate the probability that single-field inflation with a
polynomial potential can last long enough to solve the shortcomings of the
standard hot big bang model, within the semiclassical regime of loop quantum
cosmology. We conclude that, for such a class of inflationary models and for
natural values of the loop quantum cosmology parameters, a successful
inflationary scenario is highly improbable.Comment: 16 pages, 6 figures Amended version to appear in Phys. Rev.
Gravitational collapse and evolution of holographic black holes
Gravitational collapse is analyzed in the Brane-World by arguing that
regularity of five-dimensional geodesics require that stars on the brane have
an atmosphere. For the simple case of a spherically symmetric cloud of
non-dissipating dust, conditions are found for which the collapsing star
evaporates and approaches the Hawking behavior as the (apparent) horizon is
being formed. The effective energy of the star vanishes at a finite radius and
the star afterwards re-expands and "anti-evaporates". Israel junction
conditions across the brane (holographically related to the matter trace
anomaly) and the projection of the Weyl tensor on the brane (holographically
interpreted as the quantum back-reaction on the brane metric) contribute to the
total energy as, respectively, an "anti-evaporation" and an "evaporation" term.Comment: 6 pages; Talk given at QG05, Cala Gonone (Italy), September 200
Remarks on the Scalar Graviton Decoupling and Consistency of Horava Gravity
Recently Horava proposed a renormalizable gravity theory with higher
derivatives by abandoning the Lorenz invariance in UV. But there have been
confusions regarding the extra scalar graviton mode and the consistency of the
Horava model. I reconsider these problems and show that, in the Minkowski
vacuum background, the scalar graviton mode can be consistency decoupled from
the usual tensor graviton modes by imposing the (local) Hamiltonian as well as
the momentum constraints.Comment: Some clarifications regarding the projectable case added, Typos
corrected, Comments (Footnote No.9, Note Added) added, References updated,
Accepted in CQ
Two-dimensional Quantum Black Holes, Branes in BTZ and Holography
We solve semiclassical Einstein equations in two dimensions with a massive
source and we find a static, thermodynamically stable, quantum black hole
solution in the Hartle-Hawking vacuum state. We then study the black hole
geometry generated by a boundary mass sitting on a non-zero tension 1-brane
embedded in a three-dimensional BTZ black hole. We show that the two geometries
coincide and we extract, using holographic relations, information about the CFT
living on the 1-brane. Finally, we show that the quantum black hole has the
same temperature of the bulk BTZ, as expected from the holographic principle.Comment: 10 pages, 2 figures, RevTex, ``point particle of mass \mu '' changed
with ``massive boundary source'' for better clarity. Action in (50) written
in Z_2 symmetric form. Appendix clarified. Minor corrections and references
added. Version accepted for pubblication in PRD15 (2006
On-brane data for braneworld stars
Stellar structure in braneworlds is markedly different from that in ordinary
general relativity. As an indispensable first step towards a more general
analysis, we completely solve the ``on brane'' 4-dimensional Gauss and Codazzi
equations for an arbitrary static spherically symmetric star in a
Randall--Sundrum type II braneworld. We then indicate how this on-brane
boundary data should be propagated into the bulk in order to determine the full
5-dimensional spacetime geometry. Finally, we demonstrate how this procedure
can be generalized to solid objects such as planets.Comment: 5 pages, RevTeX4, v2: Main algorithm and results substantially
simplified, further discussion and references adde
Horava-Lifshitz gravity: a status report
This is intended to be a brief introduction and overview of Horava-Lifshitz
gravity. The motivation and all of the various version of the theory (to date)
are presented. The dynamics of the theory are discussed in some detail, with a
focus on low energy viability and consistency, as these have been the issues
that attracted most of the attention in the literature so far. Other properties
of the theory and developments within its framework are also covered, such as:
its relation to Einstein-aether theory, cosmology, and future perspectives.Comment: 17 pages, no figures, based on talk given at the 14th Conference on
Recent Developments in Gravity (NEBXIV), Ioannina, Greece, 8-11 Jun 2010; v2:
minor changes to match published version, references adde
Counterterms in semiclassical Horava-Lifshitz gravity
We analyze the semiclassical Ho\v{r}ava-Lifshitz gravity for quantum scalar
fields in 3+1 dimensions. The renormalizability of the theory requires that the
action of the scalar field contains terms with six spatial derivatives of the
field, i.e. in the UV, the classical action of the scalar field should preserve
the anisotropic scaling symmetry ( ,
with ) of the gravitational action. We discuss the renormalization
procedure based on adiabatic subtraction and dimensional regularization in the
weak field approximation. We verify that the divergent terms in the adiabatic
expansion of the expectation value of the energy-momentum tensor of the scalar
field contain up to six spatial derivatives, but do not contain more than two
time derivatives. We compute explicitly the counterterms needed for the
renormalization of the theory up to second adiabatic order and evaluate the
associated functions in the minimal subtraction scheme.Comment: 8 page
General Gauss-Bonnet brane cosmology
We consider 5-dimensional spacetimes of constant 3-dimensional spatial
curvature in the presence of a bulk cosmological constant. We find the general
solution of such a configuration in the presence of a Gauss-Bonnet term. Two
classes of non-trivial bulk solutions are found. The first class is valid only
under a fine tuning relation between the Gauss-Bonnet coupling constant and the
cosmological constant of the bulk spacetime. The second class of solutions are
static and are the extensions of the AdS-Schwarzchild black holes. Hence in the
absence of a cosmological constant or if the fine tuning relation is not true,
the generalised Birkhoff's staticity theorem holds even in the presence of
Gauss-Bonnet curvature terms. We examine the consequences in brane world
cosmology obtaining the generalised Friedmann equations for a perfect fluid
3-brane and discuss how this modifies the usual scenario.Comment: 20 pages, no figures, typos corrected, refs added, section IV changed
yielding novel result
Variation of G, and Vacuum Energy From Brane-World Models
In brane-world theory in five dimensions, the bulk metric is usually written
in gaussian coordinates, where and . However, the
choice is an external condition, not a requirement of the field
equations. In this paper we study the consequences of having , where and is a scalar function varying with
time, . This varying field entails the possibility of
variable fundamental physical "constants". These variations are different from
those predicted in scalar-tensor and multidimensional theories. We solve the
five-dimensional equations for a {\em fixed} brane and use the brane-world
paradigm to determine the fundamental parameters in the theory, which are the
vacuum energy , the gravitational coupling and the cosmological
term . We present specific models where these physical
quantities are variable functions of time. Different scenarios are possible but
we discuss with some detail a model for which and
, which seems to be favored by observations. Our
results are not in contradiction to previous ones in the literature. In
particular, to those where the brane is described as a domain wall moving in a
static bulk. Indeed these latter models in RS scenarios describe the
same spacetime as other solutions (with fixed brane) in gaussian coordinates
with . We conclude that the introduction of a time-varying
in brane-world theory yields a number of models that show variation in
the fundamental physical "constants" and exhibit reasonable physical
properties.Comment: In version 2 an error noticed by the author is fixed, and the
corresponding changes are made. Version 3 is identical to v2, except for a
couple of typos corrected. V3 will appear in Mod. Phys. Lett.
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