25 research outputs found
The solution of the cosmological constant problem from the inhomogeneous equation of state - a hint from modified gravity?
The cosmological constant problem is studied in a two component cosmological
model. The universe contains a cosmological constant of an arbitrary size and
sign and an additional component with an inhomogeneous equation of state. It is
shown that, in a proper parameter regime, the expansion of the universe with a
large absolute value of the cosmological constant may asymptotically tend to de
Sitter space corresponding to a small effective positive cosmological constant.
It is argued that such a behavior can be regarded as a solution of the
cosmological constant problem in this model. The mechanism behind the
relaxation of the cosmological constant is discussed. A connection with
modified gravity theories is discussed and an example of a possible realization
of the cosmological constant relaxation in f(R) modified gravity is described.Comment: 18 pages, 8 figures, version to appear in Phys. Lett.
Barotropic fluid compatible parametrizations of dark energy
Parametrizations of Equation of state parameter as a function of the scale
factor or redshift are frequently used in dark energy modeling. The question
investigated in this paper is if parametrizations proposed in the literature
are compatible with the dark energy being a barotropic fluid. The test of this
compatibility is based on the functional form of the speed of sound squared,
which for barotropic fluid dark energy follows directly from the function for
the Equation of state parameter. The requirement that the speed of sound
squared should be between 0 and speed of light squared provides constraints on
model parameters using analytical and numerical methods. It is found that this
fundamental requirement eliminates a large number of parametrizations as
barotropic fluid dark energy models and puts strong constraints on parameters
of other dark energy parametrizations.Comment: v1: 19 pages, 1 table, 4 figures. v2: minor changes, references
added. v3: version accepted in EPJ
LXCDM cosmologies: solving the cosmological coincidence problem?
We explore the possibility of having a composite (self-conserved) dark energy
(DE) whose dynamics is controlled by the quantum running of the cosmological
parameters. We find that within this scenario it is feasible to find an
explanation for the cosmological coincidence problem and at the same time a
good qualitative description of the present data.Comment: LaTeX, 7 pages, 3 figures. Prepared for DSU2006, International
Workshop on the Dark Side of the Universe, Madrid, Spain, 20-24 Jun 200