9,172 research outputs found
Is Vacuum Decay Significant in Ekpyrotic and Cyclic Models?
It has recently been argued that bubble nucleation in ekpyrotic and cyclic
cosmological scenarios can lead to unacceptable inhomogeneities unless certain
constraints are satisfied. In this paper we show that this is not the case. We
find that bubble nucleation is completely negligible in realistic models.Comment: 3 pages, 1 figure, minor revision
Generating Scale-Invariant Perturbations from Rapidly-Evolving Equation of State
Recently, we introduced an ekpyrotic model based on a single, canonical
scalar field that generates nearly scale invariant curvature fluctuations
through a purely "adiabatic mechanism" in which the background evolution is a
dynamical attractor. Despite the starkly different physical mechanism for
generating fluctuations, the two-point function is identical to inflation. In
this paper, we further explore this concept, focusing in particular on issues
of non-gaussianity and quantum corrections. We find that the degeneracy with
inflation is broken at three-point level: for the simplest case of an
exponential potential, the three-point amplitude is strongly scale dependent,
resulting in a breakdown of perturbation theory on small scales. However, we
show that the perturbative breakdown can be circumvented -- and all issues
raised in Linde et al. (arXiv:0912.0944) can be addressed -- by altering the
potential such that power is suppressed on small scales. The resulting range of
nearly scale invariant, gaussian modes can be as much as twelve e-folds, enough
to span the scales probed by microwave background and large scale structure
observations. On smaller scales, the spectrum is not scale invariant but is
observationally acceptable.Comment: 42 pages, 1 figur
Flat 3-Brane with Tension in Cascading Gravity
In the Cascading Gravity brane-world scenario, our 3-brane lies within a
succession of lower-codimension branes, each with their own induced gravity
term, embedded into each other in a higher-dimensional space-time. In the
6+1-dimensional version of this scenario, we show that a 3-brane with tension
remains flat, at least for sufficiently small tension that the weak-field
approximation is valid. The bulk solution is nowhere singular and remains in
the perturbative regime everywhere.Comment: 4 pages, 3 figures. v2: version to appear in PR
The cosmic gravitational wave background in a cyclic universe
Inflation predicts a primordial gravitational wave spectrum that is slightly
``red,'' i.e., nearly scale-invariant with slowly increasing power at longer
wavelengths. In this paper, we compute both the amplitude and spectral form of
the primordial tensor spectrum predicted by cyclic/ekpyrotic models. The
spectrum is blue and exponentially suppressed compared to inflation on long
wavelengths. The strongest observational constraint emerges from the
requirement that the energy density in gravitational waves should not exceed
around 10 per cent of the energy density at the time of nucleosynthesis.Comment: 4 pages, 3 figuer
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