268 research outputs found
The Age of the Universe and the Cosmological Constant Determined from Cosmic Microwave Background Anisotropy Measurements
If Omega_tot = 1 and structure formed from adiabatic initial conditions then
the age of the Universe, as constrained by measurements of the cosmic microwave
background (CMB), is t_0=14.0 +/- 0.5 Gyr. The uncertainty is surprisingly
small given that CMB data alone constrain neither h nor Omega_Lambda
significantly. It is due to the tight (and accidental) correlation, in these
models, of the age with the angle subtended by the sound horizon on the
last--scattering surface and thus with the well-determined acoustic peak
locations. If we assume either the HST Key Project result h = 0.72 \pm .08 or
simply that h > 0.55, we find Omega_Lambda > 0.4 at 95% confidence--another
argument for dark energy, independent of supernovae observations. Our analysis
is greatly simplified by the Monte Carlo Markov chain approach to Bayesian
inference combined with a fast method for calculating angular power spectra.Comment: 5 pages, including 2 figures and one table final published versio
Dark Matter, Modified Gravity and the Mass of the Neutrino
It has been suggested that Einstein's theory of General Relativity can be
modified to accomodate mismatches between the gravitational field and luminous
matter on a wide range of scales. Covariant theories of modified gravity
generically predict the existence of extra degrees of freedom which may be
interpreted as dark matter. We study a subclass of these theories where the
overall energy density in these extra degrees of freedom is subdominant
relative to the baryon density and show that they favour the presence of
massive neutrinos. For some specific cases (such as a flat Universes with a
cosmological constant) one finds a conservative lower bound on the neutrinos
mass of eV.Comment: 5 pages, 2 figures, 2 tables, submitted to Phys. Rev.
A note on bigravity and dark matter
We show that a class of bi-gravity theories contain solutions describing dark
matter. A particular member of this class is also shown to be equivalent to the
Eddington-Born-Infeld gravity, recently proposed as a candidate for dark
matter. Bigravity theories also have cosmological de Sitter backgrounds and we
find solutions interpolating between matter and acceleration eras.Comment: 4 pages, 1 figure, minor corrections and reference additions,
published in Phys. Rev.
Quintessence Model and Observational Constraints
The recent observations of type Ia supernovae strongly support that the
universe is accelerating now and decelerated in the recent past. By assuming a
general relation between the quintessence potential and the quintessence
kinetic energy, a general relation is found between the quintessence energy
density and the scale factor. The potential includes both the hyperbolic and
the double exponential potentials. A detailed analysis of the transition from
the deceleration phase to the acceleration phase is then performed. We show
that the current constraints on the transition time, the equation of state and
the energy density of the quintessence field are satisfied in the model.Comment: update references,add acknowledgements and correct some errors,
accepted for publication in class. and quant. gra
The cosmological behavior of Bekenstein's modified theory of gravity
We study the background cosmology governed by the Tensor-Vector-Scalar theory
of gravity proposed by Bekenstein. We consider a broad family of potentials
that lead to modified gravity and calculate the evolution of the field
variables both numerically and analytically. We find a range of possible
behaviors, from scaling to the late time domination of either the additional
gravitational degrees of freedom or the background fluid.Comment: 10 pages, 8 figures, A few typos corrected in the text and figures.
Version published in PR
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