6,235 research outputs found
Attractor solutions for general hessence dark energy
As a candidate for the dark energy, the hessence model has been recently
introduced. We discuss the critical points of this model in almost general
case, that is for arbitrary hessence potential and almost arbitrary
hessence-background matter interaction. It is shown that in all models, there
always exist some stable late-time attractors. It is shown that our general
results coincide with those solutions obtained earlier for special cases, but
some of them are new. These new solutions have two unique characteristics.
First the hessence field has finite value in these solutions and second, their
stabilities depend on the second derivative of the hessence potential.Comment: 11 pages. Add some explanations about the autonomousity of the
equations, and also a conclusion section was added. To appear in Phys. Rev. D
(2006
Dynamics of Q-Balls in an expanding universe
We analyse the evolution of light Q-balls in a cosmological background, and
find a number of interesting features. For Q-balls formed with a size
comparable to the Hubble radius, we demonstrate that there is no charge
radiation, and that the Q-ball maintains a constant physical radius. Large
expansion rates cause charge migration to the surface of the Q-ball,
corresponding to a non-homogeneous internal rotation frequency. We argue that
this is an important phenomenon as it leads to a large surface charge and
possible fragmentation of the Q-ball. We also explore the deviation of the
Q-ball profile function from the static case. By introducing a parameter
, which is the ratio of the Hubble parameter to the frequency of
oscillation of the Q-ball field, and using solutions to an analytically
approximated equation for the profile function, we determine the dependence of
the new features on the expansion rate. This allows us to gain an understanding
of when they should be considered and when they can be neglected, thereby
placing restrictions on the existence of homogeneous Q-balls in expanding
backgrounds.Comment: 5 pages, 4 figure
Gravitational instantons and internal dimensions
We Study instanton solutions in general relativity with a scalar field. The
metric ansatz we use is composed of a particular warp product of general
Einstein metrics, such as those found in a number of cosmological settings,
including string cosmology, supergravity compactifications and general Kaluza
Klein reductions. Using the Hartle-Hawking prescription the instantons we
obtain determine whether metrics involving extra compact dimensions of this
type are favoured as initial conditions for the universe. Specifically, we find
that these product metric instantons, viewed as constrained instantons, do have
a local minima in the action. These minima are then compared with the higher
dimensional version of the Hawking-Turok instantons, and we argue that the
latter always have lower action than those associated with these product
metrics.Comment: 10 pages, 5 figure
Cosmological perturbation spectra from SL(4,R)-invariant effective actions
We investigate four-dimensional cosmological vacuum solutions derived from an
effective action invariant under global SL(n,R) transformations. We find the
general solutions for linear axion field perturbations about homogeneous
dilaton-moduli-vacuum solutions for an SL(4,R)-invariant action and find the
spectrum of super-horizon perturbations resulting from vacuum fluctuations in a
pre big bang scenario. We show that for SL(n,R)-invariant actions with n>3
there exists a regime of parameter space of non-zero measure where all the
axion field spectra have positive spectral tilt, as required if light axion
fields are to provide a seed for anisotropies in the microwave background and
large-scale structure in the universe.Comment: 8 pages, 3 figures, revtex plus epsf, minor typos corrected, version
to appear in Physical Review
On the reliability of inflaton potential reconstruction
If primordial scalar and tensor perturbation spectra can be inferred from
observations of the cosmic background radiation and large-scale structure, then
one might hope to reconstruct a unique single-field inflaton potential capable
of generating the observed spectra. In this paper we examine conditions under
which such a potential can be reliably reconstructed. For it to be possible at
all, the spectra must be well fit by a Taylor series expansion. A complete
reconstruction requires a statistically-significant tensor mode to be measured
in the microwave background. We find that the observational uncertainties
dominate the theoretical error from use of the slow-roll approximation, and
conclude that the reconstruction procedure will never insidiously lead to an
irrelevant potential.Comment: 16 page LaTeX file with eight postscript figures embedded with epsf;
no special macros neede
Ambient ammonia measurements using laser photo-acoustic spectroscopy
Ammonia concentrations reached minimal levels (approximately 0.1 ppb) in early winter, followed by a sudden later winter increase. A direct relationship between ambient ammonia levels and air temperature was inferred from the data (linear correlation coefficient r=0.53). Ammonia concentrations were determined to be directly related to the absolute humidity of the air (r=0.72); a weaker relationship between ammonia concentrations and relative humidity was discovered (r=0.37). The data also indicated that ammonia levels were generally higher within continental air masses than those of maritime origin. Soil parameters such as pH and moisture content were found to have a major bearing on the release of gaseous ammonia from soils in the region
New type scalar fields for cosmic acceleration
We present a model where a non-conventional scalar field may act like dark
energy leading to cosmic acceleration. The latter is driven by an appropriate
field configuration, which result in an effective cosmological constant. The
potential role of such a scalar in the cosmological constant problem is also
discussed.Comment: 6 page
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