338 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
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
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
Stable Inflationary Dissipative Cosmologies
The stability of the de Sitter era of cosmic expansion in spatially curved
homogeneous isotropic universes is studied. The source of the gravitational
field is an imperfect fluid such that the parameters that characterize it may
change with time. In this way we extend our previous analysis for
spatially-flat spaces as well as the work of Barrow.Comment: 13 pages, LaTeX 2.09, 1 figure. To be published in International
Journal of Modern Physics
Cosmological thermodynamics and deflationary gas universe
We establish a general thermodynamic scheme for cosmic fluids with internal
self-interactions and discuss equilibrium and non-equilibrium aspects of such
systems in connection with (generalized) symmetry properties of the
cosmological dynamics. As an example we construct an exactly solvable gas
dynamical model of a ``deflationary'' transition from an initial de Sitter
phase to a subsequent Friedmann-Lema\^{\i}tre-Robertson-Walker (FLRW) period.
We demonstrate that this dynamics represents a manifestation of a conformal
symmetry of an ``optical'' metric, characterized by a specific effective
refraction index of the cosmic medium.Comment: 12 pages, to appear in PR
On homothetic cosmological dynamics
We consider the homogeneous and isotropic cosmological fluid dynamics which
is compatible with a homothetic, timelike motion, equivalent to an equation of
state . By splitting the total pressure into the sum of an
equilibrium part and a non-equilibrium part , we find that on
thermodynamical grounds this split is necessarily given by and , corresponding to a dissipative stiff (Zel'dovich) fluid.Comment: 8 pages, to be published in Class. Quantum Gra
Kinetic theory for nongeodesic particle motion: Selfinteracting equilibrium states and effective viscous fluid pressures
The particles of a classical relativistic gas are supposed to move under the
influence of a quasilinear (in the particle four-momenta), self-interacting
force inbetween elastic, binary collisions. This force which is completely
fixed by the equilibrium conditions of the gas, gives rise to an effective
viscous pressure on the fluid phenomenological level. Earlier results
concerning the possibility of accelerated expansion of the universe due to
cosmological particle production are reinterpreted. A phenomenon such as power
law inflation may be traced back to specific self-interacting forces keeping
the particles of a gas universe in states of generalized equilibrium.Comment: 16 pages, latex, uses ioplppt.sty. To appear in Class. Quantum Gra
Conformal Symmetry and Cosmological Entropy Production
Introducing an effective refraction index of an isotropic cosmic medium, we investigate the cosmological fluid dynamics which is consistent with a conformal, timelike symmetry of a corresponding ``optical'' metric. We demonstrate that this kind of symmetry is compatible with the existence of a negative viscous pressure and, consequently, with cosmological entropy production. We establish an exactly solvable model according to which the viscous pressure is a consequence of a self-interacting one-particle force which is self-consistently exerted on the microscopic particles of a relativistic gas. Furthermore, we show that a sufficiently high decay rate of the refraction index of an ultrarelativistic cosmic medium results in an inflationary expansion of the universe
Cosmic anti-friction and accelerated expansion
We explain an accelerated expansion of the present universe, suggested from
observations of supernovae of type Ia at high redshift, by introducing an
anti-frictional force that is self-consistently exerted on the particles of the
cosmic substratum. Cosmic anti-friction, which is intimately related to
``particle production'', is shown to give rise to an effective negative
pressure of the cosmic medium. While other explanations for an accelerated
expansion (cosmological constant, quintessence) introduce a component of dark
energy besides ``standard'' cold dark matter (CDM) we resort to a
phenomenological one-component model of CDM with internal self-interactions. We
demonstrate how the dynamics of the LambdaCDM model may be recovered as a
special case of cosmic anti-friction. We discuss the connection with
two-component models and obtain an attractor behavior for the ratio of the
energy densities of both components which provides a possible phenomenological
solution to the coincidence problem.Comment: 19 pages, 7 (3 new) figures, new derivation of kinetic equation with
force term, accepted by Physical Review
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