750 research outputs found
Time-Symmetrization and Isotropization of Stiff-Fluid Kantowski-Sachs Universes
It is shown that growing-entropy stiff-fluid Kantowski-Sachs universes become
time-symmetric (if they start with time-asymmetric phase) and isotropize.
Isotropization happens without any inflationary era during the evolution since
there is no cosmological term here. It seems that this approach is an
alternative to inflation since the universe gets bigger and bigger approaching
'flatness'.Comment: 9 pages, no figure
Stability of Accelerated Expansion in Nonlinear Electrodynamics
This paper is devoted to study the phase space analysis of isotropic and
homogenous universe model by taking a noninteracting mixture of electromagnetic
and viscous radiating fluids whose viscous pressure satisfies a nonlinear
version of the Israel-Stewart transport equation. We establish an autonomous
system of equations by introducing normalized dimensionless variables. In order
to analyze stability of the system, we find corresponding critical points for
different values of the parameters. We also evaluate power-law scale factor
whose behavior indicates different phases of the universe model. It is
concluded that bulk viscosity as well as electromagnetic field enhances the
stability of accelerated expansion of the isotropic and homogeneous universe
model.Comment: 17 pages, 5 figures, accepted for publication in EPJ
Cosmic time and chaos
It is shown that the Friedman cosmological models (1) with bulk viscosity dissipation, (2) with Weyssenhoff fluid (perfect fluid with macroscopic spin), (3) with a phase transition in a very early stage of the evolution, all possessing negative space-curvature, after being compactified. exhibit chaotic behaviour in asymptotic states. Geodesic flows in such models are characterized by an exponential instability; they are mixing ergodic, and have non-zero metric entropy. In fact these world models are special cases of a "chaotic evolution" described by Lockhart, Misra and Prigogine. In particular, Prigogine’s "internal time" may be defined in them. Some remarks, concerning a predictability in cosmological models with the geodesic instability, are made
Cosmology and thermodynamics of FRW universe with bulk viscous stiff fluid
We consider a cosmological model dominated by stiff fluid with a constant
bulk viscosity. We classify all the possible cases of the universe predicted by
the model and analyzing the scale factor, density as well as the curvature
scalar. We find that when the dimensionless constant bulk viscous parameter is
in the range the model began with a Big Bang, and make a
transition form the decelerating expansion epoch to an accelerating epoch, then
tends to the de Sitter phase as . The transition into the
accelerating epoch would be in the recent past, when For
the model doesn't have a Big Bang and suffered an increase in the
fluid density and scalar curvature as the universe expands, which are
eventually saturates as the scale factor in the future. We have
analyzed the model with statefinder diagnostics and find that the model is
different from CDM model but approaches CDM point as We have also analyzed the status of the generalized second law of
thermodynamics with apparent horizon as the boundary of the universe and found
that the law is generally satisfied when and for
the law is satisfied when the scale factor is larger than a
minimum value
Can a matter-dominated model with constant bulk viscosity drive the accelerated expansion of the universe?
We test a cosmological model which the only component is a pressureless fluid
with a constant bulk viscosity as an explanation for the present accelerated
expansion of the universe. We classify all the possible scenarios for the
universe predicted by the model according to their past, present and future
evolution and we test its viability performing a Bayesian statistical analysis
using the SCP ``Union'' data set (307 SNe Ia), imposing the second law of
thermodynamics on the dimensionless constant bulk viscous coefficient \zeta and
comparing the predicted age of the universe by the model with the constraints
coming from the oldest globular clusters.
The best estimated values found for \zeta and the Hubble constant Ho are:
\zeta=1.922 \pm 0.089 and Ho=69.62 \pm 0.59 km/s/Mpc with a \chi^2=314. The age
of the universe is found to be 14.95 \pm 0.42 Gyr. We see that the estimated
value of Ho as well as of \chi^2 are very similar to those obtained from LCDM
model using the same SNe Ia data set. The estimated age of the universe is in
agreement with the constraints coming from the oldest globular clusters.
Moreover, the estimated value of \zeta is positive in agreement with the second
law of thermodynamics (SLT).
On the other hand, we perform different forms of marginalization over the
parameter Ho in order to study the sensibility of the results to the way how Ho
is marginalized. We found that it is almost negligible the dependence between
the best estimated values of the free parameters of this model and the way how
Ho is marginalized in the present work. Therefore, this simple model might be a
viable candidate to explain the present acceleration in the expansion of the
universe.Comment: 31 pages, 12 figures and 2 tables. Accepted to be published in the
Journal of Cosmology and Astroparticle Physics. Analysis using the new SCP
"Union" SNe Ia dataset instead of the Gold 2006 and ESSENCE datasets and
without changes in the conclusions. Added references. Related works:
arXiv:0801.1686 and arXiv:0810.030
Multijet production in neutral current deep inelastic scattering at HERA and determination of α_{s}
Multijet production rates in neutral current deep inelastic scattering have been measured in the range of exchanged boson virtualities 10 5 GeV and –1 < η_{LAB}^{jet} < 2.5. Next-to-leading-order QCD calculations describe the data well. The value of the strong coupling constant α_{s} (M_{z}), determined from the ratio of the trijet to dijet cross sections, is α_{s} (M_{z}) = 0.1179 ± 0.0013 (stat.)_{-0.0046}^{+0.0028}(exp.)_{-0.0046}^{+0.0028}(th.)
Photoproduction of mesons associated with a leading neutron
The photoproduction of mesons associated with a leading
neutron has been observed with the ZEUS detector in collisions at HERA
using an integrated luminosity of 80 pb. The neutron carries a large
fraction, {}, of the incoming proton beam energy and is detected at
very small production angles, { mrad}, an indication of
peripheral scattering. The meson is centrally produced with
pseudorapidity {
GeV}, which is large compared to the average transverse momentum of the neutron
of 0.22 GeV. The ratio of neutron-tagged to inclusive production is
in the photon-proton
center-of-mass energy range { GeV}. The data suggest that the
presence of a hard scale enhances the fraction of events with a leading neutron
in the final state.Comment: 28 pages, 4 figures, 2 table
Observation of Scaling Violations in Scaled Momentum Distributions at HERA
Charged particle production has been measured in deep inelastic scattering
(DIS) events over a large range of and using the ZEUS detector. The
evolution of the scaled momentum, , with in the range 10 to 1280
, has been investigated in the current fragmentation region of the Breit
frame. The results show clear evidence, in a single experiment, for scaling
violations in scaled momenta as a function of .Comment: 21 pages including 4 figures, to be published in Physics Letters B.
Two references adde
Multijet production in neutral current deep inelastic scattering at HERA and determination of alpha_s
Multijet production rates in neutral current deep inelastic scattering have
been measured in the range of exchanged boson virtualities 10 < Q2 < 5000 GeV2.
The data were taken at the ep collider HERA with centre-of-mass energy sqrt(s)
= 318 GeV using the ZEUS detector and correspond to an integrated luminosity of
82.2 pb-1. Jets were identified in the Breit frame using the k_T cluster
algorithm in the longitudinally invariant inclusive mode. Measurements of
differential dijet and trijet cross sections are presented as functions of jet
transverse energy E_{T,B}{jet}, pseudorapidity eta_{LAB}{jet} and Q2 with
E_{T,B}{jet} > 5 GeV and -1 < eta_{LAB}{jet} < 2.5. Next-to-leading-order QCD
calculations describe the data well. The value of the strong coupling constant
alpha_s(M_Z), determined from the ratio of the trijet to dijet cross sections,
is alpha_s(M_Z) = 0.1179 pm 0.0013(stat.) {+0.0028}_{-0.0046}(exp.)
{+0.0064}_{-0.0046}(th.)Comment: 22 pages, 5 figure
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