306,321 research outputs found
On the Effective Equation of State of Dark Energy
In an effective field theory model with an ultraviolet momentum cutoff, there
is a relation between the effective equation of state of dark energy and the
ultraviolet cutoff scale. It implies that a measure of the equation of state of
dark energy different from minus one, does not rule out vacuum energy as dark
energy. It also indicates an interesting possibility that precise measurements
of the infrared properties of dark energy can be used to probe the ultraviolet
cutoff scale of effective quantum field theory coupled to gravity. In a toy
model with a vacuum energy dominated universe with a Planck scale cutoff, the
dark energy effective equation of state is -0.96.Comment: 7 pages, awarded honorable mention in the 2010 Gravity Research
Foundation essay competitio
Brane-Bulk energy exchange and agegraphic dark energy
We consider the agegraphic models of dark energy in a braneworld scenario
with brane-bulk energy exchange. We assume that the adiabatic equation for the
dark matter is satisfied while it is violated for the agegraphic dark energy
due to the energy exchange between the brane and the bulk. Our study shows that
with the brane-bulk interaction, the equation of state parameter of agegraphic
dark energy on the brane, , can have a transition from normal state where
to the phantom regime where , while the effective equation
of state for dark energy always satisfies .Comment: 13 pages, to appear in IJMP
On sign-changeable interaction in FLRW cosmology
We investigate an interacting two-fluid model in a spatially flat
Friedmann-Lema\^itre-Robertson-Walker (FLRW) Universe, when the energy transfer
between these two dark components is produced by a factorisable nonlinear
sign-changeable interaction depending linearly on the energy density and
quadratically on the deceleration parameter. We solve the source equation and
obtain the effective energy densities of the dark sector and their components.
We show that the effective equation of state of the dark sector includes some
of the several kind of Chaplygin gas equations of state as well as a
generalization of the polytropic equation of state. We use bayesian statistics
methods to constrain free parameters in the models during its most recent
evolution considering supernovae type Ia and measurements of the Hubble
expansion rate. The resulting constraints provide new information on
sign-changeable interactions, its equivalences and compatibility with previous
models and novel late time universe dynamics.Comment: 8 figure
Dark energy and dark matter from cosmological observations
The present status of our knowledge about the dark matter and dark energy is
reviewed. Bounds on the content of cold and hot dark matter from cosmological
observations are discussed in some detail. I also review current bounds on the
physical properties of dark energy, mainly its equation of state and effective
speed of sound.Comment: 12 pages, 4 figures, to appear in Lepton-Photon 2005 proceedings,
added figure and typos correcte
Super-acceleration on the Brane by Energy Flow from the Bulk
We consider a brane cosmological model with energy exchange between brane and
bulk. Parameterizing the energy exchange term by the scale factor and Hubble
parameter, we are able to exactly solve the modified Friedmann equation on the
brane. In this model, the equation of state for the effective dark energy has a
transition behavior changing from to , while
the equation of state for the dark energy on the brane has . Fitting data
from type Ia supernova, Sloan Digital Sky Survey and Wilkinson Microwave
Anisotropy Probe, our universe is predicted now in the state of
super-acceleration with .Comment: Revtex, 11 pages including 2 figures,v2: tpos fixed, references
added, to appear in JCA
Linear and nonlinear interactions in the dark sector
We investigate models of interacting dark matter and dark energy for the
universe in a spatially flat Friedmann-Robertson-Walker (FRW) space-time. We
find the "source equation" for the total energy density and determine the
energy density of each dark component. We introduce an effective one-fluid
description to evidence that interacting and unified models are related with
each other, analyze the effective model and obtain the attractor solutions. We
study linear and nonlinear interactions, the former comprises a linear
combination of the dark matter and dark energy densities, their first
derivatives, the total energy density, its first and second derivatives and a
function of the scale factor. The latter is a possible generalization of the
linear interaction consisting of an aggregate of the above linear combination
and a significant nonlinear term built with a rational function of the dark
matter and dark energy densities homogeneous of degree one. We solve the
evolution equations of the dark components for both interactions and examine
exhaustively several examples. There exist cases where the effective one-fluid
description produces different alternatives to the \LaCDM model and cases
where the problem of coincidence is alleviated. In addition, we find that some
nonlinear interactions yield an effective one-fluid model with a Chaplygin gas
equation of state, whereas others generate cosmological models with de Sitter
and power-law expansions. We show that a generic nonlinear interaction induces
an effective equation of state which depends on the scale factor in the same
way that the variable modified Chaplygin gas model, giving rise to the "relaxed
Chaplygin gas model".Comment: Accepted for publication in PR
Non-adiabatic Chaplygin gas
The split of a generalised Chaplygin gas with an equation of state p =
-A/\rho^{\alpha} into an interacting mixture of pressureless matter and a
dark-energy component with equation of state p_{\Lambda} = - \rho_{\Lambda}
implies the existence of non-adiabatic pressure perturbations. We demonstrate
that the square of the effective (non-adiabatic) sound speed c_s of the medium
is proportional to the ratio of the perturbations of the dark energy to those
of the dark matter. Since, as demonstrated explicitly for the particular case
\alpha = -1/2, dark-energy perturbations are negligible compared with
dark-matter perturbations on scales that are relevant for structure formation,
we find |c_s^2| << 1. Consequently, there are no oscillations or instabilities
which have plagued previous adiabatic Chaplygin-gas models.Comment: Version to appear in Physics Letters
A cosmic equation of state for the inhomogeneous Universe: can a global far-from-equilibrium state explain Dark Energy?
A system of effective Einstein equations for spatially averaged scalar
variables of inhomogeneous cosmological models can be solved by providing a
`cosmic equation of state'. Recent efforts to explain Dark Energy focus on
`backreaction effects' of inhomogeneities on the effective evolution of
cosmological parameters in our Hubble volume, avoiding a cosmological constant
in the equation of state. In this Letter it is argued that, if kinematical
backreaction effects are indeed of the order of the averaged density (or larger
as needed for an accelerating domain of the Universe), then the state of our
regional Hubble volume would have to be in the vicinity of a
far-from-equilibrium state that balances kinematical backreaction and average
density. This property, if interpreted globally, is shared by a stationary
cosmos with effective equation of state . It
is concluded that a confirmed explanation of Dark Energy by kinematical
backreaction may imply a paradigmatic change of cosmology.Comment: 7 pages, matches published version in Class. Quant. Gra
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