4,619 research outputs found
Stable Isotropic Cosmological Singularities in Quadratic Gravity
We show that, in quadratic lagrangian theories of gravity, isotropic
cosmological singularities are stable to the presence of small scalar, vector
and tensor inhomogeneities. Unlike in general relativity, a particular exact
isotropic solution is shown to be the stable attractor on approach to the
initial cosmological singularity. This solution is also known to act as an
attractor in Bianchi universes of types I, II and IX, and the results of this
paper reinforce the hypothesis that small inhomogeneous and anisotropic
perturbations of this attractor form part of the general cosmological solution
to the field equations of quadratic gravity. Implications for the existence of
a 'gravitational entropy' are also discussed.Comment: 18 pages, no figure
Bouncing Universes with Varying Constants
We investigate the behaviour of exact closed bouncing Friedmann universes in
theories with varying constants. We show that the simplest BSBM varying-alpha
theory leads to a bouncing universe. The value of alpha increases
monotonically, remaining approximately constant during most of each cycle, but
increasing significantly around each bounce. When dissipation is introduced we
show that in each new cycle the universe expands for longer and to a larger
size. We find a similar effect for closed bouncing universes in Brans-Dicke
theory, where also varies monotonically in time from cycle to cycle.
Similar behaviour occurs also in varying speed of light theories
Cosmologies with Energy Exchange
We provide a simple mathematical description of the exchange of energy
between two fluids in an expanding Friedmann universe with zero spatial
curvature. The evolution can be reduced to a single non-linear differential
equation which we solve in physically relevant cases and provide an analysis of
all the possible evolutions. Particular power-law solutions exist for the
expansion scale factor and are attractors at late times under particular
conditions. We show how a number of problems studied in the literature, such as
cosmological vacuum energy decay, particle annihilation, and the evolution of a
population of evaporating black holes, correspond to simple particular cases of
our model. In all cases we can determine the effects of the energy transfer on
the expansion scale factor. We also consider the situation in the presence of
anti-decaying fluids and so called phantom fluids which violate the dominant
energy conditions.Comment: 12 pages, 1 figur
Cosmological Constraints on a Dynamical Electron Mass
Motivated by recent astrophysical observations of quasar absorption systems,
we formulate a simple theory where the electron to proton mass ratio is allowed to vary in space-time. In such a minimal theory only
the electron mass varies, with and kept constant. We find
that changes in will be driven by the electronic energy density after
the electron mass threshold is crossed. Particle production in this scenario is
negligible. The cosmological constraints imposed by recent astronomical
observations are very weak, due to the low mass density in electrons. Unlike in
similar theories for spacetime variation of the fine structure constant, the
observational constraints on variations in imposed by the weak
equivalence principle are much more stringent constraints than those from
quasar spectra. Any time-variation in the electron-proton mass ratio must be
less than one part in since redshifts This is more than
one thousand times smaller than current spectroscopic sensitivities can
achieve. Astronomically observable variations in the electron-proton must
therefore arise directly from effects induced by varying fine structure
'constant' or by processes associated with internal proton structure. We also
place a new upper bound of on any large-scale spatial
variation of that is compatible with the isotropy of the microwave
background radiation.Comment: New bounds from weak equivalence principle experiments added,
conclusions modifie
Spatial Variations of Fundamental Constants
We show that observational limits on the possible time variation of constants
of Nature are significantly affected by allowing for both space and time
variation. Bekenstein's generalisation of Maxwell's equations to allow for
cosmological variation of is investigated in a universe containing
spherically symmetric inhomogeneities. The time variation of is
determined by the local matter density and hence limits obtained in
high-density geophysical enviroments are far more constraining than those
obtained at high redshift. This new feature is expected to be a property of a
wide class of theories for the variation of constants.Comment: 4 page
Some Late-time Asymptotics of General Scalar-Tensor Cosmologies
We study the asymptotic behaviour of isotropic and homogeneous universes in
general scalar-tensor gravity theories containing a p=-rho vacuum fluid stress
and other sub-dominant matter stresses. It is shown that in order for there to
be approach to a de Sitter spacetime at large 4-volumes the coupling function,
omega(phi), which defines the scalar-tensor theory, must diverge faster than
|phi_infty-phi|^(-1+epsilon) for all epsilon>0 as phi rightarrow phi_infty 0
for large values of the time. Thus, for a given theory, specified by
omega(phi), there must exist some phi_infty in (0,infty) such that omega ->
infty and omega' / omega^(2+epsilon) -> 0 as phi -> 0 phi_infty in order for
cosmological solutions of the theory to approach de Sitter expansion at late
times. We also classify the possible asymptotic time variations of the
gravitation `constant' G(t) at late times in scalar-tensor theories. We show
that (unlike in general relativity) the problem of a profusion of ``Boltzmann
brains'' at late cosmological times can be avoided in scalar-tensor theories,
including Brans-Dicke theory, in which phi -> infty and omega ~ o(\phi^(1/2))
at asymptotically late times.Comment: 14 page
Cosmological milestones and energy conditions
Until recently, the physically relevant singularities occurring in FRW
cosmologies had traditionally been thought to be limited to the "big bang", and
possibly a "big crunch". However, over the last few years, the zoo of
cosmological singularities considered in the literature has become considerably
more extensive, with "big rips" and "sudden singularities" added to the mix, as
well as renewed interest in non-singular cosmological events such as "bounces"
and "turnarounds". In this talk, we present an extensive catalogue of such
cosmological milestones, both at the kinematical and dynamical level. First,
using generalized power series, purely kinematical definitions of these
cosmological events are provided in terms of the behaviour of the scale factor
a(t). The notion of a "scale-factor singularity" is defined, and its relation
to curvature singularities (polynomial and differential) is explored. Second,
dynamical information is extracted by using the Friedmann equations (without
assuming even the existence of any equation of state) to place constraints on
whether or not the classical energy conditions are satisfied at the
cosmological milestones. Since the classification is extremely general, and
modulo certain technical assumptions complete, the corresponding results are to
a high degree model-independent.Comment: 8 pages, 1 table, conference proceedings for NEB XII conference in
Nafplio, Greec
The Local Effects of Cosmological Variations in Physical 'Constants' and Scalar Fields I. Spherically Symmetric Spacetimes
We apply the method of matched asymptotic expansions to analyse whether
cosmological variations in physical `constants' and scalar fields are
detectable, locally, on the surface of local gravitationally bound systems such
as planets and stars, or inside virialised systems like galaxies and clusters.
We assume spherical symmetry and derive a sufficient condition for the local
time variation of the scalar fields that drive varying constants to track the
cosmological one. We calculate a number of specific examples in detail by
matching the Schwarzschild spacetime to spherically symmetric inhomogeneous
Tolman-Bondi metrics in an intermediate region by rigorously construction
matched asymptotic expansions on cosmological and local astronomical scales
which overlap in an intermediate domain. We conclude that, independent of the
details of the scalar-field theory describing the varying `constant', the
condition for cosmological variations to be measured locally is almost always
satisfied in physically realistic situations. The proof of this statement
provides a rigorous justification for using terrestrial experiments and solar
system observations to constrain or detect any cosmological time variations in
the traditional `constants' of Nature.Comment: 30 pages, 3 figures; corrected typo
The future asymptotics of Bianchi VIII vacuum solutions
Bianchi VIII vacuum solutions to Einstein's equations are causally
geodesically complete to the future, given an appropriate time orientation, and
the objective of this article is to analyze the asymptotic behaviour of
solutions in this time direction. For the Bianchi class A spacetimes, there is
a formulation of the field equations that was presented in an article by
Wainwright and Hsu, and we analyze the asymptotic behaviour of solutions in
these variables. We also try to give the analytic results a geometric
interpretation by analyzing how a normalized version of the Riemannian metric
on the spatial hypersurfaces of homogeneity evolves.Comment: 34 pages, no figure
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