279 research outputs found

### A Note on Infinities in Eternal Inflation

In some well-known scenarios of open-universe eternal inflation, developed by
Vilenkin and co-workers, a large number of universes nucleate and thermalize
within the eternally inflating mega-universe. According to the proposal, each
universe nucleates at a point, and therefore the boundary of the nucleated
universe is a space-like surface nearly coincident with the future light cone
emanating from the point of nucleation, all points of which have the same
proper-time. This leads the authors to conclude that at the proper-time t =
t_{nuc} at which any such nucleation occurs, an infinite open universe comes
into existence. We point out that this is due entirely to the supposition of
the nucleation occurring at a single point, which in light of quantum cosmology
seems difficult to support. Even an infinitesimal space-like length at the
moment of nucleation gives a rather different result -- the boundary of the
nucleating universe evolves in proper-time and becomes infinite only in an
infinite time. The alleged infinity is never attained at any finite time.Comment: 13 pages and 6 figure

### Note on Varying Speed of Light Cosmologies

The various requirements on a consistent varying speed of light (`VSL')
theory are surveyed, giving a short check-list of issues that should be
satisfactorily handled by such theories.Comment: 6 pages; to appear in the GRG Journa

### Birkhoff Theorem and Matter

Birkhoff's theorem for spherically symmetric vacuum spacetimes is a key
theorem in studying local systems in general relativity theory. However
realistic local systems are only approximately spherically symmetric and only
approximately vacuum. In a previous paper, we showed the theorem remains
approximately true in an approximately spherically symmetric vacuum space time.
In this paper we prove the converse case: the theorem remains approximately
true in a spherically symmetric, approximately vacuum space time.Comment: 7 pages, Revtex

### Properties of Information Carrying Waves in Cosmology

Recently we studied the effects of information carrying waves propagating
through isotropic cosmologies. By information carrying we mean that the waves
have an arbitrary dependence on a function. We found that the waves introduce
shear and anisotropic stress into the universe. We then constructed explicit
examples of pure gravity wave perturbations for which the presence of this
anisotropic stress is essential and the null hypersurfaces playing the role of
the histories of the wave-fronts in the background space-time are shear-free.
Motivated by this result we now prove that these two properties are true for
all information carrying waves in isotropic cosmologies.Comment: 15 pages, Latex File, accepted for publication in Physical Review

### Physics in the Real Universe: Time and Spacetime

The Block Universe idea, representing spacetime as a fixed whole, suggests
the flow of time is an illusion: the entire universe just is, with no special
meaning attached to the present time. This view is however based on
time-reversible microphysical laws and does not represent macro-physical
behaviour and the development of emergent complex systems, including life,
which do indeed exist in the real universe. When these are taken into account,
the unchanging block universe view of spacetime is best replaced by an evolving
block universe which extends as time evolves, with the potential of the future
continually becoming the certainty of the past. However this time evolution is
not related to any preferred surfaces in spacetime; rather it is associated
with the evolution of proper time along families of world linesComment: 28 pages, including 9 Figures. Major revision in response to referee
comment

### Reply Comment: Comparison of Approaches to Classical Signature Change

We contrast the two approaches to ``classical" signature change used by
Hayward with the one used by us (Hellaby and Dray). There is (as yet) no
rigorous derivation of appropriate distributional field equations. Hayward's
distributional approach is based on a postulated modified form of the field
equations. We make an alternative postulate. We point out an important
difference between two possible philosophies of signature change --- ours is
strictly classical, while Hayward's Lagrangian approach adopts what amounts to
an imaginary proper ``time" on one side of the signature change, as is
explicitly done in quantum cosmology. We also explain why we chose to use the
Darmois-Israel type junction conditions, rather than the Lichnerowicz type
junction conditions favoured by Hayward. We show that the difference in results
is entirely explained by the difference in philosophy (imaginary versus real
Euclidean ``time"), and not by the difference in approach to junction
conditions (Lichnerowicz with specific coordinates versus Darmois with general
coordinates).Comment: 10 pages, latex, no figures. Replying to - "Comment on `Failure of
Standard Conservation Laws at a Classical Change of Signature'", S.A.
Hayward, Phys. Rev. D52, 7331-7332 (1995) (gr-qc/9606045

### LIMITS ON ANISOTROPY AND INHOMOGENEITY FROM THE COSMIC BACKGROUND RADIATION,

We consider directly the equations by which matter imposes anisotropies on
freely propagating background radiation, leading to a new way of using
anisotropy measurements to limit the deviations of the Universe from a
Friedmann-Robertson-Walker (FRW) geometry. This approach is complementary to
the usual Sachs-Wolfe approach: the limits obtained are not as detailed, but
they are more model-independent. We also give new results about combined
matter-radiation perturbations in an almost-FRW universe, and a new exact
solution of the linearised equations.Comment: 18 pages Latex

### Finding a Spherically Symmetric Cosmology from Observations in Observational Coordinates -- Advantages and Challenges

One of the continuing challenges in cosmology has been to determine the
large-scale space-time metric from observations with a minimum of assumptions
-- without, for instance, assuming that the universe is almost
Friedmann-Lema\^{i}tre-Robertson-Walker (FLRW). If we are lucky enough this
would be a way of demonstrating that our universe is FLRW, instead of
presupposing it or simply showing that the observations are consistent with
FLRW. Showing how to do this within the more general spherically symmetric,
inhomogeneous space-time framework takes us a long way towards fulfilling this
goal. In recent work researchers have shown how this can be done both in the
traditional Lema\^{i}tre-Tolman-Bondi (LTB) 3 + 1 coordinate framework, and in
the observational coordinate (OC) framework. In this paper we investigate the
stability of solutions, and the use of data in the OC field equations including
their time evolution and compare both approaches with respect to the
singularity problem at the maximum of the angular-diameter distance, the
stability of solutions, and the use of data in the field equations. This allows
a more detailed account and assessment of the OC integration procedure, and
enables a comparison of the relative advantages of the two equivalent solution
frameworks. Both formulations and integration procedures should, in principle,
lead to the same results. However, as we show in this paper, the OC procedure
manifests certain advantages, particularly in the avoidance of coordinate
singularities at the maximum of the angular-diameter distance, and in the
stability of the solutions obtained. This particular feature is what allows us
to do the best fitting of the data to smooth data functions and the possibility
of constructing analytic solutions to the field equations.Comment: 31 page

### Emergent Universe in Brane World Scenario with Schwarzschild-de Sitter Bulk

A model of an emergent universe is obtained in brane world. Here the bulk
energy is in the form of cosmological constant, while the brane consists of a
fluid satisfying an equation of state of the form $p_{b}={1/3} \rho_{b}$, which
is effectively a radiation equation of state at high energies. It is shown that
with the positive bulk cosmological constant, one of our models represents an
emergent universe.Comment: 4 pages, no figure, accepted for publication in Gen.Relt.Gra

### Light-cone averaging in cosmology: formalism and applications

We present a general gauge invariant formalism for defining cosmological
averages that are relevant for observations based on light-like signals. Such
averages involve either null hypersurfaces corresponding to a family of past
light-cones or compact surfaces given by their intersection with timelike
hypersurfaces. Generalized Buchert-Ehlers commutation rules for derivatives of
these light-cone averages are given. After introducing some adapted "geodesic
light-cone" coordinates, we give explicit expressions for averaging the
redshift to luminosity-distance relation and the so-called "redshift drift" in
a generic inhomogeneous Universe.Comment: 20 pages, 2 figures. Comments and references added, typos corrected.
Version accepted for publication in JCA

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