4,063 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

### 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

### A New Solution of The Cosmological Constant Problems

We extend the usual gravitational action principle by promoting the bare
cosmological constant (CC) from a parameter to a field which can take many
possible values. Variation leads to a new integral constraint equation which
determines the classical value of the effective CC that dominates the wave
function of the universe. In a realistic cosmological model, the expected value
of the effective CC, is calculated from measurable quantities to be O(t_U), as
observed, where t_U is the present age of the universe in Planck units,. Any
application of our model produces a falsifiable prediction for $\Lambda$ in
terms of other measurable quantities. This leads to a specific falsifiable
prediction for the observed spatial curvature parameter of Omega_k0=-0.0055.
Our testable proposal requires no fine tunings or extra dark-energy fields but
does suggest a new view of time and cosmological evolution.Comment: 5 pages; v3: version accepted by Phys. Rev. Let

### 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 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 $\mu
=m_{e}/m_{p}$ is allowed to vary in space-time. In such a minimal theory only
the electron mass varies, with $\alpha$ and $m_{p}$ kept constant. We find
that changes in $\mu$ 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 $\mu$ 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 $10^{9}$since redshifts $z\approx 1.$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 $2\times 10^{-8}$ on any large-scale spatial
variation of $\mu$ that is compatible with the isotropy of the microwave
background radiation.Comment: New bounds from weak equivalence principle experiments added,
conclusions modifie

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