3,465 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
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
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
Anisotropically Inflating Universes
We show that in theories of gravity that add quadratic curvature invariants
to the Einstein-Hilbert action there exist expanding vacuum cosmologies with
positive cosmological constant which do not approach the de Sitter universe.
Exact solutions are found which inflate anisotropically. This behaviour is
driven by the Ricci curvature invariant and has no counterpart in the general
relativistic limit. These examples show that the cosmic no-hair theorem does
not hold in these higher-order extensions of general relativity and raises new
questions about the ubiquity of inflation in the very early universe and the
thermodynamics of gravitational fields.Comment: 5 pages, further discussion and references adde
Cosmological Co-evolution of Yang-Mills Fields and Perfect Fluids
We study the co-evolution of Yang-Mills fields and perfect fluids in Bianchi
type I universes. We investigate numerically the evolution of the universe and
the Yang-Mills fields during the radiation and dust eras of a universe that is
almost isotropic. The Yang-Mills field undergoes small amplitude chaotic
oscillations, which are also displayed by the expansion scale factors of the
universe. The results of the numerical simulations are interpreted analytically
and compared with past studies of the cosmological evolution of magnetic fields
in radiation and dust universes. We find that, whereas magnetic universes are
strongly constrained by the microwave background anisotropy, Yang-Mills
universes are principally constrained by primordial nucleosynthesis and the
bound is comparatively weak, and Omega_YM < 0.105 Omega_rad.Comment: 13 pages, 5 figures, submitted to PR
The Power of General Relativity
We study the cosmological and weak-field properties of theories of gravity
derived by extending general relativity by means of a Lagrangian proportional
to . This scale-free extension reduces to general relativity when
. In order to constrain generalisations of general relativity of
this power class we analyse the behaviour of the perfect-fluid Friedmann
universes and isolate the physically relevant models of zero curvature. A
stable matter-dominated period of evolution requires or . The stable attractors of the evolution are found. By considering the
synthesis of light elements (helium-4, deuterium and lithium-7) we obtain the
bound We evaluate the effect on the power spectrum of
clustering via the shift in the epoch of matter-radiation equality. The horizon
size at matter--radiation equality will be shifted by for a value of
We study the stable extensions of the Schwarzschild
solution in these theories and calculate the timelike and null geodesics. No
significant bounds arise from null geodesic effects but the perihelion
precession observations lead to the strong bound assuming that Mercury follows a timelike geodesic. The combination of
these observational constraints leads to the overall bound on theories of this type.Comment: 26 pages and 5 figures. 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
Plane-symmetric inhomogeneous Brans-Dicke cosmology with an equation of state
We present a new exact solution in Brans-Dicke theory. The solution describes
inhomogeneous plane-symmetric perfect fluid cosmological model with an equation
of state . Some main properties of the solution are discussed.Comment: 6 pages, Late
Vacuum Structure and the Arrow of Time
We find ourselves in an extended era of entropy production. Unlike most other
observations, the arrow of time is usually regarded as a constraint on initial
conditions. I argue, however, that it primarily constrains the vacuum structure
of the theory. I exhibit simple scalar field potentials in which low-entropy
initial conditions are not necessary, or not sufficient, for an arrow of time
to arise. I argue that the string theory landscape gives rise to an arrow of
time independently of the initial entropy, assuming a plausible condition on
the lifetime of its metastable vacua. The dynamical resolution of the arrow of
time problem arises from the same structural properties of the string landscape
that allow it to solve the cosmological constant problem without producing an
empty universe, particularly its high dimensionality and the large difference
in vacuum energy between neighboring vacua.Comment: 31 pages JHEP format, 3 figure
Populating the Landscape: A Top Down Approach
We put forward a framework for cosmology that combines the string landscape
with no boundary initial conditions. In this framework, amplitudes for
alternative histories for the universe are calculated with final boundary
conditions only. This leads to a top down approach to cosmology, in which the
histories of the universe depend on the precise question asked. We study the
observational consequences of no boundary initial conditions on the landscape,
and outline a scheme to test the theory. This is illustrated in a simple model
landscape that admits several alternative inflationary histories for the
universe. Only a few of the possible vacua in the landscape will be populated.
We also discuss in what respect the top down approach differs from other
approaches to cosmology in the string landscape, like eternal inflation.Comment: 22 pages, 1 figur
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