1,928 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
A Cyclic Model of the Universe
We propose a cosmological model in which the universe undergoes an endless
sequence of cosmic epochs each beginning with a `bang' and ending in a
`crunch.' The temperature and density are finite at each transition from crunch
to bang. Instead of having an inflationary epoch, each cycle includes a period
of slow accelerated expansion (as recently observed) followed by slow
contraction. The combination produces the homogeneity, flatness, density
fluctuations and energy needed to begin the next cycle.Comment: 15 pages, 1 figure, revisions as publishe
A new kind of cyclic universe
Combining intervals of ekpyrotic (ultra-slow) contraction with a
(non-singular) classical bounce naturally leads to a novel cyclic theory of the
universe in which the Hubble parameter, energy density and temperature
oscillate periodically, but the scale factor grows by an exponential factor
from one cycle to the next. The resulting cosmology not only resolves the
homogeneity, isotropy, flatness and monopole problems and generates a nearly
scale invariant spectrum of density perturbations, but it also addresses a
number of age-old cosmological issues that big bang inflationary cosmology does
not. There may also be wider-ranging implications for fundamental physics,
black holes and quantum measurement.Comment: 7 pages, 3 figure
Dynamical Selection of the Primordial Density Fluctuation Amplitude
In inflationary models, the predicted amplitude of primordial density
perturbations Q is much larger than the observed value (~10^{-5}) for natural
choices of parameters. To explain the requisite exponential fine-tuning,
anthropic selection is often invoked, especially in cases where microphysics is
expected to produce a complex energy landscape. By contrast, we find examples
of ekpyrotic models based on heterotic M-theory for which dynamical selection
naturally favors the observed value of Q.Comment: 4 pages; v2: version to be published in PR
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