7 research outputs found
Cosmic coincidence problem and variable constants of physics
The standard model of cosmology is investigated using time dependent
cosmological constant and Newton's gravitational constant . The
total energy content is described by the modified Chaplygin gas equation of
state. It is found that the time dependent constants coupled with the modified
Chaplygin gas interpolate between the earlier matter to the later dark energy
dominated phase of the universe. We also achieve a convergence of parameter
, with minute fluctuations, showing an evolving . Thus our
model fairly alleviates the cosmic coincidence problem which demands
at present time.Comment: 27 pages, 15 figure
Regular black holes and black universes
We give a comparative description of different types of regular static,
spherically symmetric black holes (BHs) and discuss in more detail their
particular type, which we suggest to call black universes. The latter have a
Schwarzschild-like causal structure, but inside the horizon there is an
expanding Kantowski-Sachs universe and a de Sitter infinity instead of a
singularity. Thus a hypothetic BH explorer gets a chance to survive. Solutions
of this kind are naturally obtained if one considers static, spherically
symmetric distributions of various (but not all) kinds of phantom matter whose
existence is favoured by cosmological observations. It also looks possible that
our Universe has originated from phantom-dominated collapse in another universe
and underwent isotropization after crossing the horizon. An explicit example of
a black-universe solution with positive Schwarzschild mass is discussed.Comment: 13 pages, 1 figure. 6 referenses and some discussion added, misprints
correcte
Non-vacuum Solutions of Bianchi Type VI_0 Universe in f(R) Gravity
In this paper, we solve the field equations in metric f(R) gravity for
Bianchi type VI_0 spacetime and discuss evolution of the expanding universe. We
find two types of non-vacuum solutions by taking isotropic and anisotropic
fluids as the source of matter and dark energy. The physical behavior of these
solutions is analyzed and compared in the future evolution with the help of
some physical and geometrical parameters. It is concluded that in the presence
of isotropic fluid, the model has singularity at and represents
continuously expanding shearing universe currently entering into phantom phase.
In anisotropic fluid, the model has no initial singularity and exhibits the
uniform accelerating expansion. However, the spacetime does not achieve
isotropy as in both of these solutions.Comment: 20 pages, 5 figures, accepted for publication in Astrophys. Space Sc
Halo modelling in chameleon theories
We analyse modelling techniques for the large-scale structure formed in
scalar-tensor theories of constant Brans-Dicke parameter which match the
concordance model background expansion history and produce a chameleon
suppression of the gravitational modification in high-density regions. Thereby,
we use a mass and environment dependent chameleon spherical collapse model, the
Sheth-Tormen halo mass function and linear halo bias, the Navarro-Frenk-White
halo density profile, and the halo model. Furthermore, using the spherical
collapse model, we extrapolate a chameleon mass-concentration scaling relation
from a LCDM prescription calibrated to N-body simulations. We also provide
constraints on the model parameters to ensure viability on local scales. We
test our description of the halo mass function and nonlinear matter power
spectrum against the respective observables extracted from large-volume and
high-resolution N-body simulations in the limiting case of f(R) gravity,
corresponding to a vanishing Brans-Dicke parameter. We find good agreement
between the two; the halo model provides a good qualitative description of the
shape of the relative enhancement of the f(R) matter power spectrum with
respect to LCDM caused by the extra attractive gravitational force but fails to
recover the correct amplitude. Introducing an effective linear power spectrum
in the computation of the two-halo term to account for an underestimation of
the chameleon suppression at intermediate scales in our approach, we accurately
reproduce the measurements from the N-body simulations.Comment: 25 pages, 4 figures; v2: typos corrected, matches published versio