137 research outputs found
Black holes in the presence of dark energy
The new, rapidly developing field of theoretical research --- studies of dark
energy interacting with black holes (and, in particular, accreting onto black
holes) --- is reviewed. The term `dark energy' is meant to cover a wide range
of field theory models, as well as perfect fluids with various equations of
state, including cosmological dark energy. Various accretion models are
analyzed in terms of the simplest test field approximation or by allowing back
reaction on the black-hole metric. The behavior of various types of dark energy
in the vicinity of Schwarzschild and electrically charged black holes is
examined. Nontrivial effects due to the presence of dark energy in the black
hole vicinity are discussed. In particular, a physical explanation is given of
why the black hole mass decreases when phantom energy is being accreted, a
process in which the basic energy conditions of the famous theorem of
nondecreasing horizon area in classical black holes are violated. The
theoretical possibility of a signal escaping from beneath the black hole event
horizon is discussed for a number of dark energy models. Finally, the violation
of the laws of thermodynamics by black holes in the presence of noncanonical
fields is considered.Comment: 25 pages, 10 figures, review pape
Possible Explanation of the Geograv Detector Signal during the Explosion of SN 1987A in Modified Gravity Models
A change in gravity law in some regimes is predicted in the modified gravity
models that are actively discussed at present. In this paper, we consider a
possibility that the signal recorded by the Geograv resonant gravitational-wave
detector in 1987 during the explosion of SN 1987A was produced by an abrupt
change in the metric during the passage of a strong neutrino flux through the
detector. Such an impact on the detector is possible, in particular, in
extended scalar-tensor theories in which the local matter density gradient
affects the gravitational force. The first short neutrino pulse emitted at the
initial stage of stellar core collapse before the onset of neutrino opacity
could exert a major influence on the detector by exiting the detector response
at the main resonance frequency. In contrast, the influence of the subsequent
broad pulse (with a duration of several seconds) in the resonant detector is
exponentially suppressed, despite the fact that the second pulse carries an
order-of-magnitude more neutrino energy, and it could generate a signal in the
LSD neutrino detector. This explains the time delay of 1.4s between the Geograv
and LSD signals. The consequences of this effect of modified gravity for
LIGO/Virgo observations are discussed.Comment: 7 pages, 2 figures, minor corrections in the text with respect to the
published versio
Compact boson stars in K field theories
We study a scalar field theory with a non-standard kinetic term minimally
coupled to gravity. We establish the existence of compact boson stars, that is,
static solutions with compact support of the full system with self-gravitation
taken into account. Concretely, there exist two types of solutions, namely
compact balls on the one hand, and compact shells on the other hand. The
compact balls have a naked singularity at the center. The inner boundary of the
compact shells is singular, as well, but it is, at the same time, a Killing
horizon. These singular, compact shells therefore resemble black holes.Comment: Latex, 45 pages, 25 figures, some references and comments adde
On the Extra Mode and Inconsistency of Horava Gravity
We address the consistency of Horava's proposal for a theory of quantum
gravity from the low-energy perspective. We uncover the additional scalar
degree of freedom arising from the explicit breaking of the general covariance
and study its properties. The analysis is performed both in the original
formulation of the theory and in the Stueckelberg picture. A peculiarity of the
new mode is that it satisfies an equation of motion that is of first order in
time derivatives. At linear level the mode is manifest only around spatially
inhomogeneous and time-dependent backgrounds. We find two serious problems
associated with this mode. First, the mode develops very fast exponential
instabilities at short distances. Second, it becomes strongly coupled at an
extremely low cutoff scale. We also discuss the "projectable" version of
Horava's proposal and argue that this version can be understood as a certain
limit of the ghost condensate model. The theory is still problematic since the
additional field generically forms caustics and, again, has a very low strong
coupling scale. We clarify some subtleties that arise in the application of the
Stueckelberg formalism to Horava's model due to its non-relativistic nature.Comment: Discussion expanded; a figure added; accepted to JHE
k-Essence, superluminal propagation, causality and emergent geometry
The k-essence theories admit in general the superluminal propagation of the
perturbations on classical backgrounds. We show that in spite of the
superluminal propagation the causal paradoxes do not arise in these theories
and in this respect they are not less safe than General Relativity.Comment: 34 pages, 5 figure
The Imperfect Fluid behind Kinetic Gravity Braiding
We present a standard hydrodynamical description for non-canonical scalar
field theories with kinetic gravity braiding. In particular, this picture
applies to the simplest galileons and k-essence. The fluid variables not only
have a clear physical meaning but also drastically simplify the analysis of the
system. The fluid carries charges corresponding to shifts in field space. This
shift-charge current contains a spatial part responsible for diffusion of the
charges. Moreover, in the incompressible limit, the equation of motion becomes
the standard diffusion equation. The fluid is indeed imperfect because the
energy flows neither along the field gradient nor along the shift current. The
fluid has zero vorticity and is not dissipative: there is no entropy
production, the energy-momentum is exactly conserved, the temperature vanishes
and there is no shear viscosity. Still, in an expansion around a perfect fluid
one can identify terms which correct the pressure in the manner of bulk
viscosity. We close by formulating the non-trivial conditions for the
thermodynamic equilibrium of this imperfect fluid.Comment: 23 pages plus appendices. New version includes extended discussion on
diffusion and dynamics in alternative frames, as well as additional
references. v3 reflects version accepted for publication in JHEP: minor
comments added regarding suitability to numerical approache
On Isotropic Turbulence in the Dark Fluid Universe
As first part of this work, experimental information about the decay of
isotropic turbulence in ordinary hydrodynamics, u^2(t) proportional to
t^{-6/5}, is used as input in FRW equations in order to investigate how an
initial fraction f of turbulent kinetic energy in the cosmic fluid influences
the cosmological development in the late, quintessence/phantom, universe. First
order perturbative theory to the first order in f is employed. It turns out
that both in the Hubble factor, and in the energy density, the influence from
the turbulence fades away at late times. The divergences in these quantities
near the Big Rip behave essentially as in a non-turbulent fluid. However, for
the scale factor, the turbulence modification turns out to diverge
logarithmically. As second part of our work, we consider the full FRW equation
in which the turbulent part of the dark energy is accounted for by a separate
term. It is demonstrated that turbulence occurrence may change the future
universe evolution due to dissipation of dark energy. For instance,
phantom-dominated universe becomes asymptotically a de Sitter one in the
future, thus avoiding the Big Rip singularity.Comment: 10 pages, no figures, significant revision. Matches published versio
Cosmic coincidence problem and variable constants of physics
The standard model of cosmology is investigated using time dependent
cosmological constant and Newton's gravitational constant . The
total energy content is described by the modified Chaplygin gas equation of
state. It is found that the time dependent constants coupled with the modified
Chaplygin gas interpolate between the earlier matter to the later dark energy
dominated phase of the universe. We also achieve a convergence of parameter
, with minute fluctuations, showing an evolving . Thus our
model fairly alleviates the cosmic coincidence problem which demands
at present time.Comment: 27 pages, 15 figure
Answering a Basic Objection to Bang/Crunch Holography
The current cosmic acceleration does not imply that our Universe is basically
de Sitter-like: in the first part of this work we argue that, by introducing
matter into *anti-de Sitter* spacetime in a natural way, one may be able to
account for the acceleration just as well. However, this leads to a Big Crunch,
and the Euclidean versions of Bang/Crunch cosmologies have [apparently]
disconnected conformal boundaries. As Maldacena and Maoz have recently
stressed, this seems to contradict the holographic principle. In the second
part we argue that this "double boundary problem" is a matter not of geometry
but rather of how one chooses a conformal compactification: if one chooses to
compactify in an unorthodox way, then the appearance of disconnectedness can be
regarded as a *coordinate effect*. With the kind of matter we have introduced
here, namely a Euclidean axion, the underlying compact Euclidean manifold has
an unexpectedly non-trivial topology: it is in fact one of the 75 possible
underlying manifolds of flat compact four-dimensional Euclidean spaces.Comment: 29 pages, 3 figures, added references and comparison with "cyclic"
cosmology, JHEP versio
Non-vacuum Solutions of Bianchi Type VI_0 Universe in f(R) Gravity
In this paper, we solve the field equations in metric f(R) gravity for
Bianchi type VI_0 spacetime and discuss evolution of the expanding universe. We
find two types of non-vacuum solutions by taking isotropic and anisotropic
fluids as the source of matter and dark energy. The physical behavior of these
solutions is analyzed and compared in the future evolution with the help of
some physical and geometrical parameters. It is concluded that in the presence
of isotropic fluid, the model has singularity at and represents
continuously expanding shearing universe currently entering into phantom phase.
In anisotropic fluid, the model has no initial singularity and exhibits the
uniform accelerating expansion. However, the spacetime does not achieve
isotropy as in both of these solutions.Comment: 20 pages, 5 figures, accepted for publication in Astrophys. Space Sc
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