198 research outputs found
The Scale Factor Potential Approach to Inflation
We propose a new approach to investigate inflation in a model-independent
way, and in particular to elaborate the involved observables, through the
introduction of the "scale factor potential". Through its use one can
immediately determine the inflation end, which corresponds to its first (and
global) minimum. Additionally, we express the inflationary observables in terms
of its logarithm, using as independent variable the e-folding number. As an
example, we construct a new class of scalar potentials that can lead to the
desired spectral index and tensor-to-scalar ratio, namely
and for 60 -folds, in agreement with observations.Comment: Accepted for Publishing in EPJ
Aspects of Horava-Lifshitz cosmology
We review some general aspects of Horava-Lifshitz cosmology. Formulating it
in its basic version, we extract the cosmological equations and we use
observational data in order to constrain the parameters of the theory. Through
a phase-space analysis we extract the late-time stable solutions, and we show
that eternal expansion, and bouncing and cyclic behavior can arise naturally.
Concerning the effective dark energy sector we show that it can describe the
phantom phase without the use of a phantom field. However, performing a
detailed perturbation analysis, we see that Horava-Lifshitz gravity in its
basic version suffers from instabilities. Therefore, suitable generalizations
are required in order for this novel theory to be a candidate for the
description of nature.Comment: 10 pages, 4 figures, invited talk given at the 2nd International
Workshop on Dark Matter, Dark Energy and Matter-Antimatter Assymetry,
National Tsing Hua University, Hsinchu, Taiwan, November 5-6, 201
On an exact hydrodynamic solution for the elliptic flow
Looking for the underlying hydrodynamic mechanisms determining the elliptic
flow we show that for an expanding relativistic perfect fluid the transverse
flow may derive from a solvable hydrodynamic potential, if the entropy is
transversally conserved and the corresponding expansion "quasi-stationary",
that is mainly governed by the temperature cooling. Exact solutions for the
velocity flow coefficients and the temperature dependence of the spatial
and momentum anisotropy are obtained and shown to be in agreement with the
elliptic flow features of heavy-ion collisions.Comment: 10 pages, 4 figure
Interacting Three Fluid System and Thermodynamics of the Universe Bounded by the Event Horizon
The work deals with the thermodynamics of the universe bounded by the event
horizon. The matter in the universe has three constituents namely dark energy,
dark matter and radiation in nature and interaction between then is assumed.
The variation of entropy of the surface of the horizon is obtained from unified
first law while matter entropy variation is calculated from the Gibbss' law.
Finally, validity of the generalized second law of thermodynamics is examined
and conclusions are written point wise.Comment: 7 page
The Cosmological Constant and Horava-Lifshitz Gravity
Horava-Lifshitz theory of gravity with detailed balance is plagued by the
presence of a negative bare (or geometrical) cosmological constant which makes
its cosmology clash with observations. We argue that adding the effects of the
large vacuum energy of quantum matter fields, this bare cosmological constant
can be approximately compensated to account for the small observed (total)
cosmological constant. Even though we cannot address the fine-tuning problem in
this way, we are able to establish a relation between the smallness of observed
cosmological constant and the length scale at which dimension 4 corrections to
the Einstein gravity become significant for cosmology. This scale turns out to
be approximately 5 times the Planck length for an (almost) vanishing observed
cosmological constant and we therefore argue that its smallness guarantees that
Lorentz invariance is broken only at very small scales. We are also able to
provide a first rough estimation for the infrared values of the parameters of
the theory and .Comment: 9 pages, Late
Restoring New Agegraphic Dark Energy in RS II Braneworld
Motivated by recent works [1,2], we investigate new agegraphic model of dark
energy in the framework of RS II braneworld. We also include the case of
variable gravitational constant in our model. Furthermore, we establish
correspondence between the new agegraphic dark energy with other dark energy
candidates based on scalar fields.Comment: 13 pages, accepted for publication in IJT
Viscous cosmology for early-And late-Time universe
© World Scientific Publishing Company. From a hydrodynamicist's point of view the inclusion of viscosity concepts in the macroscopic theory of the cosmic fluid would appear most natural, as an ideal fluid is after all an abstraction (exluding special cases such as superconductivity). Making use of modern observational results for the Hubble parameter plus standard Friedmann formalism, we may extrapolate the description of the universe back in time up to the inflationary era, or we may go to the opposite extreme and analyze the probable ultimate fate of the universe. In this review, we discuss a variety of topics in cosmology when it is enlarged in order to contain a bulk viscosity. Various forms of this viscosity, when expressed in terms of the fluid density or the Hubble parameter, are discussed. Furthermore, we consider homogeneous as well as inhomogeneous equations of state. We investigate viscous cosmology in the early universe, examining the viscosity effects on the various inflationary observables. Additionally, we study viscous cosmology in the late universe, containing current acceleration and the possible future singularities, and we investigate how one may even unify inflationary and late-Time acceleration. Finally, we analyze the viscosity-induced crossing through the quintessence-phantom divide, we examine the realization of viscosity-driven cosmological bounces, and we briefly discuss how the Cardy-Verlinde formula is affected by viscosity
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
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
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