990 research outputs found
Matter Bounce in Horava-Lifshitz Cosmology
Horava-Lifshitz gravity, a recent proposal for a UV-complete renormalizable
gravity theory, may lead to a bouncing cosmology. In this note we argue that
Horava-Lifshitz cosmology may yield a concrete realization of the matter bounce
scenario, and thus give rise to an alternative to inflation for producing a
scale-invariant spectrum of cosmological perturbations. In this scenario,
quantum vacuum fluctuations exit the Hubble radius in the pre-bounce phase and
the spectrum is transformed into a scale-invariant one on super-Hubble scales
before the bounce because the long wavelength modes undergo squeezing of their
wave-functions for a longer period of time than shorter wavelength modes. The
scale-invariance of the spectrum of curvature fluctuations is preserved during
and after the bounce. A distinctive prediction of this scenario is the
amplitude and shape of the bispectrum.Comment: 6 pages, 1 figure, a couple of minor wording change
Perturbations in a Bouncing Brane Model
The question of how perturbations evolve through a bounce in the Cyclic and
Ekpyrotic models of the Universe is still a matter of ongoing debate. In this
report we show that the collision between boundary branes is in most cases
singular even in the full 5-D formalism, and that first order perturbation
theory breaks down for at least one perturbation variable. Only in the case
that the boundary branes approach each other with constant velocity shortly
before the bounce, can a consistent, non singular solution be found. It is then
possible to follow the perturbations explicitly until the actual collision. In
this case, we find that if a scale invariant spectrum developed on the hidden
brane, it will get transferred to the visible brane during the bounce.Comment: 15 pages, minor modifications, a few typos correcte
Thermal Equilibrium of String Gas in Hagedorn Universe
The thermal equilibrium of string gas is necessary to activate the
Brandenberger-Vafa mechanism, which makes our observed 4-dimensional universe
enlarge. Nevertheless, the thermal equilibrium is not realized in the original
setup, a problem that remains as a critical defect. We study thermal
equilibrium in the Hagedorn universe, and explore possibilities for avoiding
the issue aforementioned flaw. We employ a minimal modification of the original
setup, introducing a dilaton potential. Two types of potential are
investigated: exponential and double-well potentials. For the first type, the
basic evolutions of universe and dilaton are such that both the radius of the
universe and the dilaton asymptotically grow in over a short time, or that the
radius converges to a constant value while the dilaton rolls down toward the
weak coupling limit. For the second type, in addition to the above solutions,
there is another solution in which the dilaton is stabilized at a minimum of
potential and the radius grows in proportion to . Thermal equilibrium is
realized for both cases during the initial phase. These simple setups provide
possible resolutions of the difficulty.Comment: 23 pages,19 figure
Processing of Cosmological Perturbations in a Cyclic Cosmology
The evolution of the spectrum of cosmological fluctuations from one cycle to
the next is studied. It is pointed out that each cycle leads to a reddening of
the spectrum. This opens up new ways to generate a scale-invariant spectrum of
curvature perturbations. The large increase in the amplitude of the
fluctuations quickly leads to a breakdown of the linear theory. More generaly,
we see that, after including linearized cosmological perturbations, a cyclic
universe cannot be truly cyclic.Comment: 5 pages, 1 figur
Producing a Scale-Invariant Spectrum of Perturbations in a Hagedorn Phase of String Cosmology
We study the generation of cosmological perturbations during the Hagedorn
phase of string gas cosmology. Using tools of string thermodynamics we provide
indications that it may be possible to obtain a nearly scale-invariant spectrum
of cosmological fluctuations on scales which are of cosmological interest
today. In our cosmological scenario, the early Hagedorn phase of string gas
cosmology goes over smoothly into the radiation-dominated phase of standard
cosmology, without having a period of cosmological inflation.Comment: 4 pages, 1 figur
Dynamical Relaxation of the Cosmological Constant and Matter Creation in the Universe
In this Letter we discuss the issues of the graceful exit from inflation and
of matter creation in the context of a recent scenario \cite{RHBrev} in which
the back-reaction of long wavelength cosmological perturbations induces a
negative contribution to the cosmological constant and leads to a dynamical
relaxation of the bare cosmological constant. The initially large cosmological
constant gives rise to primordial inflation, during which cosmological
perturbations are stretched beyond the Hubble radius. The cumulative effect of
the long wavelength fluctuations back-reacts on the background geometry in a
form which corresponds to the addition of a negative effective cosmological
constant to the energy-momentum tensor. In the absence of an effective scalar
field driving inflation, whose decay can reheat the Universe, the challenge is
to find a mechanism which produces matter at the end of the relaxation process.
In this Letter, we point out that the decay of a condensate representing the
order parameter for a ``flat'' direction in the field theory moduli space can
naturally provide a matter generation mechanism. The order parameter is
displaced from its vacuum value by thermal or quantum fluctuations, it is
frozen until the Hubble constant drops to a sufficiently low value, and then
begins to oscillate about its ground state. During the period of oscillation it
can decay into Standard Model particles similar to how the inflaton decays in
scalar-field-driven models of inflation.Comment: 6 page
On the new string theory inspired mechanism of generation of cosmological perturbations
Recently a non-inflationary mechanism of generation of scale-free
cosmological perturbations of metric was proposed by Brandenberger, Nayeri, and
Vafa in the context of the string gas cosmology. We discuss various problems of
their model and argue that the cosmological perturbations of metric produced in
this model have blue spectrum with a spectral index n = 5, which strongly
disagrees with observations. We conclude that this model in its present form is
not a viable alternative to inflationary cosmology.Comment: 11 pages, 1 figur
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