4,555 research outputs found
The Paths of Quintessence
The structure of the dark energy equation of state phase plane holds
important information on the nature of the physics. We explain the bounds of
the freezing and thawing models of scalar field dark energy in terms of the
tension between the steepness of the potential vs. the Hubble drag.
Additionally, we extend the phase plane structure to modified gravity theories,
examine trajectories of models with certain properties, and categorize regions
in terms of scalar field hierarchical parameters, showing that dark energy is
generically not a slow roll phenomenon.Comment: 12 pages, 7 figures; matches PRD versio
Shifting the Universe: Early Dark Energy and Standard Rulers
The presence of dark energy at high redshift influences both the cosmic sound
horizon and the distance to last scattering of the cosmic microwave background.
We demonstrate that through the degeneracy in their ratio, early dark energy
can lie hidden in the CMB temperature and polarization spectra, leading to an
unrecognized shift in the sound horizon. If the sound horizon is then used as a
standard ruler, as in baryon acoustic oscillations, then the derived
cosmological parameters can be nontrivially biased. Fitting for the absolute
ruler scale (just as supernovae must be fit for the absolute candle magnitude)
removes the bias but decreases the leverage of the BAO technique by a factor 2.Comment: 6 pages, 3 figure
Fingerprinting dark energy
Dark energy perturbations are normally either neglected or else included in a
purely numerical way, obscuring their dependence on underlying parameters like
the equation of state or the sound speed. However, while many different
explanations for the dark energy can have the same equation of state, they
usually differ in their perturbations so that these provide a fingerprint for
distinguishing between different models with the same equation of state. In
this paper we derive simple yet accurate approximations that are able to
characterize a specific class of models (encompassing most scalar-field models)
which is often generically called "dark energy". We then use the approximate
solutions to look at the impact of the dark energy perturbations on the dark
matter power spectrum and on the integrated Sachs-Wolfe effect in the cosmic
microwave background radiation.Comment: 11 pages, 5 figures, minor changes to match published versio
Comparison of organoleptic acceptability of liquid and fresh diets
Organoleptic acceptability of liquid and fresh diets for space flight feedin
Induced Gravity and the Attractor Dynamics of Dark Energy/Dark Matter
Attractor solutions that give dynamical reasons for dark energy to act like
the cosmological constant, or behavior close to it, are interesting
possibilities to explain cosmic acceleration. Coupling the scalar field to
matter or to gravity enlarges the dynamical behavior; we consider both
couplings together, which can ameliorate some problems for each individually.
Such theories have also been proposed in a Higgs-like fashion to induce gravity
and unify dark energy and dark matter origins. We explore restrictions on such
theories due to their dynamical behavior compared to observations of the cosmic
expansion. Quartic potentials in particular have viable stability properties
and asymptotically approach general relativity.Comment: 11 pages, 10 figures, accepted in JCAP, results unchanged, an
explanation added on perfect fluids for general spinor Lagrangian
Constraining Modified Gravity with Euclid
Future proposed satellite missions as Euclid can offer the opportunity to
test general relativity on cosmic scales through mapping of the galaxy weak
lensing signal. In this paper we forecast the ability of these experiments to
constrain modified gravity scenarios as those predicted by scalar-tensor and
theories. We found that Euclid will improve constraints expected from
the PLANCK satellite on these modified gravity models by two orders of
magnitude. We discuss parameter degeneracies and the possible biases introduced
by modified gravity
Is Dark Energy Dynamical? Prospects for an Answer
Recent data advances offer the exciting prospect of a first look at whether
dark energy has a dynamical equation of state or not. While formally theories
exist with a constant equation of state, they are nongeneric -- Einstein's
cosmological constant is a notable exception. So limits on the time variation,
w', directly tell us crucial physics. Two recent improvements in supernova data
from the Hubble Space Telescope allow important steps forward in constraining
the dynamics of dark energy, possessing the ability to exclude models with
w'\ga 1, if the universe truly has a cosmological constant. These data bring us
much closer to the ``systematics'' era, where further improvements will come
predominantly from more accurate, not merely more, observations. We examine the
possible gains and point out the complementary roles of space and ground based
observations in the near future. To achieve the leap to precision understanding
of dark energy in the next generation will require specially designed space
based measurements; we estimate the confidence level of detection of dynamics
(e.g. distinguishing between and ) will be ~1.8\sigma after the
ongoing generation, improving to more than 6.5\sigma in the dedicated space
generation.Comment: 6 pages, 2 figures; version accepted to Phys. Rev.
Global perspective of nitrate flux in ice cores
The relationships between the concentration and the flux of chemical species (Cl-, NO3 - , SO42-, Na +, K + , NH4 + , Mg 2+ , Ca 2+) versus snow accumulation rate were examined at GISP2 and 20D in Greenland, Mount Logan from the St. Elias Range, Yukon Territory, Canada, and Sentik Glacier from the northwest end of the Zanskar Range in the Indian Himalayas. At all sites, only nitrate flux is significantly (a = 0.05) related to snow accumulation rate. Of all the chemical series, only nitrate concentration data are normally distributed. Therefore we suggest that nitrate concentration in snow is affected by postdepositionaJ exchange with the atmosphere over a broad range of environmental conditions. The persistent summer maxima in nitrate observed in Greenland snow over the entire range of record studied (the last 800 years) may be mainly due to NO• released from peroxyacetyl nitrate by thermal decomposition in the presence of higher OH concentrations in summer. The late winter/early spring nitrate peak observed in modern Greenland snow may be related to the buildup of anthropogenically derived N Oy in the Arctic troposphere during the long polar winter
Quinstant Dark Energy Predictions for Structure Formation
We explore the predictions of a class of dark energy models, quinstant dark
energy, concerning the structure formation in the Universe, both in the linear
and non-linear regimes. Quinstant dark energy is considered to be formed by
quintessence and a negative cosmological constant. We conclude that these
models give good predictions for structure formation in the linear regime, but
fail to do so in the non-linear one, for redshifts larger than one.Comment: 9 pages, 14 figures, "Accepted for publication in Astrophysics &
Space Science
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