2,235 research outputs found
Notes on Matter in Horava-Lifshitz Gravity
We investigate the dynamics of a scalar field governed by the Lifshitz-type
action which should appear naturally in Horava-Lifshitz gravity. The wave of
the scalar field may propagate with any speed without an upper bound. To
preserve the causality, the action cannot have a generic form. Due to the
superluminal propagation, a formation of a singularity may cause the breakdown
of the predictability of the theory. To check whether such a catastrophe could
occur in Horava-Lifshitz gravity, we investigate the dynamics of a dust. It
turns out that the dust does not collapse completely to form a singularity in a
generic situation, but expands again after it attains a maximum energy density.Comment: 14 pages, references adde
Warped compactification on curved manifolds
The characterization of a six- (or seven)-dimensional internal manifold with
metric as having positive, zero or negative curvature is expected to be an
important aspect of warped compactifications in supergravity. In this context,
Douglas and Kallosh recently pointed out that a compact internal space with
negative curvature could help to construct four-dimensional de Sitter solutions
only if the extra dimensions are strongly warped or there are large stringy
corrections. That is, the problem of finding 4-dimensional de Sitter solutions
is well posed, if all extra dimensions are physically compact, which is called
a no-go theorem. Here, we show that the above conclusion does not extend to a
general class of warped compactifications in classical supergravity that allow
a non-compact direction or cosmological solutions for which the internal space
is asymptotic to a cone over a product of compact Einstein spaces or spheres.
For clarity, we present classical solutions that compactify higher-dimensional
spacetime to produce a Robertson--Walker universe with de Sitter-type expansion
plus one extra non-compact direction. Such models are found to admit both an
effective four-dimensional Newton constant that remains finite and a
normalizable zero-mode graviton wavefunction. We also exhibit the possibility
of obtaining 4D de Sitter solutions by including the effect of fluxes (p-form
field strengths).Comment: 24 pages, 1 figure; v5 significant changes in the presentation,
published (journal) versio
Detailed balance condition and ultraviolet stability of scalar field in Horava-Lifshitz gravity
Detailed balance and projectability conditions are two main assumptions when
Horava recently formulated his theory of quantum gravity - the Horava-Lifshitz
(HL) theory. While the latter represents an important ingredient, the former
often believed needs to be abandoned, in order to obtain an ultraviolet stable
scalar field, among other things. In this paper, because of several attractive
features of this condition, we revisit it, and show that the scalar field can
be stabilized, if the detailed balance condition is allowed to be softly
broken. Although this is done explicitly in the non-relativistic general
covariant setup of Horava-Melby-Thompson with an arbitrary coupling constant
, generalized lately by da Silva, it is also true in other versions of
the HL theory. With the detailed balance condition softly breaking, the number
of independent coupling constants can be still significantly reduced. It is
remarkable to note that, unlike other setups, in this da Silva generalization,
there exists a master equation for the linear perturbations of the scalar field
in the flat Friedmann-Robertson-Walker background.Comment: Some typos are corrected. To appear in JCA
Particle Kinematics in Horava-Lifshitz Gravity
We study the deformed kinematics of point particles in the Horava theory of
gravity. This is achieved by considering particles as the optical limit of
fields with a generalized Klein-Gordon action. We derive the deformed geodesic
equation and study in detail the cases of flat and spherically symmetric
(Schwarzschild-like) spacetimes. As the theory is not invariant under local
Lorenz transformations, deviations from standard kinematics become evident even
for flat manifolds, supporting superluminal as well as massive luminal
particles. These deviations from standard behavior could be used for
experimental tests of this modified theory of gravity.Comment: Added references, corrected a typing erro
Particle Probe of Horava-Lifshitz Gravity
Kehagias-Sfetsos black hole in Ho\v{r}ava-Lifshitz gravity is probed through
particle geodesics. Gravitational force of KS black hole becomes weaker than
that of Schwarzschild around horizon and interior space. Particles can be
always scattered or trapped in new closed orbits, unlike those falling forever
in Schwarzschild black. The properties of null and timelike geodesics are
classified with values of coupling constants. The precession rates of the
orbits are evaluated. The time trajectories are also classified under different
values of coupling constants for both null and timelike geodesics. Physical
phenomena that may be observable are discussed.Comment: 10 pages, 8 figure
On Horava-Lifshitz "Black Holes"
The most general spherically symmetric solution with zero shift is found in
the non-projectable Horava-Lifshitz class of theories with general coupling
constants. It contains as special cases, spherically symmetric solutions found
by other authors earlier. It is found that the generic solution has
conventional (AdS, dS or flat) asymptotics with a universal 1/r tail. There are
several special cases where the asymptotics differ, including the detailed
balance choice of couplings. The conventional thermodynamics of this general
class of solutions is established by calculating the energy, temperature and
entropy. Although several of the solutions have conventional horizons, for
particles with ultra-luminal dispersion relations such solutions appear to be
horizonless.Comment: Latex 41 pages, 5 figure
Fluctuations in a Ho\v{r}ava-Lifshitz Bouncing Cosmology
Ho\v{r}ava-Lifshitz gravity is a potentially UV complete theory with
important implications for the very early universe. In particular, in the
presence of spatial curvature it is possible to obtain a non-singular bouncing
cosmology. The bounce is realized as a consequence of higher order spatial
curvature terms in the gravitational action. Here, we extend the study of
linear cosmological perturbations in Ho\v{r}ava-Lifshitz gravity coupled to
matter in the case when spatial curvature is present. As in the case without
spatial curvature, we find that there is no extra dynamical degree of freedom
for scalar metric perturbations. We study the evolution of fluctuations through
the bounce and show that the solutions remain non-singular throughout. If we
start with quantum vacuum fluctuations on sub-Hubble scales in the contracting
phase, and if the contracting phase is dominated by pressure-less matter, then
for and in the infrared limit the perturbations at late times are
scale invariant. Thus, Ho\v{r}ava-Lifshitz gravity can provide a realization of
the ``matter bounce'' scenario of structure formation.Comment: 19 page
The Black Hole and Cosmological Solutions in IR modified Horava Gravity
Recently Horava proposed a renormalizable gravity theory in four dimensions
which reduces to Einstein gravity with a non-vanishing cosmological constant in
IR but with improved UV behaviors. Here, I study an IR modification which
breaks "softly" the detailed balance condition in Horava model and allows the
asymptotically flat limit as well. I obtain the black hole and cosmological
solutions for "arbitrary" cosmological constant that represent the analogs of
the standard Schwartzschild-(A)dS solutions which can be asymptotically (A)dS
as well as flat and I discuss some thermodynamical properties. I also obtain
solutions for FRW metric with an arbitrary cosmological constant. I study its
implication to the dark energy and find that it seems to be consistent with
current observational data.Comment: Footnote 5 about the the very meaning of the horizons and Hawking
temperature is added; Accepted in JHE
Horava-Lifshitz f(R) Gravity
This paper is devoted to the construction of new type of f(R) theories of
gravity that are based on the principle of detailed balance. We discuss two
versions of these theories with and without the projectability condition.Comment: 22 pages, references adde
Caustic avoidance in Horava-Lifshitz gravity
There are at least four versions of Horava-Lishitz gravity in the literature.
We consider the version without the detailed balance condition with the
projectability condition and address one aspect of the theory: avoidance of
caustics for constant time hypersurfaces. We show that there is no caustic with
plane symmetry in the absence of matter source if \lambda\ne 1. If \lambda=1 is
a stable IR fixed point of the renormalization group flow then \lambda is
expected to deviate from 1 near would-be caustics, where the extrinsic
curvature increases and high-energy corrections become important. Therefore,
the absence of caustics with \lambda\ne 1 implies that caustics cannot form
with this symmetry in the absence of matter source. We argue that inclusion of
matter source will not change the conclusion. We also argue that caustics with
codimension higher than one will not form because of repulsive gravity
generated by nonlinear higher curvature terms. These arguments support our
conjecture that there is no caustic for constant time hypersurfaces. Finally,
we discuss implications to the recently proposed scenario of ``dark matter as
integration constant''.Comment: 19 pages; extended to general z \geq 3, typos corrected (v2); version
accepted for publication in JCAP (v3
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