176 research outputs found
Friedmann Equations and Thermodynamics of Apparent Horizons
With the help of a masslike function which has dimension of energy and equals
to the Misner-Sharp mass at the apparent horizon, we show that the first law of
thermodynamics of the apparent horizon can be derived from the
Friedmann equation in various theories of gravity, including the Einstein,
Lovelock, nonlinear, and scalar-tensor theories. This result strongly suggests
that the relationship between the first law of thermodynamics of the apparent
horizon and the Friedmann equation is not just a simple coincidence, but rather
a more profound physical connection.Comment: no figures, V2: re-organized and re-writtend, main results and
conclusion unchanged, Phys. Rev. Lett. in pres
Energy conditions and current acceleration of the universe
The energy conditions provide a very promising model-independent study of the
current acceleration of the universe. However, in order to connect these
conditions with observations, one often needs first to integrate them, and then
find the corresponding constraints on some observational variables, such as the
distance modulus. Those integral forms can be misleading, and great caution is
needed when one interprets them physically. A typical example is that the
transition point of the deceleration parameter is at about in the CDM model. However, with the same model when we consider
the dimensionless Hubble parameter , which involves the integration of
, we find that does not cross the line of before . Therefore, to get the correct result, we cannot use the latter to determine
the transition point. With these in mind, we carefully study the constraints
from the energy conditions, and find that, among other things, the current
observational data indeed strongly indicate that our universe has ocne
experienced an accelerating expansion phase between the epoch of galaxy
formation and the present.Comment: revtex4, five figures. Corrected some typos and added new references.
Physics Letters B652, 63-68 (2007
Non-equilibrium condensation process in holographic superconductor with nonlinear electrodynamics
We study the non-equilibrium condensation process in a holographic
superconductor with nonlinear corrections to the U(1) gauge field. We start
with an asymptotic Anti-de-Sitter(AdS) black hole against a complex scalar
perturbation at the initial time, and solve the dynamics of the gravitational
systems in the bulk. When the black hole temperature T is smaller than a
critical value Tc, the scalar perturbation grows exponentially till saturation,
the final state of spacetime approaches to a hairy black hole. In the bulk
theory, we find the clue of the influence of nonlinear corrections in the gauge
field on the process of the scalar field condensation. We show that the bulk
dynamics in the non-equilibrium process is completely consistent with the
observations on the boundary order parameter. Furthermore we examine the time
evolution of horizons in the bulk non-equilibrium transformation process from
the bald AdS black hole to the AdS hairy hole. Both the evolution of apparent
and event horizons show that the original AdS black hole configuration requires
more time to finish the transformation to become a hairy black hole if there is
nonlinear correction to the electromagnetic field. We generalize our
non-equilibrium discussions to the holographic entanglement entropy and find
that the holographic entanglement entropy can give us further understanding of
the influence of the nonlinearity in the gauge field on the scalar
condensation. In our analysis, we also compare the effect of different models
on the corrections to the gauge field on the formation of holographic
superconductor.Comment: 22 pages,accepted for publication in JHE
Cosmological constant and late transient acceleration of the universe in the Horava-Witten Heterotic M-Theory on S^{1}/Z_{2}
Orbifold branes are studied in the framework of the 11-dimensional
Horava-Witten heterotic M-Theory. It is found that the effective cosmological
constant can be easily lowered to its current observational value by the
mechanism of large extra dimensions. The domination of this constant over the
evolution of the universe is only temporarily. Due to the interaction of the
bulk and the branes, the universe will be in its decelerating expansion phase
again in the future, whereby all problems connected with a far future de Sitter
universe are resolved.Comment: latex4 file, one figure. Version to be published in Physics Letters
Thermodynamics of an accelerated expanding universe
We investigate the laws of thermodynamics in an accelerating universe driven
by dark energy with a time-dependent equation of state. In the case we consider
that the physically relevant part of the Universe is that envelopped by the
dynamical apparent horizon, we have shown that both the first law and second
law of thermodynamics are satisfied. On the other hand, if the boundary of the
Universe is considered to be the cosmological event horizon the thermodynamical
description based on the definitions of boundary entropy and temperature breaks
down. No parameter redefinition can rescue the thermodynamics laws from such a
fate, rendering the cosmological event horizon unphysical from the point of
view of the laws of thermodynamics.Comment: 13 pages, 2 figure
Thermodynamical properties of dark energy
We have investigated the thermodynamical properties of dark energy. Assuming
that the dark energy temperature and considering that the volume
of the Universe enveloped by the apparent horizon relates to the temperature,
we have derived the dark energy entropy. For dark energy with constant equation
of state and the generalized Chaplygin gas, the derived entropy can be
positive and satisfy the entropy bound. The total entropy, including those of
dark energy, the thermal radiation and the apparent horizon, satisfies the
generalized second law of thermodynamics. However, for the phantom with
constant equation of state, the positivity of entropy, the entropy bound, and
the generalized second law cannot be satisfied simultaneously.Comment: 5 two column pages, 2 figures; v2: discussion on thermal equilibrium
with the horizon is added, v3: minor corrections, published in PR
On Relation between Capability and Leadership of College Students: Based on âAble Personâ Hypothesis
It is of necessity for institutions of higher education, given âability-orientedâ education idea to embody the concept of putting ability first and promote the leadership advancement of college students in developing themselves and the entire society with the starting point of capability. Leadership of college students showcases their capability and demonstrates as a collection of abile quality and this ability meets the need of modern society for college students to shoulder their social responsibilities, besides, leadership also consists of ability quality for college students to achieve their own development. Therefore, this paper strives to construct further the relationship of capability and leadership of college students following clarifying the relation of capability and leadership
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