4,556 research outputs found
Cosmological Bounds on Spatial Variations of Physical Constants
We derive strong observational limits on any possible large-scale spatial
variation in the values of physical 'constants' whose space-time evolution is
driven by a scalar field. The limits are imposed by the isotropy of the
microwave background on large angular scales in theories which describe space
and time variations in the fine structure constant, the electron-proton mass
ratio, and the Newtonian gravitational constant, G. Large-scale spatial
fluctuations in the fine structure constant are bounded by 2x10^-9 and
1.2x10^-8 in the BSBM and VSL theories respectively, fluctuations in the
electron-proton mass ratio by 9x10^-5 in the BM theory and fluctuations in G by
3.6x10^-10 in Brans-Dicke theory. These derived bounds are significantly
stronger than any obtainable by direct observations of astrophysical objects at
the present time.Comment: 13 pages, 1 table, typos corrected, refs added. Published versio
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
Dynamical study of the empty Bianchi type I model in generalised scalar-tensor theory
A dynamical study of the generalised scalar-tensor theory in the empty
Bianchi type I model is made. We use a method from which we derive the sign of
the first and second derivatives of the metric functions and examine three
different theories that can all tend towards relativistic behaviours at late
time. We determine conditions so that the dynamic be in expansion and
decelerated at late time.Comment: 18 pages, 3 figures, to appear in General Relativity and Gravitatio
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
Newtonian nonlinear hydrodynamics and magnetohydrodynamics
We use covariant methods to analyse the nonlinear evolution of
self-gravitating, non-relativistic media. The formalism is first applied to
imperfect fluids, aiming at the kinematic effects of viscosity, before extended
to inhomogeneous magnetised environments. The nonlinear electrodynamic formulae
are derived and successively applied to electrically resistive and to highly
conductive fluids. By nature, the covariant equations isolate the magnetic
effects on the kinematics and the dynamics of the medium, combining
mathematical transparency and physical clarity. Employing the Newtonian
analogue of the relativistic 1+3 covariant treatment, also facilitates the
direct comparison with the earlier relativistic studies and helps to identify
the differences in an unambiguous way. The purpose of this work is to set the
framework and take a first step towards the detailed analytical study of
complex nonlinear systems, like non-relativistic astrophysical plasmas and
collapsing protogalactic clouds.Comment: Typos corrected, references added and updated (MNRAS in press
Cosmological dynamics of exponential gravity
We present a detailed investigation of the cosmological dynamics based on
gravity. We apply the dynamical system approach to both
the vacuum and matter cases and obtain exact solutions and their stability in
the finite and asymptotic regimes. The results show that cosmic histories exist
which admit a double de-Sitter phase which could be useful for describing the
early and the late-time accelerating universe.Comment: 17 pages LaTeX, 3 figure
Evidence of vorticity and shear at large angular scales in the WMAP data: a violation of cosmological isotropy?
Motivated by the large-scale asymmetry observed in the cosmic microwave
background sky, we consider a specific class of anisotropic cosmological models
-- Bianchi type VII_h -- and compare them to the WMAP first-year data on large
angular scales. Remarkably, we find evidence of a correlation which is ruled
out as a chance alignment at the 3sigma level. The best fit Bianchi model
corresponds to x=0.55, Omega_0=0.5, a rotation axis in the direction
(l,b)=(222degr,-62degr), shear (sigma/H)_0=2.4e-10 and a right--handed
vorticity (omega/H)_0=6.1e-10. Correcting for this component greatly reduces
the significance of the large-scale power asymmetry, resolves several anomalies
detected on large angular scales (ie. the low quadrupole amplitude and
quadrupole/octopole planarity and alignment), and can account for a
non--Gaussian "cold spot" on the sky. Despite the apparent inconsistency with
the best-fit parameters required in inflationary models to account for the
acoustic peaks, we consider the results sufficiently provocative to merit
further consideration.Comment: 4 pages, 3 figures; emulateapj.cls; ApJL accepted version plus fixed
error in vorticity calculation (sqrt(2) off in Table 1, abstract, and
conclusions); basic conclusions unchange
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