709 research outputs found
Generalized second law of thermodynamics in f(T) gravity
We investigate the validity of the generalized second law (GSL) of
gravitational thermodynamics in the framework of f(T) modified teleparallel
gravity. We consider a spatially flat FRW universe containing only the
pressureless matter. The boundary of the universe is assumed to be enclosed by
the Hubble horizon. For two viable f(T) models containing
and , we first
calculate the effective equation of state and deceleration parameters. Then, we
investigate the null and strong energy conditions and conclude that a sudden
future singularity appears in both models. Furthermore, using a cosmographic
analysis we check the viability of two models. Finally, we examine the validity
of the GSL and find that for both models it is satisfied from the early times
to the present epoch. But in the future, the GSL is violated for the special
ranges of the torsion scalar T.Comment: 16 pages, 10 figures, accepted by JCAP 201
Horava-Lifshitz Dark Energy
We formulate Horava-Lifshitz cosmology with an additional scalar field that
leads to an effective dark energy sector. We find that, due to the inherited
features from the gravitational background, Horava-Lifshitz dark energy
naturally presents very interesting behaviors, possessing a varying
equation-of-state parameter, exhibiting phantom behavior and allowing for a
realization of the phantom divide crossing. In addition, Horava-Lifshitz dark
energy guarantees for a bounce at small scale factors and it may trigger the
turnaround at large scale factors, leading naturally to cyclic cosmology.Comment: 17 pages, no figures, version published at EJP
A Comparison of the LVDP and {\Lambda}CDM Cosmological Models
We compare the cosmological kinematics obtained via our law of linearly
varying deceleration parameter (LVDP) with the kinematics obtained in the
{\Lambda}CDM model. We show that the LVDP model is almost indistinguishable
from the {\Lambda}CDM model up to the near future of our universe as far as the
current observations are concerned, though their predictions differ
tremendously into the far future.Comment: 6 pages, 5 figures, 1 table, matches the version to be published in
International Journal of Theoretical Physic
A note on Friedmann equation of FRW universe in deformed Horava-Lifshitz gravity from entropic force
With entropic interpretation of gravity proposed by Verlinde, we obtain the
Friedmann equation of the Friedmann-Robertson-Walker universe for the deformed
Ho\v{r}ava-Lifshitz gravity. It is shown that, when the parameter of
Ho\v{r}ava-Lifshitz gravity , the modified Friedmann
equation will go back to the one in Einstein gravity. This results may imply
that the entropic interpretation of gravity is effective for the deformed
Ho\v{r}ava-Lifshitz gravity.Comment: 9 pages, no figure
Unconventional Cosmology
I review two cosmological paradigms which are alternative to the current
inflationary scenario. The first alternative is the "matter bounce", a
non-singular bouncing cosmology with a matter-dominated phase of contraction.
The second is an "emergent" scenario, which can be implemented in the context
of "string gas cosmology". I will compare these scenarios with the inflationary
one and demonstrate that all three lead to an approximately scale-invariant
spectrum of cosmological perturbations.Comment: 45 pages, 10 figures; invited lectures at the 6th Aegean Summer
School "Quantum Gravity and Quantum Cosmology", Chora, Naxos, Greece, Sept.
12 - 17 2012, to be publ. in the proceedings; these lecture notes form an
updated version of arXiv:1003.1745 and arXiv:1103.227
Matter bounce cosmology with a generalized single field: Non-Gaussianity and an extended no-go theorem
Large scale structure and cosmologyTheoretical Physic
Matter bounce cosmology with a generalized single field: Non-Gaussianity and an extended no-go theorem
Large scale structure and cosmologyTheoretical Physic
When primordial black holes from sound speed resonance meet a stochastic background of gravitational waves
Theoretical Physic
Does entropic force always imply the Newtonian force law?
We study the entropic force by introducing a bound between
entropy and area which was derived by imposing the non-gravitational collapse
condition. In this case, applying a modified entropic force to this system does
not lead to the Newtonian force law.Comment: 11 pages, version to appear in EPJ
Cardy-Verlinde formula of Kehagias-Sfetsos black hole
In this paper, we have shown that the entropy of the Kehagias-Sfetsos black
hole in Hoava-Lifshitz (HL) gravity can be expressed by the
Cardy-Verlinde formula. The later is supposed to be an entropy formula of
conformal field theory in any dimension.Comment: 10 pages, accepted by IJT
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