50 research outputs found
Updated F(T) gravity constraints from high redshift cosmography
In the last dozen years a wide and variegated mass of observational data
revealed that the universe is now expanding at an accelerated rate. In the
absence of a well-based theory to interpret the observations, cosmography
provides information about the evolution of the Universe from measured
distances, only assuming that the geometry of the can be described by the
Friedmann-Lemaitre-Robertson -Walker metric. We perform a high-redshift
analysis allows us to put constraints on the cosmographic parameters up to the
5fth order, thus inducing indirect constraints on any gravity theory. Here we
are interested in the so called teleparallel gravity theory, f(T). Actually we
use the analytical expressions of the present day values of f(T) and its
derivatives as functions of the cosmographic parameters to map the cosmography
region of confidences into confidence ranges for f(T) and its derivative.
Moreover, we show how these can be used to test some teleparallel gravity
models without solving the dynamical equations. Our analysis is based on the
Union2 Type Ia Supernovae (SNIa) data set, a set of 28 measurements of the
Hubble parameter, the Hubble diagram constructed from some Gamma Ray Bursts
(GRB) luminosity distance indicators, and gaussian priors on the distance from
the Baryon Acoustic Oscillations (BAO), and the Hubble constant h. To perform
our statistical analysis and to explore the probability distributions of the
cosmographic parameters we use the Markov Chain Monte Carlo Method (MCMC).Comment: International Journal of Modern Physics D, 20 pages, 5 figure
Cosmological parameters from lenses distance ratio
Strong lensing provides popular techniques to investigate the mass
distribution of intermediate redshift galaxies, testing galaxy evolution and
formation scenarios. It especially probes the background cosmic expansion,
hence constraining cosmological parameters. The measurement of Einstein radii
and central velocity dispersions indeed allows to trace the ratio D_s/D_ls
between the distance D_s from the observer to the source and the distance D_ls
from the lens to the source. We present an improved method to explicitly
include the two - component structure in the galaxy lens modeling, in order to
analyze the role played by the redshift and the model dependence on a nuisance
parameter, F_E, which is usually marginalized in the cosmological applications.
We show how to deal with these problems and carry on a Fisher matrix analysis
to infer the accuracy on cosmological parameters achieved by this method.Comment: 8 pages, 2 figures, 2 tables, accepted for publication on Monthly
Notices of Royal Astronomical Societ
Observational tests of the Glavan, Prokopec and Starobinsky model of dark energy
In the last dozens of years different data sets revealed the accelerated
expansion of the Universe which is driven by the so called dark energy, that
now dominates the total amount of matter-energy in the Universe. In a recent
paper Glavan, Prokopec and Starobinsky propose an interesting model of dark
energy, which traces the Universe evolution from the very early quantum era to
the present time. Here we perform a high-redshift analysis to check if this new
model is compatible with present day observational data and compare predictions
of this model with that of the standard CDM cosmological model. In our
analysis we use only the most reliable observational data, namely the distances
to selected SNIa, GRB Hubble diagram, and direct measurements of the
Hubble constant. Moreover we consider also non {\it geometric} data related to
the growth rate of density perturbations. We explore the probability
distributions of the cosmological parameters for both models. To build up their
own regions of confidence, we maximize the appropriate likelihood functions
using the Markov chain Monte Carlo (MCMC) method. Our statistical analysis
indicates that these very different models of dark energy are compatible with
present day observational data and the GPS model seems slightly favored with
respect to the CDM model. However to further restrict different models
of dark energy it will be necessary to increase the precision of the Hubble
diagram at high redshifts and to perform more detailed analysis of the
influence of dark energy on the process of formation of large scale structure.Comment: accepted for publication on EPJ
Cosmology with gamma-ray bursts: I. The Hubble diagram through the calibrated - correlation
Gamma-ray bursts are the most energetic explosions in the Universe. They are
detectable up to very high redshifts, therefore can be used to study the
expansion rate of the Universe and to investigate the observational properties
of dark energy, provided that empirical correlations between spectral and
intensity properties are appropriately calibrated. We used the type Ia
supernova luminosity distances to calibrate the correlation between the peak
photon energy, , and the isotropic equivalent radiated energy, in GRBs. With this correlation, we tested the reliability of applying
GRBs to measure cosmological parameters and to obtain indications on the basic
properties and evolution of dark energy. Using 162 GRBs with measured redshifts
and spectra, we applied a local regression technique to calibrate the - correlation against the type Ia SN data to build a calibrated GRB
Hubble diagram. We tested the possible redshift dependence of the correlation
and its effect on the Hubble diagram. Finally, we used the GRB Hubble diagram
to investigate the dark energy EOS. For this, we focused on the so-called
Chevalier-Polarski-Linder (CPL) parametrization of the dark energy EOS and
implemented the Markov chain Monte Carlo (MCMC) method to efficiently sample
the space of cosmological parameters. Our analysis shows once more that the
- correlation has no significant redshift dependence.
Therefore the high-redshift GRBs can be used as a cosmological tool to
determine the basic cosmological parameters and to test different models of
dark energy in the redshift region (), which is unexplored by the
SNIa and baryonic acoustic oscillations data. Our updated calibrated Hubble
diagram of GRBs provides some marginal indication (at level) of an
evolving dark energy EOS.Comment: 12 pages, 11 figure
Cosmology with gamma-ray bursts: II Cosmography challenges and cosmological scenarios for the accelerated Universe
Context. Explaining the accelerated expansion of the Universe is one of the
fundamental challenges in physics today. Cosmography provides information about
the evolution of the universe derived from measured distances, assuming only
that the space time ge- ometry is described by the
Friedman-Lemaitre-Robertson-Walker metric, and adopting an approach that
effectively uses only Taylor expansions of basic observables. Aims. We perform
a high-redshift analysis to constrain the cosmographic expansion up to the
fifth order. It is based on the Union2 type Ia supernovae data set, the
gamma-ray burst Hubble diagram, a data set of 28 independent measurements of
the Hubble param- eter, baryon acoustic oscillations measurements from galaxy
clustering and the Lyman-{\alpha} forest in the SDSS-III Baryon Oscillation
Spectroscopic Survey (BOSS), and some Gaussian priors on h and {\Omega}M .
Methods. We performed a statistical analysis and explored the probability
distributions of the cosmographic parameters. By building up their regions of
confidence, we maximized our likelihood function using the Markov chain Monte
Carlo method. Results. Our high-redshift analysis confirms that the expansion
of the Universe currently accelerates; the estimation of the jerk parameter
indicates a possible deviation from the standard {\Lambda}CDM cosmological
model. Moreover, we investigate implications of our results for the
reconstruction of the dark energy equation of state (EOS) by comparing the
standard technique of cosmography with an alternative approach based on
generalized Pad\'e approximations of the same observables. Because these
expansions converge better, is possible to improve the constraints on the
cosmographic parameters and also on the dark matter EOS. Conclusions. The
estimation of the jerk and the DE parameters indicates at 1{\sigma} a possible
deviation from the {\Lambda}CDM cosmological model.Comment: 10 pages, 7 figures, accepted for publication in A &
Slope evolution of GRB correlations and cosmology
Gamma -ray bursts (GRBs) observed up to redshifts can be used as
possible probes to test cosmological models. Here we show how changes of the
slope of the {\it luminosity -break time } correlation in GRB
afterglows, hereafter the LT correlation, affect the determination of the
cosmological parameters. With a simulated data set of 101 GRBs with a central
value of the correlation slope that differs on the intrinsic one by a
factor, we find an overstimated value of the matter density parameter,
, compared to the value obtained with SNe Ia, while the Hubble
constant, , best fit value is still compatible in 1 compared to
other probes. We show that this compatibility of is due to the large
intrinsic scatter associated with the simulated sample. Instead, if we consider
a subsample of high luminous GRBs (), we find that both the evaluation
of and are not more compatible in 1 and is
underestimated by the . However, the sample choice reduces
dramatically the intrinsic scatter of the correlation, thus possibly
identifying this sample as the standard canonical `GRBs' confirming previous
results presented in Dainotti et al. (2010,2011). Here, we consider the LT
correlation as an example, but this reasoning can be extended also for all
other GRB correlations. In literature so far GRB correlations are not corrected
for redshift evolution and selection biases, therefore we are not aware of
their intrinsic slopes and consequently how far the use of the observed
correlations can influence the derived `best' cosmological settings. Therefore,
we conclude that any approach that involves cosmology should take into
consideration only intrinsic correlations not the observed ones.Comment: 8 pages, 4 figures, accepted to MNRAS Main Journa
Geometric-Phase Waveplates for Free-Form Dark Hollow Beams
We demonstrate the possibility of creating optical beams with phase singularities engraved into exotic intensity landscapes imitating the shapes of a large variety of diverse plane curves. To achieve this aim, we have developed a method for directly encoding the geometric properties of a selected curve into a single azimuthal phase factor without passing through indirect encryption methods involving lengthy numerical procedures. The outcome is utilized to mold the optic axis distribution of a liquid-crystal-based inhomogeneous waveplate. The latter is finally used to sculpt the wavefront of an input optical gaussian beam via the Pancharatnam-Berry phase