6,469 research outputs found
Interacting entropy-corrected holographic dark energy with apparent horizon as an infrared cutoff
In this work we consider the entropy-corrected version of interacting
holographic dark energy (HDE), in the non-flat universe enclosed by apparent
horizon. Two corrections of entropy so-called logarithmic 'LEC' and power-law
'PLEC' in HDE model with apparent horizon as an IR-cutoff are studied. The
ratio of dark matter to dark energy densities , equation of state parameter
and deceleration parameter are obtained. We show that the cosmic
coincidence is satisfied for both interacting models. By studying the effect of
interaction in EoS parameter, we see that the phantom divide may be crossed and
also find that the interacting models can drive an acceleration expansion at
the present and future, while in non-interacting case, this expansion can
happen only at the early time. The graphs of deceleration parameter for
interacting models, show that the present acceleration expansion is preceded by
a sufficiently long period deceleration at past. Moreover, the thermodynamical
interpretation of interaction between LECHDE and dark matter is described. We
obtain a relation between the interaction term of dark components and thermal
fluctuation in a non-flat universe, bounded by the apparent horizon. In
limiting case, for ordinary HDE, the relation of interaction term versus
thermal fluctuation is also calculated.Comment: 20 pages, 8 figures, figures changed, some Ref. is added, changed
some sentences, accepted by General relativity and gravitation (GERG
Solutions to the Cosmic Initial Entropy Problem without Equilibrium Initial Conditions
The entropy of the observable universe is increasing. Thus, at earlier times
the entropy was lower. However, the cosmic microwave background radiation
reveals an apparently high entropy universe close to thermal and chemical
equilibrium. A two-part solution to this cosmic initial entropy problem is
proposed. Following Penrose, we argue that the evenly distributed matter of the
early universe is equivalent to low gravitational entropy. There are two
competing explanations for how this initial low gravitational entropy comes
about. (1) Inflation and baryogenesis produce a virtually homogeneous
distribution of matter with a low gravitational entropy. (2) Dissatisfied with
explaining a low gravitational entropy as the product of a 'special' scalar
field, some theorists argue (following Boltzmann) for a 'more natural' initial
condition in which the entire universe is in an initial equilibrium state of
maximum entropy. In this equilibrium model, our observable universe is an
unusual low entropy fluctuation embedded in a high entropy universe. The
anthropic principle and the fluctuation theorem suggest that this low entropy
region should be as small as possible and have as large an entropy as possible,
consistent with our existence. However, our low entropy universe is much larger
than needed to produce observers, and we see no evidence for an embedding in a
higher entropy background. The initial conditions of inflationary models are as
natural as the equilibrium background favored by many theorists.Comment: 10 pages, 5 figure
Cosmology
In these lectures we first concentrate on the cosmological problems which,
hopefully, have to do with the new physics to be probed at the LHC: the nature
and origin of dark matter and generation of matter-antimatter asymmetry. We
give several examples showing the LHC cosmological potential. These are WIMPs
as cold dark matter, gravitinos as warm dark matter, and electroweak
baryogenesis as a mechanism for generating matter-antimatter asymmetry. In the
remaining part of the lectures we discuss the cosmological perturbations as a
tool for studying the epoch preceeding the conventional hot stage of the
cosmological evolution.Comment: 47 pages, set of lectures given at the 2011 European School of
High-Energy Physics, Cheile Gradistei, Romania, 7-20 Sep 2011, edited by C.
Grojean, M. Mulder
Thermodynamics, stability and Hawking-Page transition of Kerr black holes from R\'enyi statistics
Thermodynamics of rotating black holes described by the R\'enyi formula as
equilibrium and zeroth law compatible entropy function is investigated. We show
that similarly to the standard Boltzmann approach, isolated Kerr black holes
are stable with respect to axisymmetric perturbations in the R\'enyi model. On
the other hand, when the black holes are surrounded by a bath of thermal
radiation, slowly rotating black holes can also be in stable equilibrium with
the heat bath at a fixed temperature, in contrast to the Boltzmann description.
For the question of possible phase transitions in the system, we show that a
Hawking-Page transition and a first order small black hole/large black hole
transition occur, analogous to the picture of rotating black holes in AdS
space. These results confirm the similarity between the R\'enyi-asymptotically
flat and Boltzmann-AdS approaches to black hole thermodynamics in the rotating
case as well. We derive the relations between the thermodynamic parameters
based on this correspondence.Comment: 29 pages, 20 figure
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