8,784 research outputs found
Observational Bounds on Modified Gravity Models
Modified gravity provides a possible explanation for the currently observed
cosmic accelaration. In this paper, we study general classes of modified
gravity models. The Einstein-Hilbert action is modified by using general
functions of the Ricci and the Gauss-Bonnet scalars, both in the metric and in
the Palatini formalisms. We do not use an explicit form for the functions, but
a general form with a valid Taylor expansion up to second order about redshift
zero in the Riemann-scalars. The coefficients of this expansion are then
reconstructed via the cosmic expansion history measured using current
cosmological observations. These are the quantities of interest for theoretical
considerations relating to ghosts and instabilities. We find that current data
provide interesting constraints on the coefficients. The next-generation dark
energy surveys should shrink the allowed parameter space for modifed gravity
models quite dramatically.Comment: 23 pages, 5 figures, uses RevTe
Centrality, system size and energy dependences of charged-particle pseudo-rapidity distribution
Utilizing the three-fireball picture within the quark combination model, we
study systematically the charged particle pseudorapidity distributions in both
Au+Au and Cu+Cu collision systems as a function of collision centrality and
energy, 19.6, 62.4, 130 and 200 GeV, in full pseudorapidity
range. We find that: (i)the contribution from leading particles to
distributions increases with the decrease of the collision
centrality and energy respectively; (ii)the number of the leading particles is
almost independent of the collision energy, but it does depend on the nucleon
participants ; (iii)if Cu+Cu and Au+Au collisions at the same
collision energy are selected to have the same , the resulting of
charged particle distributions are nearly identical, both in the
mid-rapidity particle density and the width of the distribution. This is true
for both 62.4 GeV and 200 GeV data. (iv)the limiting fragmentation phenomenon
is reproduced. (iiv) we predict the total multiplicity and pseudorapidity
distribution for the charged particles in Pb+Pb collisions at TeV. Finally, we give a qualitative analysis of the
and as function of
and from RHIC to LHC.Comment: 12 pages, 8 figure
Probing deviations from General Relativity with the Euclid spectroscopic survey
We discuss the ability of the planned Euclid mission to detect deviations
from General Relativity using its extensive redshift survey of more than 50
Million galaxies. Constraints on the gravity theory are placed measuring the
growth rate of structure within 14 redshift bins between z=0.7 and z=2. The
growth rate is measured from redshift-space distortions, i.e. the anisotropy of
the clustering pattern induced by coherent peculiar motions. This is performed
in the overall context of the Euclid spectroscopic survey, which will
simultaneously measure the expansion history of the universe, using the power
spectrum and its baryonic features as a standard ruler, accounting for the
relative degeneracies of expansion and growth parameters. The resulting
expected errors on the growth rate in the different redshift bins, expressed
through the quantity f\sigma_8, range between 1.3% and 4.4%. We discuss the
optimisation of the survey configuration and investigate the important
dependence on the growth parameterisation and the assumed cosmological model.
We show how a specific parameterisation could actually drive the design towards
artificially restricted regions of the parameter space. Finally, in the
framework of the popular "\gamma -parameterisation", we show that the Euclid
spectroscopic survey alone will already be able to provide substantial evidence
(in Bayesian terms) if the growth index differs from the GR value \gamma=0.55
by at least \sim 0.13. This will combine with the comparable inference power
provided by the Euclid weak lensing survey, resulting in Euclid's unique
ability to provide a decisive test of modified gravity.Comment: 18 pages, 15 figures, accepted by MNRA
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