5,832 research outputs found
A Strong Constraint on Ever-Present Lambda
We show that the causal set approach to creating an ever-present cosmological
'constant' in the expanding universe is strongly constrained by the isotropy of
the microwave background. Fluctuations generated by stochastic lambda
generation which are consistent with COBE and WMAP observations are far too
small to dominate the expansion dynamics at z<1000 and so cannot explain the
observed late-time acceleration of the universe. We also discuss other
observational constraints from the power spectrum of galaxy clustering and show
that the theoretical possibility of ever-present lambda arises only in 3+1
dimensional space-times.Comment: 5 pages, minor additions, published versio
Simple Types of Anisotropic Inflation
We display some simple cosmological solutions of gravity theories with
quadratic Ricci curvature terms added to the Einstein-Hilbert lagrangian which
exhibit anisotropic inflation. The Hubble expansion rates are constant and
unequal in three orthogonal directions. We describe the evolution of the
simplest of these homogeneous and anisotropic cosmological models from its
natural initial state and evaluate the deviations they will create from
statistical isotropy in the fluctuations produced during a period of
anisotropic inflation. The anisotropic inflation is not a late-time attractor
in these models but the rate of approach to a final isotropic de Sitter state
is slow and is conducive to the creation of observable anisotropic statistical
effects in the microwave background. The statistical anisotropy would not be
scale invariant and the level of statistical anisotropy will grow with scale.Comment: 8pages, 3 figs v2:refs added, typos fixe
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
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
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
Unusual Features of Varying Speed of Light Cosmologies
We contrast features of simple varying speed of light (VSL) cosmologies with
inflationary universe models. We present new features of VSL cosmologies and
show that they face problems explaining the cosmological isotropy problem. We
also find that if c falls fast enough to solve the flatness and horizon
problems then the quantum wavelengths of massive particle states and the radii
of primordial black holes can grow to exceed the scale of the particle horizon.
This may provide VSL cosmologies with a self-reproduction property. The
constraint of entropy increase is also discussed. The new problems described in
the this letter provide a set of bench tests for more sophisticated VSL
theories to pass.Comment: expanded version, 12 page
- …