41,771 research outputs found
Dark matter interacts with variable vacuum energy
We investigate a spatially flat Friedmann-Robertson-Walker (FRW) scenario
with two interacting components, dark matter and variable vacuum energy (VVE)
densities, plus two decoupled components, one is a baryon term while the other
behaves as a radiation component. We consider a linear interaction in the
derivative dark component density. We apply the method to the
observational Hubble data for constraining the cosmological parameters and
analyze the amount of dark energy in the radiation era for the model. It turns
out that our model fulfills the severe bound of at level, so is consistent with the recent analysis that
includes cosmic microwave background anisotropy measurements from Planck
survey, the future constraints achievable by Euclid and CMBPol experiments,
reported for the behavior of the dark energy at early times, and fulfills the
stringent bound at level in the
big-bang nucleosynthesis epoch. We also examine the cosmic age problem at high
redshift associated with the old quasar APM 08279+5255 and estimate the age of
the universe today.Comment: 8 pages, 12 figures. arXiv admin note: text overlap with
arXiv:1310.5335 by other author
Extended tachyon field using form invariance symmetry
In this work we illustrate how form-invariance transformations (FIT) can be
used to construct phantom and complementary tachyon cosmologies from standard
tachyon field universes. We show how these transformations act on the Hubble
expansion rate, the energy density, and pressure of the tachyon field. The FIT
generate new cosmologies from a known "seed" one, in particular from the
ordinary tachyon field we obtain two types of tachyon species, denominated
phantom and complementary tachyon. We see that the FIT allow us to pass from a
non-stable cosmology to a stable one and vice-versa, as appeared in the
literature. Finally, as an example, we apply the transformations to a
cosmological fluid with an inverse square potential, , and
generate the extended tachyon field.Comment: 4 pages, 0 figure
Measuring the transition to homogeneity with photometric redshift surveys
We study the possibility of detecting the transition to homogeneity using
photometric redshift catalogs. Our method is based on measuring the fractality
of the projected galaxy distribution, using angular distances, and relies only
on observable quantites. It thus provides a way to test the Cosmological
Principle in a model-independent unbiased way. We have tested our method on
different synthetic inhomogeneous catalogs, and shown that it is capable of
discriminating some fractal models with relatively large fractal dimensions, in
spite of the loss of information due to the radial projection. We have also
studied the influence of the redshift bin width, photometric redshift errors,
bias, non-linear clustering, and surveyed area, on the angular homogeneity
index H2 ({\theta}) in a {\Lambda}CDM cosmology. The level to which an upcoming
galaxy survey will be able to constrain the transition to homogeneity will
depend mainly on the total surveyed area and the compactness of the surveyed
region. In particular, a Dark Energy Survey (DES)-like survey should be able to
easily discriminate certain fractal models with fractal dimensions as large as
D2 = 2.95. We believe that this method will have relevant applications for
upcoming large photometric redshift surveys, such as DES or the Large Synoptic
Survey Telescope (LSST).Comment: 14 pages, 14 figure
Anomalous Gauge Boson Couplings in the e^+ e^- -> ZZ Process
We discuss experimental aspects related to the process and to the search for anomalous ZZV couplings
(V) at LEP2 and future colliders. We
present two possible approaches for a realistic study of the reaction and
discuss the differences between them. We find that the optimal method to study
double Z resonant production and to quantify the presence of anomalous
couplings requires the use of a complete four-fermion final-state calculation.Comment: 28 pages, 12 figures, final version for Phys. Rev.
Power spectra in extended tachyon cosmologies
In the present work the power spectrum of a particular class of tachyon
fields is compared with the one corresponding to a cosmological constant model.
This is done for different barotropic indexes and the background
space time is assumed to be of the spatially flat Friedmann-Robertson-Walker
type. The differential equation describing the perturbations is solved
numerically and the power spectrum at the scale factor value is plotted
for each case. The result is that the power spectrum of the standard tachyon
field differs in many magnitude orders from the CDM. However, the one
with , which corresponds to a complementary tachyon field,
coincides fairly well with the concordance model. Therefore, we conclude that
the perturbed solutions constitute an effective method to distinguish between
the different values for the tachionization CDM model and
the fiducial model. The Statefinder parameters , measuring the
deviations of the analysed model from the concordance model, are also
explicitly calculated. Our result suggest that, depending on the value of
, these models can explain the observed expansion history or the
perturbation power spectrum of the universe, but they may have problems in
describing both features simultaneously.Comment: 7 pages, 7 figures, accepted for publication in General Relativity
and Gravitatio
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