764 research outputs found
Brans-Dicke Theory as a Unified Model for Dark Matter - Dark Energy
The Brans-Dicke (BD) theory of gravity is taken as a possible theory of
k-essence coupled to gravity. It then has been realized that the BD scalar
field does indeed play a role of a k-essence, but in a very unique way which
distinguishes it from other k-essence fields studied in the literature. That
is, first in the BD scalar field-dominated era when the contribution from this
k-essence overwhelms those from other types of matter, the BD theory predicts
the emergence of a yet-unknown {\it zero acceleration} epoch which is an
intermediate stage acting as a ``crossing bridge'' between the decelerating
matter-dominated era and the accelerating phase. Upon realizing this, next,
closer study of the effects of this k-essence on the evolutionary behavior of
the matter-dominated and the accelerating eras has been performed. The result
of the study indicates that the BD scalar field appears to interpolate {\it
smoothly} between these two late-time stages by speeding up the expansion rate
of the matter-dominated era somewhat while slowing down that of the
accelerating phase to some degree. Thus with the newly found BD scalar
field-dominated era in between these two, the late-time of the universe
evolution appears to be mixed sequence of the three stages.Comment: 25 pages, Mon. Not. R. Astron. Soc.(MNRAS), in pres
General conditions for scale-invariant perturbations in an expanding universe
We investigate the general properties of expanding cosmological models which
generate scale-invariant curvature perturbations in the presence of a variable
speed of sound. We show that in an expanding universe, generation of a
super-Hubble, nearly scale-invariant spectrum of perturbations over a range of
wavelengths consistent with observation requires at least one of three
conditions: (1) accelerating expansion, (2) a speed of sound faster than the
speed of light, or (3) super-Planckian energy density.Comment: 4 pages, RevTe
The effect of curvature in thawing models
We study the evolution of spatial curvature for thawing class of dark energy
models. We examine the evolution of the equation of state parameter, ,
as a function of the scale factor , for the case in which the scalar field
evolve in nearly flat scalar potential. We show that all such models
provide the corresponding approximate analytical expressions for
and . We present observational
constraints on these models.Comment: 14 pages, 6 figures. Accepted for publication in Phys. Lett.
Can We See Lorentz-Violating Vector Fields in the CMB?
We investigate the perturbation theory of a fixed-norm, timelike
Lorentz-violating vector field. After consistently quantizing the vector field
to put constraints on its parameters, we compute the primordial spectra of
perturbations generated by inflation in the presence of this vector field. We
find that its perturbations are sourced by the perturbations of the inflaton;
without the inflaton perturbation the vector field perturbations decay away
leaving no primordial spectra of perturbations. Since the inflaton perturbation
does not have a spin-1 component, the vector field generically does not
generate any spin-1 ``vector-type'' perturbations. Nevertheless, it will modify
the amplitude of both the spin-0 ``scalar-type'' and spin-2 ``tensor-type''
perturbation spectra, leading to violations of the inflationary consistency
relationship.Comment: 36 pages, 1 fig, RevTex4, Submitted to PR
Near Scale Invariance with Modified Dispersion Relations
We describe a novel mechanism to seed a nearly scale invariant spectrum of
adiabatic perturbations during a non-inflationary stage. It relies on a
modified dispersion relation that contains higher powers of the spatial
momentum of matter perturbations. We implement this idea in the context of a
massless scalar field in an otherwise perfectly homogeneous universe. The
couplings of the field to background scalars and tensors give rise to the
required modification of its dispersion relation, and the couplings of the
scalar to matter result in an adiabatic primordial spectrum. This work is meant
to explicitly illustrate that it is possible to seed nearly scale invariant
primordial spectra without inflation, within a conventional expansion history.Comment: 7 pages and no figures. Uses RevTeX
Some Aspects of Brane Inflation
The inflaton potential in four-dimensional theory is rather arbitrary, and
fine-tuning is required generically. By contrast, inflation in the brane world
scenario has the interesting feature that the inflaton potential is motivated
from higher dimensional gravity, or more generally, from bulk modes or string
theory. We emphasize this feature with examples. We also consider the impact on
the spectrum of density perturbation from a velocity-dependent potential
between branes in the brane inflationary scenario. It is likely that such a
potential can have an observable effect on the ratio of tensor to scalar
perturbations.Comment: 15 pages, 1 figure, references added, minor typos correcte
On A Cosmological Invariant as an Observational Probe in the Early Universe
k-essence scalar field models are usually taken to have lagrangians of the
form with some general function of
. Under certain conditions this lagrangian
in the context of the early universe can take the form of that of an oscillator
with time dependent frequency. The Ermakov invariant for a time dependent
oscillator in a cosmological scenario then leads to an invariant quadratic form
involving the Hubble parameter and the logarithm of the scale factor. In
principle, this invariant can lead to further observational probes for the
early universe. Moreover, if such an invariant can be observationally verified
then the presence of dark energy will also be indirectly confirmed.Comment: 4 pages, Revte
Compact boson stars in K field theories
We study a scalar field theory with a non-standard kinetic term minimally
coupled to gravity. We establish the existence of compact boson stars, that is,
static solutions with compact support of the full system with self-gravitation
taken into account. Concretely, there exist two types of solutions, namely
compact balls on the one hand, and compact shells on the other hand. The
compact balls have a naked singularity at the center. The inner boundary of the
compact shells is singular, as well, but it is, at the same time, a Killing
horizon. These singular, compact shells therefore resemble black holes.Comment: Latex, 45 pages, 25 figures, some references and comments adde
Where does Cosmological Perturbation Theory Break Down?
We apply the effective field theory approach to the coupled metric-inflaton
system, in order to investigate the impact of higher dimension operators on the
spectrum of scalar and tensor perturbations in the short-wavelength regime. In
both cases, effective corrections at tree-level become important when the
Hubble parameter is of the order of the Planck mass, or when the physical wave
number of a cosmological perturbation mode approaches the square of the Planck
mass divided by the Hubble constant. Thus, the cut-off length below which
conventional cosmological perturbation theory does not apply is likely to be
much smaller than the Planck length. This has implications for the
observability of "trans-Planckian" effects in the spectrum of primordial
perturbations.Comment: 25 pages, uses FeynM
Preheating in Derivatively-Coupled Inflation Models
We study preheating in theories where the inflaton couples derivatively to
scalar and gauge fields. Such couplings may dominate in natural models of
inflation, in which the flatness of the inflaton potential is related to an
approximate shift symmetry of the inflaton. We compare our results with
previously studied models with non-derivative couplings. For sufficiently heavy
scalar matter, parametric resonance is ineffective in reheating the universe,
because the couplings of the inflaton to matter are very weak. If scalar matter
fields are light, derivative couplings lead to a mild long-wavelength
instability that drives matter fields to non-zero expectation values. In this
case however, long-wavelength fluctuations of the light scalar are produced
during inflation, leading to a host of cosmological problems. In contrast,
axion-like couplings of the inflaton to a gauge field do not lead to production
of long-wavelength fluctuations during inflation. However, again because of the
weakness of the couplings to the inflaton, parametric resonance is not
effective in producing gauge field quanta.Comment: 10 pages, 9 figure
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