46 research outputs found
Hints for families of GRBs improving the Hubble diagram
As soon as their extragalactic origins were established, the hope to make
Gamma - Ray Bursts (GRBs) standardizeable candles to probe the very high - z
universe has opened the search for scaling relations between redshift
independent observable quantities and distance dependent ones. Although some
remarkable success has been achieved, the empirical correlations thus found are
still affected by a significant intrinsic scatter which downgrades the
precision in the inferred GRBs Hubble diagram. We investigate here whether this
scatter may come from fitting together objects belonging to intrinsically
different classes. To this end, we rely on a cladistics analysis to partition
GRBs in homogenous families according to their rest frame properties. Although
the poor statistics prevent us from drawing a definitive answer, we find that
both the intrinsic scatter and the coefficients of the \,-\,
and \,-\, correlations significantly change depending on which
subsample is fitted. It turns out that the fit to the full sample leads to a
scaling relation which approximately follows the diagonal of the region
delimited by the fits to each homogenous class. We therefore argue that a
preliminary identification of the class a GRB belongs to is necessary in order
to select the right scaling relation to be used in order to not bias the
distance determination and hence the Hubble diagram.Comment: 10 pages, 6 figures, 4 tables, accepted for publication on MNRA
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
An updated analysis of two classes of f(R) theories of gravity
The observed accelerated cosmic expansion can be a signature of
fourth\,-\,order gravity theories, where the acceleration of the Universe is a
consequence of departures from Einstein General Relativity, rather than the
sign of the existence of a fluid with negative pressure. In the
fourth\,-\,order gravity theories, the gravity Lagrangian is described by an
analytic function of the scalar curvature subject to the demanding
conditions that no detectable deviations from standard GR is observed on the
Solar System scale. Here we consider two classes of theories able to
pass Solar System tests and investigate their viability on cosmological scales.
To this end, we fit the theories to a large dataset including the combined
Hubble diagram of Type Ia Supernovae and Gamma Ray Bursts, the Hubble parameter
data from passively evolving red galaxies, Baryon Acoustic Oscillations
extracted from the seventh data release of the Sloan Digital Sky Survey (SDSS)
and the distance priors from the Wilkinson Microwave Anisotropy Probe seven
years (WMAP7) data. We find that both classes of fit very well this
large dataset with the present\,-\,day values of the matter density, Hubble
constant and deceleration parameter in agreement with previous estimates;
however, the strong degeneracy among the parameters prevents us from
strongly constraining their values. We also derive the growth factor , with the matter density
perturbation, and show that it can still be well approximated by . We finally constrain (on some representative
scales) and investigate its redshift dependence to see whether future data can
discriminate between these classes of theories and standard dark energy
models.Comment: 27 pages, 5 figures, 1 table, accepted for publication on JCAP. Note
that this paper updates and supersedes preprint arXiv:0907.468
Cosmography of f(R) - brane cosmology
Cosmography is a useful tool to constrain cosmological models, in particular
dark energy models. In the case of modified theories of gravity, where the
equations of motion are generally quite complicated, cosmography can contribute
to select realistic models without imposing arbitrary choices a priori. Indeed,
its reliability is based on the assumptions that the universe is homogeneous
and isotropic on large scale and luminosity distance can be "tracked" by the
derivative series of the scale factor a(t). We apply this approach to induced
gravity brane-world models where an f(R)-term is present in the brane effective
action. The virtue of the model is to self-accelerate the normal and healthy
DGP branch once the f(R)-term deviates from the Hilbert-Einstein action. We
show that the model, coming from a fundamental theory, is consistent with the
LCDM scenario at low redshift. We finally estimate the cosmographic parameters
fitting the Union2 Type Ia Supernovae (SNeIa) dataset and the distance priors
from Baryon Acoustic Oscillations (BAO) and then provide constraints on the
present day values of f(R) and its second and third derivatives.Comment: 9 pages, 3 figures, to appear in Phys. Rev.
Testing the distance duality relation with present and future data
The assumptions that "light propagates along null geodesics of the spacetime
metric" and "the number of photons is conserved along the light path" lead to
the distance duality relation (DDR), ,
with and the luminosity and angular diameter distances to a
source at redshift . In order to test the DDR, we follow the usual strategy
comparing the angular diameter distances of a set of clusters, inferred from X
- ray and radio data, with the luminosity distance at the same cluster redshift
using the local regression technique to estimate from Type Ia
Supernovae (SNeIa) Hubble diagram. In order to both strengthen the constraints
on the DDR and get rid of the systematics related to the unknown cluster
geometry, we also investigate the possibility to use Baryon Acoustic
Oscillations (BAO) to infer from future BAO surveys. As a test case,
we consider the proposed Euclid mission investigating the precision can be
afforded on from the expected SNeIa and BAO data. We find that the
combination of BAO and the local regression coupled allows to reduce the errors
on by a factor two if one is forced and future data are used. On the other hand, although the
statistical error on is not significantly reduced, the constraints on
this quantity will be nevertheless ameliorated thanks to the reduce impact of
systematics.Comment: 10 pages, 1 figure, 1 table, accepted for publication on Physical
Review
Measurement of the halo bias from stacked shear profiles of galaxy clusters
We present the observational evidence of the 2-halo term in the stacked shear
profile of a sample of about 1200 optically selected galaxy clusters based on
imaging data and the public shear catalog from the CFHTLenS. We find that the
halo bias, a measure of the correlated distribution of matter around galaxy
clusters, has amplitude and correlation with galaxy cluster mass in very good
agreement with the predictions based on the LCDM standard cosmological model.
The mass-concentration relation is flat but higher than theoretical
predictions. We also confirm the close scaling relation between the optical
richness of galaxy clusters and their mass.Comment: 5 pages, 4 figures. In press on ApJ Letter
Constraining massive gravity with recent cosmological data
A covariant formulation of a theory with a massive graviton and no negative
energy state has been recently proposed as an alternative to the usual General
Relativity framework. For a spatially flat homogenous and isotropic universe,
the theory introduces modified Friedmann equations where the standard matter
term is supplemented by four effective fluids mimicking dust, cosmological
constant, quintessence and stiff matter, respectively. We test the viability of
this massive gravity formulation by contrasting its theoretical prediction to
the Hubble diagram as traced by Type Ia Supernovae (SNeIa) and Gamma Ray Bursts
(GRBs), the measurements from passively evolving galaxies, Baryon
Acoustic Oscillations (BAOs) from galaxy surveys and the distance priors from
the Cosmic Microwave Background Radiation (CMBR) anisotropy spectrum. It turns
out that the model is indeed able to very well fit this large dataset thus
offering a viable alternative to the usual dark energy framework. We finally
set stringent constraints on its parameters also narrowing down the allowed
range for the graviton mass.Comment: 10 pages, 1 figure, 2 tables, accepted for publication on Physical
Review
Increasing the lensing figure of merit through higher order convergence moments
The unprecedented quality, the increased data set, and the wide area of ongoing and near future weak lensing surveys allows one to move beyond the standard two points statistics, thus making it worthwhile to investigate higher order probes. As an interesting step toward this direction, we explore the use of higher order moments (HOM) of the convergence field as a way to increase the lensing figure of merit (FoM). To this end, we rely on simulated convergence to first show that HOM can be measured and calibrated so that it is indeed possible to predict them for a given cosmological model provided suitable nuisance parameters are introduced and then marginalized over. We then forecast the accuracy on cosmological parameters from the use of HOM alone and in combination with standard shear power spectra tomography. It turns out that HOM allow one to break some common degeneracies, thus significantly boosting the overall FoM. We also qualitatively discuss possible systematics and how they can be dealt with
Come Euclid ci aiuterà a scoprire la storia dell'Universo
Ametà del secolo scorso Allan Sandage, uno dei maggiori astronomi del tempo, era solito dire che la cosmologia si riduceva alla ricerca di due numeri: la costante di Hubble e il parametro di decelerazione. La prima ci dice quanto velocemente l'universo si sta espandendo, mentre il secondo permette di rispondere alla domanda se questa espansione stia accelerando o rallentando. Settanta anni dopo, quella frase ha lo stesso valore di un reperto archeologico. Ci mostra quale fosse il pensiero dominante di un'epoca e ci fa capire quanto cammino abbiamo fatto da allora. Quei due numeri oggi li conosciamo e, soprattutto, sappiamo che cosa li determina. Conosciamo, quindi, dove dobbiamo guardare per rispondere alla domanda di Sandage su quale sia il valore dell'accelerazione dell'espansione dell'universo