465 research outputs found
Conformal symmetry and deflationary gas universe
We describe the ``deflationary'' evolution from an initial de Sitter phase to
a subsequent Friedmann-Lema\^{\i}tre-Robertson-Walker (FLRW) period as a
specific non-equilibrium configuration of a self-interacting gas. The
transition dynamics corresponds to a conformal, timelike symmetry of an
``optical'' metric, characterized by a refraction index of the cosmic medium
which continously decreases from a very large initial value to unity in the
FLRW phase.Comment: 10 pages, to appear in "Exact Solutions and Scalar Fields in Gravity:
Recent Developments", ed. by A. Macias, J. Cervantes-Cota, and C.
L\"ammerzahl, Kluwer Academic Publishers 200
Scaling cosmology with variable dark-energy equation of state
Interactions between dark matter and dark energy which result in a power-law
behavior (with respect to the cosmic scale factor) of the ratio between the
energy densities of the dark components (thus generalizing the LCDM model) have
been considered as an attempt to alleviate the cosmic coincidence problem
phenomenologically. We generalize this approach by allowing for a variable
equation of state for the dark energy within the CPL-parametrization. Based on
analytic solutions for the Hubble rate and using the Constitution and Union2
SNIa sets, we present a statistical analysis and classify different interacting
and non-interacting models according to the Akaike (AIC) and the Bayesian (BIC)
information criteria. We do not find noticeable evidence for an alleviation of
the coincidence problem with the mentioned type of interaction.Comment: 21 pages, 11 figures, 11 tables, discussion improve
Cosmological particle production, causal thermodynamics, and inflationary expansion
Combining the equivalence between cosmological particle creation and an
effective viscous fluid pressure with the fact that the latter represents a
dynamical degree of freedom within the second-order Israel-Stewart theory for
imperfect fluids, we reconsider the possibility of accelerated expansion in
fluid cosmology. We find an inherent self-limitation for the magnitude of an
effective bulk pressure which is due to adiabatic (isentropic) particle
production. For a production rate which depends quadratically on the Hubble
rate we confirm the existence of solutions which describe a smooth transition
from inflationary to noninflationary behavior and discuss their interpretation
within the model of a decaying vacuum energy density. An alternative
formulation of the effective imperfect fluid dynamics in terms of a minimally
coupled scalar field is given. The corresponding potential is discussed and an
entropy equivalent for the scalar field is found.Comment: 16 pages, revtex file, submitted to Phys. Rev.
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
Generalised equilibrium of cosmological fluids in second-order thermodynamics
Combining the second-order entropy flow vector of the causal Israel-Stewart
theory with the conformal Killing-vector property of , where
is the four-velocity of the medium and T its equilibrium temperature, we
investigate generalized equilibrium states for cosmological fluids with
nonconserved particle number. We calculate the corresponding equilibrium
particle production rate and show that this quantity is reduced compared with
the results of the previously studied first-order theory. Generalized
equilibrium for massive particles turns out to be compatible with a dependence
of the fluid energy density on the scale factor a
of the Robertson-Walker metric and may be regarded as a realization of
so-called K-matter.Comment: 17 pages, iopfts.tex file, submitted to Class. Quantum Gra
Cosmology with Ricci-type dark energy
We consider the dynamics of a cosmological substratum of pressureless matter
and holographic dark energy with a cutoff length proportional to the Ricci
scale. Stability requirements for the matter perturbations are shown to single
out a model with a fixed relation between the present matter fraction
and the present value of the equation-of-state
parameter of the dark energy. This model has the same number of free parameters
as the CDM model but it has no CDM limit. We discuss the
consistency between background observations and the mentioned
stability-guaranteeing parameter combination.Comment: 6 pages, 3 figures, submitted to the Proceedings of the CosmoSurII
conference, Valpara\'iso, Chile, 27 - 31 May 201
Is the cosmological dark sector better modeled by a generalized Chaplygin gas or by a scalar field?
Both scalar fields and (generalized) Chaplygin gases have been widely used
separately to characterize the dark sector of the Universe. Here we investigate
the cosmological background dynamics for a mixture of both these components and
quantify the fractional abundances that are admitted by observational data from
supernovae of type Ia and from the evolution of the Hubble rate. Moreover, we
study how the growth rate of (baryonic) matter perturbations is affected by the
dark-sector perturbations.Comment: 20 pages, 9 figures, substantially revised, section on matter
perturbations added, accepted for publication in EPJ
Collisional equilibrium, particle production and the inflationary universe
Particle production processes in the expanding universe are described within
a simple kinetic model. The equilibrium conditions for a Maxwell-Boltzmann gas
with variable particle number are investigated. We find that radiation and
nonrelativistic matter may be in equilibrium at the same temperature provided
the matter particles are created at a rate that is half the expansion rate.
Using the fact that the creation of particles is dynamically equivalent to a
nonvanishing bulk pressure we calculate the backreaction of this process on the
cosmological dynamics. It turns out that the `adiabatic' creation of massive
particles with an equilibrium distribution for the latter necessarily implies
power-law inflation. Exponential inflation in this context is shown to become
inconsistent with the second law of thermodynamics after a time interval of the
order of the Hubble time.Comment: 19 pages, latex, no figures, to appear in Phys.Rev.
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