78 research outputs found
Dark Energy: A Unifying View
Different models of the cosmic substratum which pretend to describe the
present stage of accelerated expansion of the Universe like the CDM
model or a Chaplygin gas, can be seen as special realizations of a holographic
dark energy cosmology if the option of an interaction between pressurless dark
matter and dark energy is taken seriously. The corresponding interaction
strength parameter plays the role of a cosmological constant. Differences occur
at the perturbative level. In particular, the pressure perturbations are
intrinsically non-adiabatic.Comment: 9 pages, selected for "Honorable Mention" by the Gravity Research
Foundatio
Cosmic evolution during primordial black hole evaporation
Primordial black holes with a narrow mass range are regarded as a
nonrelativistic fluid component with an equation of state for dust. The impact
of the black hole evaporation on the dynamics of the early universe is studied
by resorting to a two-fluid model. We find periods of intense radiation
reheating in the initial and final stages of the evaporation.Comment: 12 pages, Revtex, two figures, to appear in Phys.Rev.
Holographic Dark Energy and Present Cosmic Acceleration
We review the notion of holographic dark energy and assess its significance
in the light of the well documented cosmic acceleration at the present time. We
next propose a model of holographic dark energy in which the infrared cutoff is
set by the Hubble scale. The model accounts for the aforesaid acceleration and,
by construction, is free of the cosmic coincidence problem.Comment: 6 pages. To appear in the Proceedings of the XXVIII Spanish
Relativity Meeting. Key words: Cosmology, Holography, Late accelerated
expansion, Dark energ
Self-interacting gas in a gravitational wave field
We investigate a relativistic self-interacting gas in the field of an
external {\it pp} gravitational wave. Based on symmetry considerations we ask
for those forces which are able to compensate the imprint of the gravitational
wave on the macroscopic 4-acceleration of the gaseous fluid. We establish an
exactly solvable toy model according to which the stationary states which
characterize such a situation have negative entropy production and are
accompanied by instabilities of the microscopic particle motion. These features
are similar to those which one encounters in phenomena of self-organization in
many-particle systems.Comment: 17 pages, to be published in the GRG-Journa
Thermodynamic equilibrium in the expanding universe
We show that a relativistic gas may be at ``global'' equilibrium in the
expanding universe for any equation of state , provided
that the gas particles move under the influence of a self-interacting,
effective one-particle force in between elastic binary collisions. In the
force-free limit we recover the equilibrium conditions for ultrarelativistic
matter which imply the existence of a conformal timelike Killing vector.Comment: 10 pages, Latex, to appear in GR
"Understanding" cosmological bulk viscosity
A universe consisting of two interacting perfect fluids with the same
4-velocity is considered. A heuristic mean free time argument is used to show
that the system as a whole cannot be perfect as well but neccessarily implies a
nonvanishing bulk viscosity. A new formula for the latter is derived and
compared with corresponding results of radiative hydrodynamics.Comment: 12 pages, Latex file, to appear in MNRA
Inflation in a self-interacting gas universe
We show that a de Sitter spacetime is a solution of Einstein's field
equations with the energy momentum tensor of a self-interacting, classical
Maxwell-Boltzmann gas in collisional equilibrium. The self-interaction is
described by a four-force which is quadratic in the (spatially projected)
particle four-momenta. This force does not preserve the particle number and
gives rise to an exponential increase in the comoving entropy of the universe
while the temperature of the latter remains constant. These properties of a gas
universe are related to the existence of a ``projector-conformal'' timelike
Killing vector representing a symmetry which is ``in between'' the symmetries
characterized by a Killing vector and those characterized by a conformal
Killing vector.Comment: 10 pages, Revtex, Journal reference: Phys.Rev.D58 063503 (1998
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