12 research outputs found
Braneworld models with a non-minimally coupled phantom bulk field: a simple way to obtain the -1-crossing at late times
We investigate general braneworld models, with a non-minimally coupled
phantom bulk field and arbitrary brane and bulk matter contents. We show that
the effective dark energy of the brane-universe acquires a dynamical nature, as
a result of the non-minimal coupling which provides a mechanism for an indirect
"bulk-brane interaction" through gravity. For late-time cosmological evolution
and without resorting to special ansatzes or to specific areas of the parameter
space, we show that the -1-crossing of its equation-of-state parameter is
general and can be easily achieved. As an example we provide a simple, but
sufficiently general, approximate analytical solution, that presents the
crossing behavior.Comment: 11 pages, 2 figure
The phase portrait of a matter bounce in Horava-Lifshitz cosmology
The occurrence of a bounce in FRW cosmology requires modifications of general
relativity. An example of such a modification is the recently proposed
Horava-Lifshitz theory of gravity, which includes a ``dark radiation'' term
with a negative coefficient in the analog of the Friedmann equation. This paper
describes a phase space analysis of models of this sort with the aim of
determining to what extent bouncing solutions can occur. A simplification,
valid in the relevant region, allows a reduction of the dimension of phase
space so that visualization in three dimensions is possible. It is found that a
bounce is possible, but not generic in models under consideration. Apart from
previously known bouncing solutions some new ones are also described. Other
interesting solutions found include ones which describe a novel sort of
oscillating universes.Comment: 14 pages, 8 figure
Modified F(R) Horava-Lifshitz gravity: a way to accelerating FRW cosmology
We propose a general approach for the construction of modified gravity which
is invariant under foliation-preserving diffeomorphisms. Special attention is
paid to the formulation of modified Ho\v{r}ava-Lifshitz gravity (FRHL),
whose Hamiltonian structure is studied. It is demonstrated that the
spatially-flat FRW equations of FRHL are consistent with the constraint
equations. The analysis of de Sitter solutions for several versions of FRHL
indicates that the unification of the early-time inflation with the late-time
acceleration is possible. It is shown that a special choice of parameters for
FRHL leads to the same spatially-flat FRW equations as in the case of
traditional -gravity. Finally, an essentially most general modified
Ho\v{r}ava-Lifshitz gravity is proposed, motivated by its fully
diffeomorphism-invariant counterpart, with the restriction that the action does
not contain derivatives higher than the second order with respect to the time
coordinate.Comment: LaTeX 11 pages. v4: Some errors have been correcte
Horava-Lifshitz Dark Energy
We formulate Horava-Lifshitz cosmology with an additional scalar field that
leads to an effective dark energy sector. We find that, due to the inherited
features from the gravitational background, Horava-Lifshitz dark energy
naturally presents very interesting behaviors, possessing a varying
equation-of-state parameter, exhibiting phantom behavior and allowing for a
realization of the phantom divide crossing. In addition, Horava-Lifshitz dark
energy guarantees for a bounce at small scale factors and it may trigger the
turnaround at large scale factors, leading naturally to cyclic cosmology.Comment: 17 pages, no figures, version published at EJP
New agegraphic dark energy in Horava-Lifshitz cosmology
We investigate the new agegraphic dark energy scenario in a universe governed
by Horava-Lifshitz gravity. We consider both the detailed and non-detailed
balanced version of the theory, we impose an arbitrary curvature, and we allow
for an interaction between the matter and dark energy sectors. Extracting the
differential equation for the evolution of the dark energy density parameter
and performing an expansion of the dark energy equation-of-state parameter, we
calculate its present and its low-redshift value as functions of the dark
energy and curvature density parameters at present, of the Horava-Lifshitz
running parameter , of the new agegraphic dark energy parameter ,
and of the interaction coupling . We find that
and . Although this analysis indicates that the
scenario can be compatible with observations, it does not enlighten the
discussion about the possible conceptual and theoretical problems of
Horava-Lifshitz gravity.Comment: 17 pages, no figures, version published at JCA
Horava-Lifshitz Holography
We derive the detailed balance condition as a solution to the Hamilton-Jacobi
equation in the Horava-Lifshitz gravity. This result leads us to propose the
existence of the d-dimensional quantum field theory on the future boundary of
the (d+1)-dimensional Horava-Lifshitz gravity from the viewpoint of the
holographic renormalization group. We also obtain a Ricci flow equation of the
boundary theory as the holographic RG flow, which is the Hamilton equation in
the bulk gravity, by tuning parameters in the theory.Comment: 7 page
Observational constraints on Horava-Lifshitz cosmology
We use observational data from Type Ia Supernovae (SNIa), Baryon Acoustic
Oscillations (BAO), and Cosmic Microwave Background (CMB), along with
requirements of Big Bang Nucleosynthesis (BBN), to constrain the cosmological
scenarios governed by Horava-Lifshitz gravity. We consider both the detailed
and non-detailed balance versions of the gravitational sector, and we include
the matter and radiation sectors. We conclude that the detailed-balance
scenario cannot be ruled out from the observational point of view, however the
corresponding likelihood contours impose tight constraints on the involved
parameters. The scenario beyond detailed balance is compatible with
observational data, and we present the corresponding stringent constraints and
contour-plots of the parameters. Although this analysis indicates that
Horava-Lifshitz cosmology can be compatible with observations, it does not
enlighten the discussion about its possible conceptual and theoretical
problems.Comment: 11 pages, 6 figures, version published in JCA
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
Geometrothermodynamics in Horava-Lifshitz gravity
We investigate the thermodynamic geometries of the most general static,
spherically symmetric, topological black holes of the Ho\v{r}ava--Lifshitz
gravity. In particular, we show that a Legendre invariant metric derived in the
context of geometrothermodynamics for the equilibrium manifold reproduces
correctly the phase transition structure of these black holes. Moreover, the
limiting cases in which the mass, the entropy or the Hawking temperature vanish
are also accompanied by curvature singularities which indicate the limit of
applicability of the thermodynamics and the geometrothermodynamics of black
holes. The Einstein limit and the case of a black hole with flat horizon are
also investigated.Comment: Preliminary draf