238 research outputs found
Observational constraints on the unified dark matter and dark energy model based on the quark bag model
In this work we investigate if a small fraction of quarks and gluons, which
escaped hadronization and survived as a uniformly spread perfect fluid, can
play the role of both dark matter and dark energy. This fluid, as developed in
\citep{Brilenkov}, is characterized by two main parameters: , related to
the amount of quarks and gluons which act as dark matter; and , acting
as the cosmological constant. We explore the feasibility of this model at
cosmological scales using data from type Ia Supernovae (SNeIa), Long Gamma-Ray
Bursts (LGRB) and direct observational Hubble data. We find that: (i) in
general, cannot be constrained by SNeIa data nor by LGRB or H(z) data;
(ii) can be constrained quite well by all three data sets,
contributing with to the energy-matter content; (iii) when a
strong prior on (only) baryonic matter is assumed, the two parameters of the
model are constrained successfully.Comment: 10 pages, 6 figures, 3 table
Revisiting a model-independent dark energy reconstruction method
Model independent reconstructions of dark energy have received some
attention. The approach that addresses the reconstruction of the dimensionless
coordinate distance and its two first derivatives using a polynomial fit in
different redshift windows is well developed
\cite{DalyDjorgovski1,DalyDjorgovski2,DalyDjorgovski3}. In this work we offer
new insights into the problem by focusing on two types of observational probes:
SNeIa and GRBs. Our results allow to highlight some of the intrinsic weaknesses
of the method. One of the directions we follow is to consider updated
observational samples. Our results indicate than conclusions on the main dark
energy features as drawn from this method are intimately related to the
features of the samples themselves (which are not quite ideal). This is
particularly true of GRBs, which manifest themselves as poor performers in this
context. In contrast to original works, we conclude they cannot be used for
cosmological purposes, and the state of the art does not allow to regard them
on the same quality basis as SNeIa. The next direction we contribute to is the
question of how the adjusting of some parameters (window width, overlap,
selection criteria) affect the results. We find again there is a considerable
sensitivity to these features. Then, we try to establish what is the current
redshift range for which one can make solid predictions on dark energy
evolution. Finally, we strengthen the former view that this model is modest in
the sense it provides only a picture of the global trend. But, on the other
hand, we believe it offers an interesting complement to other approaches given
that it works on minimal assumptions.Comment: revtex4-1, 17 page
Varying constants entropic--CDM cosmology
We formulate the basic framework of thermodynamical entropic force cosmology
which allows variation of the gravitational constant and the speed of light
. Three different approaches to the formulation of the field equations are
presented. Some cosmological solutions for each framework are given and one of
them is tested against combined observational data (supernovae, BAO, and CMB).
From the fit of the data it is found that the Hawking temperature numerical
coefficient is two to four orders of magnitude less than usually
assumed on the geometrical ground theoretical value of and that it is
also compatible with zero. Besides, in the entropic scenario we observationally
test that the fit of the data is allowed for the speed of light growing and
the gravitational constant diminishing during the evolution of the
universe. We also obtain a bound on the variation of to be which is at least one order of magnitude weaker than the
quasar spectra observational bound.Comment: Matched with published version. some changes in Section VII, 15 page
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