5 research outputs found
The Lemaitre Model and the Generalisation of the Cosmic Mass
We consider the spherically symmetric metric with a comoving perfect fluid
and non-zero pressure -- the Lemaitre metric -- and present it in the form of a
calculational algorithm. We use it to review the definition of mass, and to
look at the apparent horizon relations on the observer's past null cone. We
show that the introduction of pressure makes it difficult to separate the mass
from other physical parameters in an invariant way. Under the usual mass
definition, the apparent horizon relation, that relates the diameter distance
to the cosmic mass, remains the same as in the Lemaitre-Tolman case.Comment: latex, 16 pages, Revision has minor changes due to referee's
comments
Imitating accelerated expansion of the Universe by matter inhomogeneities - corrections of some misunderstandings
A number of misunderstandings about modeling the apparent accelerated
expansion of the Universe, and about the `weak singularity' are clarified: 1.
Of the five definitions of the deceleration parameter given by Hirata and
Seljak (HS), only is a correct invariant measure of
acceleration/deceleration of expansion. The and are unrelated to
acceleration in an inhomogeneous model. 2. The averaging over directions
involved in the definition of does not correspond to what is done in
observational astronomy. 3. HS's equation (38) connecting to the flow
invariants gives self-contradictory results when applied at the centre of
symmetry of the Lema\^{\i}tre-Tolman (L-T) model. The intermediate equation
(31) that determines is correct, but approximate, so it cannot be used
for determining the sign of the deceleration parameter. Even so, at the centre
of symmetry of the L-T model, it puts no limitation on the sign of .
4. The `weak singularity' of Vanderveld {\it et al.} is a conical profile of
mass density at the centre - a perfectly acceptable configuration. 5. The
so-called `critical point' in the equations of the `inverse problem' for a
central observer in an L-T model is a manifestation of the apparent horizon - a
common property of the past light cones in zero-lambda L-T models, perfectly
manageable if the equations are correctly integrated.Comment: 15 pages. Completely rewritten to match the published version. We
added discussion of 2 key papers cited by VFW and identified more clearly the
assumptions, approximations and mistakes that led to certain misconceptions
Cosmological Backreaction from Perturbations
We reformulate the averaged Einstein equations in a form suitable for use
with Newtonian gauge linear perturbation theory and track the size of the
modifications to standard Robertson-Walker evolution on the largest scales as a
function of redshift for both Einstein de-Sitter and Lambda CDM cosmologies. In
both cases the effective energy density arising from linear perturbations is of
the order of 10^-5 the matter density, as would be expected, with an effective
equation of state w ~ -1/19. Employing a modified Halofit code to extend our
results to quasilinear scales, we find that, while larger, the deviations from
Robertson-Walker behaviour remain of the order of 10^-5.Comment: 15 pages, 8 figures; replaced by version accepted by JCA
Apparent and average acceleration of the Universe
In this paper we consider the relation between the volume deceleration
parameter obtained within the Buchert averaging scheme and the deceleration
parameter derived from the supernova observation. This work was motivated by
recent findings that showed that there are models which despite
have volume deceleration parameter . This opens the possibility
that backreaction and averaging effects may be used as an interesting
alternative explanation to the dark energy phenomenon.
We have calculated in some Lema\^itre--Tolman models. For those
models which are chosen to be realistic and which fit the supernova data, we
find that , while those models which we have been able to find
which exhibit turn out to be unrealistic. This indicates that
care must be exercised in relating the deceleration parameter to observations.Comment: 15 pages, 5 figures; matches published versio