214 research outputs found
Quantum Singularities in Horava-Lifshitz Cosmology
The recently proposed Horava-Lifshitz (HL) theory of gravity is analyzed from
the quantum cosmology point of view. By employing usual quantum cosmology
techniques, we study the quantum Friedmann-Lemaitre-Robertson-Walker (FLRW)
universe filled with radiation in the context of HL gravity. We find that this
universe is quantum mechanically nonsingular in two different ways: the
expectation value of the scale factor never vanishes and, if we
abandon the detailed balance condition suggested by Horava, the quantum
dynamics of the universe is uniquely determined by the initial wave packet and
no boundary condition at is indeed necessary.Comment: 13 pages, revtex, 1 figure. Final version to appear in PR
Tunneling probability for the birth of an universe with radiation in Horava-Lifshitz theory
In the present work, we study the birth of a homogeneous and isotropic
Friedmann Lemaitre Robertson Walker (FLRW) cosmological model, considering
Horava Lifshitz (HL) as the gravitational theory. The matter content of the
model is a radiation perfect fluid. In order to study the birth of the universe
in the present model, we consider the quantum cosmology mechanism of creation
from nothing. In that mechanism, the universe appears after the wavefunction
associated to that universe tunnels through a potential barrier. We started
studying the classical model. We draw the phase portrait of the model and
identify qualitatively all types of dynamical behaviors associated to it. Then,
we write the Hamiltonian of the model and apply the Dirac quantization
procedure to quantize a constrained theory. We find the appropriate
Wheeler-DeWitt equation and solve it using the Wentzel Kramers Brillouin (WKB)
approximation. Using the WKB solution, to the Wheeler DeWitt equation, we
compute the tunneling probabilities for the birth of that universe (TPWKB).
Since the WKB wavefunction depends on the radiation energy (E) and the free
parameters coming from the HL theory (gc, gr, gs, gLambda), we compute the
behavior of TPWKB as a function of E and all the HL parameters gc, gr, gs,
gLambda.Comment: The paper has 20 pages and 9 figure
Lorentz symmetry is relevant
We set up a covariant renormalisation group equation on a foliated spacetime
which preserves background diffeomorphism symmetry. As a first application of
the new formalism, we study the effect of quantum fluctuations in Lorentz
symmetry breaking theories of quantum gravity. It is found that once a small
breaking is introduced e.g. at the Planck scale, quantum fluctuations enhance
this breaking at low energies. A numerical analysis shows that the
magnification is of order unity for trajectories compatible with a small
cosmological constant. The immediate consequence is that the stringent
observational constraints on Lorentz symmetry breaking are essentially
scale-independent and must be met even at the Planck scale.Comment: 10 pages, 1 figur
Hypothesis on the Nature of Time
We present numerical evidence that fictitious diffusing particles in the
causal dynamical triangulation (CDT) approach to quantum gravity exceed the
speed of light on small distance scales. We argue this superluminal behaviour
is responsible for the appearance of dimensional reduction in the spectral
dimension. By axiomatically enforcing a scale invariant speed of light we show
that time must dilate as a function of relative scale, just as it does as a
function of relative velocity. By calculating the Hausdorff dimension of CDT
diffusion paths we present a seemingly equivalent dual description in terms of
a scale dependent Wick rotation of the metric. Such a modification to the
nature of time may also have relevance for other approaches to quantum gravity.Comment: 15 pages, 4 figures. Conforms with version to be published in PRD.
Clarifications and references adde
Mimetic gravity: a review of recent developments and applications to cosmology and astrophysics
Mimetic gravity is a Weyl-symmetric extension of General Relativity, related
to the latter by a singular disformal transformation, wherein the appearance of
a dust-like perfect fluid can mimic cold dark matter at a cosmological level.
Within this framework, it is possible to provide an unified geometrical
explanation for dark matter, the late-time acceleration, and inflation, making
it a very attractive theory. In this review, we summarize the main aspects of
mimetic gravity, as well as extensions of the minimal formulation of the model.
We devote particular focus to the reconstruction technique, which allows the
realization of any desired expansionary history of the Universe by an accurate
choice of potential, or other functions defined within the theory (as in the
case of mimetic gravity). We briefly discuss cosmological perturbation
theory within mimetic gravity. As a case study within which we apply the
concepts previously discussed, we study a mimetic Ho\v{r}ava-like theory, of
which we explore solutions and cosmological perturbations in detail. Finally,
we conclude the review by discussing static spherically symmetric solutions
within mimetic gravity, and apply our findings to the problem of galactic
rotation curves. Our review provides an introduction to mimetic gravity, as
well as a concise but self-contained summary of recent findings, progresses,
open questions, and outlooks on future research directions.Comment: 68 pages, invited review to appear in Advances in High Energy Physic
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