167 research outputs found
CDM Accelerating Cosmology as an Alternative to LCDM model
A new accelerating cosmology driven only by baryons plus cold dark matter
(CDM) is proposed in the framework of general relativity. In this model the
present accelerating stage of the Universe is powered by the negative pressure
describing the gravitationally-induced particle production of cold dark matter
particles. This kind of scenario has only one free parameter and the
differential equation governing the evolution of the scale factor is exactly
the same of the CDM model. For a spatially flat Universe, as predicted
by inflation (), it is found that the
effectively observed matter density parameter is ,
where is the constant parameter specifying the CDM particle creation
rate. The supernovae test based on the Union data (2008) requires so that as independently derived from weak
gravitational lensing, the large scale structure and other complementary
observations.Comment: 6 pages, 3 figure
Deflationary Models Driven by Matter Creation
A nonsingular deflationary cosmology driven by adiabatic matter creation is
proposed. In this scenario there is no preinflationary stage as happens in
conventional inflationary models. Deflation starts from a de Sitter spacetime
characterized by an arbitrary time scale , which also pins down an
initial value for the temperature of the universe. The model evolves
continuously towards a slightly modified Friedman-Robertson-Walker universe.
The horizon and other well known problems of the standard model are then solved
but, unlike in microscopic models of inflation, there is no supercooling and
subsequent reheating. Entropy generation is concomitant with deflation and if
is of the order of the Planck time, the present day value of the
radiation temperature is deduced. It is also shown that the ``age problem''
does not exist here. In particular, the theoretically favored FRW flat model is
old enough to agree with the observations even given the high values of
suggested by recent measurements.Comment: 17 pages, 2 figures(available from the authors), uses LATE
Transport coefficients and nonextensive statistics
We discuss the basic transport phenomena in gases and plasmas obeying the
-nonextensive velocity distribution (power-law). Analytical expressions for
the thermal conductivity () and viscosity () are derived by
solving the Boltzmann equation in the relaxation-time approximation. The
available experimental results to the ratio {}/ constrains the
-parameter on the interval . In the extensive limiting
case, the standard transport coefficients based on the local Gaussian
distribution are recovered, and due to a surprising cancellation, the electric
conductivity of a neutral plasma is not modified.Comment: 14 pages, 3 figures, REVTEX, submitted to PR
Bayesian Analysis and Constraints on Kinematic Models from Union SNIa
The kinematic expansion history of the universe is investigated by using the
307 supernovae type Ia from the Union Compilation set. Three simple model
parameterizations for the deceleration parameter (constant, linear and abrupt
transition) and two different models that are explicitly parametrized by the
cosmic jerk parameter (constant and variable) are considered. Likelihood and
Bayesian analyses are employed to find best fit parameters and compare models
among themselves and with the flat CDM model. Analytical expressions
and estimates for the deceleration and cosmic jerk parameters today ( and
) and for the transition redshift () between a past phase of cosmic
deceleration to a current phase of acceleration are given. All models
characterize an accelerated expansion for the universe today and largely
indicate that it was decelerating in the past, having a transition redshift
around 0.5. The cosmic jerk is not strongly constrained by the present
supernovae data. For the most realistic kinematic models the
confidence limits imply the following ranges of values: ,
and , which are compatible with the
CDM predictions, , and .
We find that even very simple kinematic models are equally good to describe the
data compared to the concordance CDM model, and that the current
observations are not powerful enough to discriminate among all of them.Comment: 13 pages. Matches published versio
Inhomogeneous models of interacting dark matter and dark energy
We derive and analyze a class of spherically symmetric cosmological models
whose source is an interactive mixture of inhomogeneous cold dark matter (DM)
and a generic homogeneous dark energy (DE) fluid. If the DE fluid corresponds
to a quintessense scalar field, the interaction term can be associated with a
well motivated non--minimal coupling to the DM component. By constructing a
suitable volume average of the DM component we obtain a Friedman evolution
equation relating this average density with an average Hubble scalar, with the
DE component playing the role of a repulsive and time-dependent term.
Once we select an ``equation of state'' linking the energy density () and
pressure () of the DE fluid, as well as a free function governing the radial
dependence, the models become fully determinate and can be applied to known
specific DE sources, such as quintessense scalar fields or tachyonic fluids.
Considering the simple equation of state with , we show that the free parameters and boundary conditions can be selected
for an adequate description of a local DM overdensity evolving in a suitable
cosmic background that accurately fits current observational data. While a DE
dominated scenario emerges in the asymptotic future, with total and
tending respectively to 1 and -1/2 for all cosmic observers, the effects of
inhomogeneity and anisotropy yield different local behavior and evolution rates
for these parameters in the local overdense region. We suggest that the models
presented can be directly applied to explore the effects of various DE
formalisms on local DM cosmological inhomogeneities.Comment: 15 pages, revtex4, 10 eps figure
An Accelerating Cosmology Without Dark Energy
The negative pressure accompanying gravitationally-induced particle creation
can lead to a cold dark matter (CDM) dominated, accelerating Universe (Lima et
al. 1996) without requiring the presence of dark energy or a cosmological
constant. In a recent study Lima et al. (2008, LSS) demonstrated that particle
creation driven cosmological models are capable of accounting for the SNIa
observations of the recent transition from a decelerating to an accelerating
Universe. Here we test the evolution of such models at high redshift using the
constraint on z_eq, the redshift of the epoch of matter radiation equality,
provided by the WMAP constraints on the early Integrated Sachs-Wolfe effect.
Since the contribution of baryons and radiation was ignored in the work of LSS,
we include them in our study of this class of models. The parameters of these
more realistic models with continuous creation of CDM is tested and constrained
at widely-separated epochs (z = z_eq and z = 0) in the evolution of the
Universe. This comparison reveals a tension between the high redshift CMB
constraint on z_eq and that which follows from the low redshift SNIa data,
challenging the viability of this class of models.Comment: 12 pages, 8 figure
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.
Big-Bang Cosmology with Photon Creation
The temperature evolution law is determined for an expanding FRW type
Universe with a mixture of matter and radiation where "adiabatic" creation of
photons has taken place. Taking into account this photon creation we discuss
the physical conditions for having a hot big bang Universe. We also compare our
results to the ones obtained from the standard FRW model.Comment: 9 pages, no figures, LaTex (RevTex). Minor corrections on the cover
page and reference
The hydrostatic equilibrium and Tsallis equilibrium for self-gravitating systems
Self-gravitating systems are generally thought to behavior non-extensively
due to the long-range nature of gravitational forces. We obtain a relation
between the nonextensive parameter q of Tsallis statistics, the temperature
gradient and the gravitational potential based on the equation of hydrostatic
equilibrium of self-gravitating systems. It is suggested that the nonextensive
parameter in Tsallis statistics has a clear physical meaning with regard to the
non-isothermal nature of the systems with long-range interactions and Tsallis
equilibrium distribution for the self-gravitating systems describes the
property of hydrostatic equilibrium of the systems.Comment: 7 pages, 9 Reference
Higher spin fields and the problem of cosmological constant
The cosmological evolution of free massless vector or tensor (but not gauge)
fields minimally coupled to gravity is analyzed. It is shown that there are
some unstable solutions for these fields in De Sitter background. The back
reaction of the energy-momentum tensor of such solutions to the original
cosmological constant exactly cancels the latter and the expansion regime
changes from the exponential to the power law one. In contrast to the
adjustment mechanism realized by a scalar field the gravitational coupling
constant in this model is time-independent and the resulting cosmology may
resemble the realistic one.Comment: 15 pages, Latex twic
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