1,249 research outputs found
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
DSR as an explanation of cosmological structure
Deformed special relativity (DSR) is one of the possible realizations of a
varying speed of light (VSL). It deforms the usual quadratic dispersion
relations so that the speed of light becomes energy dependent, with preferred
frames avoided by postulating a non-linear representation of the Lorentz group.
The theory may be used to induce a varying speed of sound capable of generating
(near) scale-invariant density fluctuations, as discussed in a recent Letter.
We identify the non-linear representation of the Lorentz group that leads to
scale-invariance, finding a universal result. We also examine the higher order
field theory that could be set up to represent it
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Institute on Domestic Violence and Sexual Assault (IDVSA
A Dynamical Solution to the Problem of a Small Cosmological Constant and Late-time Cosmic Acceleration
Increasing evidence suggests that most of the energy density of the universe
consists of a dark energy component with negative pressure, a ``cosmological
constant" that causes the cosmic expansion to accelerate. In this paper, we
address the puzzle of why this component comes to dominate the universe only
recently rather than at some much earlier epoch. We present a class of theories
based on an evolving scalar field where the explanation is based entirely on
internal dynamical properties of the solutions. In the theories we consider,
the dynamics causes the scalar field to lock automatically into a negative
pressure state at the onset of matter-domination such that the present epoch is
the earliest possible time, consistent with nucleosynthesis restrictions, when
it can start to dominate.Comment: 5 pages, 3 figure
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
Natural variation in snow depth and snow melt timing in the High Arctic have implications for soil and plant nutrient status and vegetation composition
Snow cover is a key component in Arctic ecosystems and will likely be affected by
changes in winter precipitation. Increased snow depth and consequent later snowmelt leads
to greater microbial mineralization in winter, improving soil and vegetation nutrient
status. We studied areas with naturally differing snow depths and date of snowmelt in
Adventdalen, Svalbard. Soil properties, plant leaf nutrient status, and species composition
along with the normalized difference vegetation index (NDVI) were compared for three
snowmelt regimes (Early, Mid, and Late). We showed that (1) Late regimes (snow beds) had
wetter soils, higher pH, and leaves of Bistorta vivipara (L.) Delarbre and Salix polaris
Wahlenb. had higher concentration of nutrients (nitrogen and δ15N). Little to no difference
was found in soil nutrient concentrations between snowmelt regimes. (2) Late regimes had
highest NDVI values, whereas those of Early and Mid regimes were similar. (3) Vegetation
composition differed between Early and Late regimes, with Dryas octopetala L. and Luzula
arcuata subsp. confusa (Lange) characterizing the former and Equisetum arvense L. and
Eriophorum scheuchzeri Hoppe the latter. (4) Trends for plant nutrient contents were similar
to those found in a nearby snow manipulation experiment. Snow distribution and time of
snowmelt played an important role in determining regional environmental heterogeneity,
patchiness in plant community distribution, their species composition, and plant
phenology
Essentials of k-essence
We recently introduced the concept of "k-essence" as a dynamical solution for
explaining naturally why the universe has entered an epoch of accelerated
expansion at a late stage of its evolution. The solution avoids fine-tuning of
parameters and anthropic arguments. Instead, k-essence is based on the idea of
a dynamical attractor solution which causes it to act as a cosmological
constant only at the onset of matter-domination. Consequently, k-essence
overtakes the matter density and induces cosmic acceleration at about the
present epoch. In this paper, we present the basic theory of k-essence and
dynamical attractors based on evolving scalar fields with non-linear kinetic
energy terms in the action. We present guidelines for constructing concrete
examples and show that there are two classes of solutions, one in which cosmic
acceleration continues forever and one in which the acceleration has finite
duration.Comment: 14 pages, 11 figure
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
Space-time evolution induced by spinor fields with canonical and non-canonical kinetic terms
We study spinor field theories as an origin to induce space-time evolution.
Self-interacting spinor fields with canonical and non-canonical kinetic terms
are considered in a Friedman-Robertson-Walker universe. The deceleration
parameter is calculated by solving the equation of motion and the Friedman
equation, simultaneously. It is shown that the spinor fields can accelerate and
decelerate the universe expansion. To construct realistic models we discuss the
contributions from the dynamical symmetry breaking.Comment: 16 pages, 19 figure
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