1,720,980 research outputs found
Standardizing the gamma-ray bursts with the Amati Ep,i-Eiso relation: the updated Hubble diagram and implications for cosmography
No full textPECULIAR VELOCITIES FIELD AND THE MATTER DENSITY OF THE UNIVERSE IN SCALAR FIELD QUINTESSENCE MODELS
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High redshift cosmography: new results and implication for dark energy
No full textThe explanation of the accelerated expansion of the Universe poses one of the most
fundamental questions in physics and cosmology today. If the acceleration is driven
by some form of dark energy, and in the absence of a well-based theory to interpret
the observations, one can try to constrain the parameters describing the kinematical
state of the universe using a cosmographic approach, which is fundamental in that it
requires only a minimal set of assumptions, namely to specify the metric, and it does
not rely on the dynamical equations for gravity. Our high-redshift analysis allows us
to put constraints on the cosmographic expansion up to the fifth order. It is based
on the Union2 Type Ia Supernovae (SNIa) data set, the Hubble diagram constructed
from some Gamma Ray Bursts luminosity distance indicators, and gaussian priors on
the distance from the Baryon Acoustic Oscillations (BAO), and the Hubble constant
h (these priors have been included in order to help break the degeneracies among
model parameters). To perform our statistical analysis and to explore the probability
distributions of the cosmographic parameters we use the Markov Chain Monte Carlo
Method (MCMC). We finally investigate implications of our results for the dark en-
ergy, in particular, we focus on the parametrization of the dark energy equation of state
(EOS). Actually, a possibility to investigate the nature of dark energy lies in measuring
the dark energy equation of state, w, and its time (or redshift) dependence at high ac-
curacy. However, since w(z) is not directly accessible to measurement, reconstruction
methods are needed to extract it reliably from observations. Here we investigate differ-
ent models of dark energy, described through several parametrizations of the equation
of state, by comparing the cosmographic and the EOS series. The main results are:
a) even if relying on a mathematical approximate assumption such as the scale factor
series expansion in terms of time, cosmography can be extremely useful in assessing
dynamical properties of the Universe; b) the deceleration parameter clearly confirms
the present acceleration phase; c) the MCMC method provides stronger constraints
for parameter estimation, in particular for higher order cosmographic parameters (the
jerk and the snap), with respect to those presented in the literature; d) both the esti-
mation of the jerk and the DE parameters, reflect the possibility of a deviation from
the LambdaCDM cosmological model; e) there are indications that the dark energy equa-
tion of state is evolving for all the parametrizations that we considered; f) the q(z)
reconstruction provided by our cosmographic analysis allows a transient acceleration
Scalar field, nonminimal coupling, and cosmology
No full textWe study the dynamics of a flat Friedmann-Robertson-Walker universe filled with a self-interacting scalar field nonminimally coupled to the gravitational field. Dynamical equations for the system can be derived from a pointlike Lagrangian. For this system an additional Noether symmetry exists provided that the coupling constant xi is equal to 0 or 1/6. When xi = 1/6 the scalar potential has to be constant. In this case we obtain an exact solution. We also analyze the behavior of the scalar field when xi is not 0 or 1/6. Most of the considered solutions are unphysical but there exists a very interesting case in which the effective cosmological constant is rapidly changing, which might lead to inflation
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