7 research outputs found
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
Correspondence between kinematical backreaction and scalar field cosmologies - the `morphon field'
Spatially averaged inhomogeneous cosmologies in classical general relativity
can be written in the form of effective Friedmann equations with sources that
include backreaction terms. In this paper we propose to describe these
backreaction terms with the help of a homogeneous scalar field evolving in a
potential; we call it the `morphon field'. This new field links classical
inhomogeneous cosmologies to scalar field cosmologies, allowing to reinterpret,
e.g., quintessence scenarios by routing the physical origin of the scalar field
source to inhomogeneities in the Universe. We investigate a one-parameter
family of scaling solutions to the backreaction problem. Subcases of these
solutions (all without an assumed cosmological constant) include
scale-dependent models with Friedmannian kinematics that can mimic the presence
of a cosmological constant or a time-dependent cosmological term. We explicitly
reconstruct the scalar field potential for the scaling solutions, and discuss
those cases that provide a solution to the Dark Energy and coincidence
problems. In this approach, Dark Energy emerges from morphon fields, a
mechanism that can be understood through the proposed correspondence: the
averaged cosmology is characterized by a weak decay (quintessence) or growth
(phantom quintessence) of kinematical fluctuations, fed by `curvature energy'
that is stored in the averaged 3-Ricci curvature. We find that the late-time
trajectories of those models approach attractors that lie in the future of a
state that is predicted by observational constraints.Comment: 36 pages and 6 Figures, matches published version in Class.Quant.Gra
On globally static and stationary cosmologies with or without a cosmological constant and the Dark Energy problem
In the framework of spatially averaged inhomogeneous cosmologies in classical
General Relativity, effective Einstein equations govern the regional and the
global dynamics of averaged scalar variables of cosmological models. A
particular solution may be characterized by a cosmic equation of state. In this
paper it is pointed out that a globally static averaged dust model is
conceivable without employing a compensating cosmological constant. Much in the
spirit of Einstein's original model we discuss consequences for the global, but
also for the regional properties of this cosmology. We then consider the wider
class of globally stationary cosmologies that are conceivable in the presented
framework. All these models are based on exact solutions of the averaged
Einstein equations and provide examples of cosmologies in an out-of-equilibrium
state, which we characterize by an information-theoretical measure. It is shown
that such cosmologies preserve high-magnitude kinematical fluctuations and so
tend to maintain their global properties. The same is true for a
driven cosmos in such a state despite of exponential expansion. We
outline relations to inflationary scenarios, and put the Dark Energy problem
into perspective. Here, it is argued, on the grounds of the discussed
cosmologies, that a classical explanation of Dark Energy through backreaction
effects is theoretically conceivable, if the matter-dominated Universe emerged
from a non-perturbative state in the vicinity of the stationary solution. We
also discuss a number of caveats that furnish strong counter arguments in the
framework of structure formation in a perturbed Friedmannian model.Comment: 33 pages, matches published version in Class. Quant. Gra
Accelerated expansion from structure formation
We discuss the physics of backreaction-driven accelerated expansion. Using
the exact equations for the behaviour of averages in dust universes, we explain
how large-scale smoothness does not imply that the effect of inhomogeneity and
anisotropy on the expansion rate is small. We demonstrate with an analytical
toy model how gravitational collapse can lead to acceleration. We find that the
conjecture of the accelerated expansion being due to structure formation is in
agreement with the general observational picture of structures in the universe,
and more quantitative work is needed to make a detailed comparison.Comment: 44 pages, 1 figure. Expanded treatment of topics from the Gravity
Research Foundation contest essay astro-ph/0605632. v2: Added references,
clarified wordings. v3: Published version. Minor changes and corrections,
added a referenc