96 research outputs found
Viable f(T) models are practically indistinguishable from LCDM
We investigate the cosmological predictions of several models, with up
to two parameters, at both the background and the perturbation levels. Using
current cosmological observations (geometric supernovae type Ia, cosmic
microwave background and baryonic acoustic oscillation and dynamical growth
data) we impose constraints on the distortion parameter, which quantifies the
deviation of these models from the concordance cosmology at the
background level. In addition we constrain the growth index predicted
in the context of these models using the latest perturbation growth data in the
context of three parametrizations for . The evolution of the best fit
effective Newton constant, which incorporates the -gravity effects, is
also obtained along with the corresponding error regions. We show
that all the viable parameter sectors of the gravity models considered
practically reduce these models to CDM. Thus, the degrees of freedom
that open up to CDM in the context of gravity models are not
utilized by the cosmological data leading to an overall disfavor of these
models.Comment: 16 pages, 9 figures, changes match published versio
Testing LCDM with the Growth Function \delta(a): Current Constraints
We have compiled a dataset consisting of 22 datapoints at a redshift range
(0.15,3.8) which can be used to constrain the linear perturbation growth rate
f=\frac{d\ln\delta}{d\ln a}. Five of these data-points constrain directly the
growth rate f through either redshift distortions or change of the power
spectrum with redshift. The rest of the datapoints constrain f indirectly
through the rms mass fluctuation \sigma_8(z) inferred from Ly-\alpha at various
redshifts. Our analysis tests the consistency of the LCDM model and leads to a
constraint of the Wang-Steinhardt growth index \gamma (defined from
f=\Omega_m^\gamma) as \gamma=0.67^{+0.20}_{-0.17}. This result is clearly
consistent at with the value \gamma={6/11}=0.55 predicted by LCDM. A
first order expansion of the index \gamma in redshift space leads to similar
results.We also apply our analysis on a new null test of LCDM which is similar
to the one recently proposed by Chiba and Nakamura (arXiv:0708.3877) but does
not involve derivatives of the expansion rate . This also leads to the
fact that LCDM provides an excellent fit to the current linear growth data.Comment: 7 pages, 4 figures. Added comments on the data of Table I (after eq.
(2.16)). Corrected a typo on eq. (2.15). The mathematica files with the
numerical analysis of this study may be found at
http://nesseris.physics.uoi.gr/growth/growth.ht
Cosmic Acceleration Data and Bulk-Brane Energy Exchange
We consider a braneworld model with bulk-brane energy exchange. This allows
for crossing of the w=-1 phantom divide line without introducing phantom energy
with quantum instabilities. We use the latest SnIa data included in the Gold06
dataset to provide an estimate of the preferred parameter values of this
braneworld model. We use three fitting approaches which provide best fit
parameter values and hint towards a bulk energy component that behaves like
relativistic matter which is propagating in the bulk and is moving at a speed v
along the fifth dimension, while the bulk-brane energy exchange component
corresponds to negative pressure and signifies energy flowing from the bulk
into the brane. We find that the best fit effective equation of state parameter
marginally crosses the phantom divide line w=-1. Thus, we have
demonstrated both the ability of this class of braneworld models to provide
crossing of the phantom divide and also that cosmological data hint towards
natural values for the model parameters.Comment: 12 pages, 2 figures, added comments, references update
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