37 research outputs found
Geometry of the quasi-hyperbolic Szekeres models
Geometric properties of the quasi-hyperbolic Szekeres models are discussed
and related to the quasi-spherical Szekeres models. Typical examples of shapes
of various classes of 2-dimensional coordinate surfaces are shown in graphs;
for the hyperbolically symmetric subcase and for the general quasi-hyperbolic
case. An analysis of the mass function is carried out in parallel to an
analogous analysis for the quasi-spherical models. This leads to the conclusion
that determines the density of rest mass averaged over the whole space
of constant time.Comment: 19 pages, 13 figures. This version matches the published tex
Apparent horizons in the quasi-spherical Szekeres models
The notion of an apparent horizon (AH) in a collapsing object can be carried
over from the Lema\^{\i}tre -- Tolman (L--T) to the quasispherical Szekeres
models in three ways: 1. Literally by the definition -- the AH is the boundary
of the region, in which every bundle of null geodesics has negative expansion
scalar. 2. As the locus, at which null lines that are as nearly radial as
possible are turned toward decreasing areal radius . These lines are in
general nongeodesic. The name "absolute apparent horizon" (AAH) is proposed for
this locus. 3. As the boundary of a region, where null \textit{geodesics} are
turned toward decreasing . The name "light collapse region" (LCR) is
proposed for this region (which is 3-dimensional in every space of constant
); its boundary coincides with the AAH. The AH and AAH coincide in the L--T
models. In the quasispherical Szekeres models, the AH is different from (but
not disjoint with) the AAH. Properties of the AAH and LCR are investigated, and
the relations between the AAH and the AH are illustrated with diagrams using an
explicit example of a Szekeres metric. It turns out that an observer who is
already within the AH is, for some time, not yet within the AAH. Nevertheless,
no light signal can be sent through the AH from the inside. The analogue of the
AAH for massive particles is also considered.Comment: 14 pages, 9 figures, includes little extensions and style corrections
made after referee's comments, the text matches the published versio
Volume averaging in the quasispherical Szekeres model
This paper considers the volume averaging in the quasispherical Szekeres
model. The volume averaging became of considerable interest after it was shown
that the volume acceleration calculated within the averaging framework can be
positive even though the local expansion rate is always decelerating. This
issue was intensively studied within spherically symmetric models. However,
since our Universe is not spherically symmetric similar analysis is needed in
non symmetrical models. This papers presents the averaging analysis within the
quasispherical Szekeres model which is a non-symmetrical generalisation of the
spherically symmetric Lema\^itre--Tolman family of models. Density distribution
in the quasispherical Szekeres has a structure of a time-dependent mass dipole
superposed on a monopole. This paper shows that when calculating the volume
acceleration, , within the Szekeres model, the dipole does not
contribute to the final result, hence only depends on a monopole
configuration. Thus, the volume averaging within the Szekeres model leads to
literally the same solutions as obtained within the Lema\^itre--Tolman model.Comment: 8 pages; calculation of the spatial Ricci scalar added; accepted for
publication in Gen. Rel. Gra
Generalized Swiss-Cheese Cosmologies II: Spherical Dust
The generalized Swiss - cheese model, consisting of a Lema\^itre - Tolman
(inhomogeneous dust) region matched, by way of a comoving boundary surface,
onto a Robertson-Walker background of homogeneous dust, has become a standard
construction in modern cosmology. Here we ask if this construction can be made
more realistic by introducing some evolution of the boundary surface. The
answer we find is no. To maintain a boundary surface using the Darmois - Israel
junction conditions, as opposed to the introduction of a surface layer, the
boundary must remain exactly comoving. The options are to drop the assumption
of dust or allow the development of surface layers. Either option fundamentally
changes the original construction.Comment: 5 pages revtex 4.1 Final form to appear in Phys. Rev.
A (giant) void is not mandatory to explain away dark energy with a Lemaitre - Tolman model
Lema\^itre - Tolman (L--T) toy models with a central observer have been used
to study the effect of large scale inhomogeneities on the SN Ia dimming. Claims
that a giant void is mandatory to explain away dark energy in this framework
are currently dominating. Our aim is to show that L-T models exist that
reproduce a few features of the CDM model, but do not contain the
giant cosmic void. We propose to use two sets of data - the angular diameter
distance together with the redshift-space mass-density and the angular diameter
distance together with the expansion rate - both defined on the past null cone
as functions of the redshift. We assume that these functions are of the same
form as in the CDM model. Using the Mustapha-Hellaby-Ellis algorithm,
we numerically transform these initial data into the usual two L-T arbitrary
functions and solve the evolution equation to calculate the mass distribution
in spacetime. For both models, we find that the current density profile does
not exhibit a giant void, but rather a giant hump. However, this hump is not
directly observable, since it is in a spacelike relation to a present observer.
The alleged existence of the giant void was a consequence of the L-T models
used earlier because their generality was limited a priori by needless
simplifying assumptions, like, for example, the bang-time function being
constant. Instead, one can feed any mass distribution or expansion rate history
on the past light cone as initial data to the L-T evolution equation. When a
fully general L-T metric is used, the giant void is not implied.Comment: 25 pages, 20 figures, substantially revised to match the published
version, figs. 9 and 19 changed to match the caption
Radial asymptotics of Lemaitre-Tolman-Bondi dust models
We examine the radial asymptotic behavior of spherically symmetric
Lemaitre-Tolman-Bondi dust models by looking at their covariant scalars along
radial rays, which are spacelike geodesics parametrized by proper length
, orthogonal to the 4-velocity and to the orbits of SO(3). By introducing
quasi-local scalars defined as integral functions along the rays, we obtain a
complete and covariant representation of the models, leading to an initial
value parametrization in which all scalars can be given by scaling laws
depending on two metric scale factors and two basic initial value functions.
Considering regular "open" LTB models whose space slices allow for a diverging
, we provide the conditions on the radial coordinate so that its
asymptotic limit corresponds to the limit as . The "asymptotic
state" is then defined as this limit, together with asymptotic series expansion
around it, evaluated for all metric functions, covariant scalars (local and
quasi-local) and their fluctuations. By looking at different sets of initial
conditions, we examine and classify the asymptotic states of parabolic,
hyperbolic and open elliptic models admitting a symmetry center. We show that
in the radial direction the models can be asymptotic to any one of the
following spacetimes: FLRW dust cosmologies with zero or negative spatial
curvature, sections of Minkowski flat space (including Milne's space), sections
of the Schwarzschild--Kruskal manifold or self--similar dust solutions.Comment: 44 pages (including a long appendix), 3 figures, IOP LaTeX style.
Typos corrected and an important reference added. Accepted for publication in
General Relativity and Gravitatio
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
Generalized Swiss-cheese cosmologies: Mass scales
We generalize the Swiss-cheese cosmologies so as to include nonzero linear
momenta of the associated boundary surfaces. The evolution of mass scales in
these generalized cosmologies is studied for a variety of models for the
background without having to specify any details within the local
inhomogeneities. We find that the final effective gravitational mass and size
of the evolving inhomogeneities depends on their linear momenta but these
properties are essentially unaffected by the details of the background model.Comment: 10 pages, 14 figures, 1 table, revtex4, Published form (with minor
corrections
Conditions for spontaneous homogenization of the Universe
The present-day Universe appears to be homogeneous on very large scales. Yet
when the casual structure of the early Universe is considered, it becomes
apparent that the early Universe must have been highly inhomogeneous. The
current paradigm attempts to answer this problem by postulating the inflation
mechanism However, inflation in order to start requires a homogeneous patch of
at least the horizon size. This paper examines if dynamical processes of the
early Universe could lead to homogenization. In the past similar studies seem
to imply that the set of initial conditions that leads to homogenization is of
measure zero. This essay proves contrary: a set of initial conditions for
spontaneous homogenization of cosmological models can form a set of non-zero
measure.Comment: 7 pages. Fifth Award in the 2010 Gravity Research Foundation essay
competitio
Covariant coarse-graining of inhomogeneous dust flow in General Relativity
A new definition of coarse-grained quantities describing the dust flow in
General Relativity is proposed. It assigns the coarse--grained expansion, shear
and vorticity to finite-size comoving domains of fluid in a covariant,
coordinate-independent manner. The coarse--grained quantities are all
quasi-local functionals, depending only on the geometry of the boundary of the
considered domain. They can be thought of as relativistic generalizations of
simple volume averages of local quantities in a flat space. The procedure is
based on the isometric embedding theorem for S^2 surfaces and thus requires the
boundary of the domain in question to have spherical topology and positive
scalar curvature. We prove that in the limit of infinitesimally small volume
the proposed quantities reproduce the local expansion, shear and vorticity. In
case of irrotational flow we derive the time evolution for the coarse-grained
quantities and show that its structure is very similar to the evolution
equation for their local counterparts. Additional terms appearing in it may
serve as a measure of the backreacton of small-scale inhomogeneities of the
flow on the large-scale motion of the fluid inside the domain and therefore the
result may be interesting in the context of the cosmological backreaction
problem. We also consider the application of the proposed coarse-graining
procedure to a number of known exact solutions of Einstein equations with dust
and show that it yields reasonable results.Comment: 17 pages, 5 figures. Version accepted in Classical and Quantum
Gravity