521 research outputs found
Emergent Universe from A Composition of Matter, Exotic Matter and Dark Energy
A specific class of flat Emergent Universe (EU) is considered and its
viability is tested in view of the recent observations. Model parameters are
constrained from Stern data for Hubble Parameter and Redshift ( vs. )
and from a model independent measurement of BAO peak parameter. It is noted
that a composition of Exotic matter, dust and dark energy, capable of producing
an EU, can not be ruled out with present data. Evolution of other relevant
cosmological parameters, viz. density parameter (), effective equation
of state (EOS) parameter () are also shown.Comment: 5 pages, 5 figures (accepted in MNRAS
17-01 An Intersection Database Enhances Blind Pedestrians\u27 Access to Complex Signalized Intersections: Stage 2 Analysis & Database Development
This project investigated the effects of providing verbal descriptions of intersections and crosswalks on the performance of street-crossing subtasks by individuals who are totally blind. The intersections included crosswalks that varied widely in geometric and operational characteristics, including the presence or absence of accessibility features. In the no-database condition, participants used their typical street-crossing procedures. In the database-condition, participants additionally listened to database-generated descriptions of the intersections and crosswalks before crossing. The database descriptions had significant positive effects on some subtasks (primarily “crossing” subtasks such as deciding when to cross) and not others (primarily “wayfinding” subtasks such as remaining in the crosswalk)
Challenges for Superstring Cosmology
We consider whether current notions about superstring theory below the Planck
scale are compatible with cosmology. We find that the anticipated form for the
dilaton interaction creates a serious roadblock for inflation and makes it
unlikely that the universe ever reaches a state with zero cosmological constant
and time-independent gravitational constant.Comment: 14 pages, 2 figures available as eps files on reques
Cosmic Background Radiation Temperature Anisotropy: Position of First Doppler Peak
The purpose of the Cosmic Background Radiation (CBR) experiments is to
measure the temperature anisotropy via the autocorrelation function. The
partial wave corresponding to the first Doppler peak caused by
baryon-photon oscillations at the surface of last scattering depends on the
present density and the cosmological constant contribution
. We discuss this dependence on the basis of perspicuous
figures.Comment: 16 pages LaTeX including four figure
Bianchi type I universe with viscous fluid: A qualitative analysis
The nature of cosmological solutions for a homogeneous, anisotropic Universe
given by a Bianchi type-I (BI) model in the presence of a Cosmological constant
is investigated by taking into account dissipative process due to
viscosity. The system in question is thoroughly studied both analytically and
numerically. It is shown the viscosity, as well as the term exhibit
essential influence on the character of the solutions. In particular a negative
gives rise to an ever-expanding Universe, whereas, a suitable choice
of initial conditions plus a positive can result in a
singularity-free oscillatory mode of expansion. For some special cases it is
possible to obtain oscillations in the exponential mode of expansion of the BI
model even with a negative , where oscillations arise by virtue of
viscosity.Comment: RevTex, 16 pages, 32 figure
Density Perturbations in the Ekpyrotic Scenario
We study the generation of density perturbations in the ekpyrotic scenario
for the early universe, including gravitational backreaction. We expose
interesting subtleties that apply to both inflationary and ekpyrotic models.
Our analysis includes a detailed proposal of how the perturbations generated in
a contracting phase may be matched across a `bounce' to those in an expanding
hot big bang phase. For the physical conditions relevant to the ekpyrotic
scenario, we re-obtain our earlier result of a nearly scale-invariant spectrum
of energy density perturbations. We find that the perturbation amplitude is
typically small, as desired to match observation.Comment: 36 pages, compressed and RevTex file, one postscript figure file.
Minor typographical and numerical errors corrected, discussion added. This
version to appear in Physical Review
Emergent universe in a Jordan-Brans-Dicke theory
In this paper we study emergent universe model in the context of a self
interacting Jordan-Brans-Dicke theory. The model presents a stable past eternal
static solution which eventually enters a phase where the stability of this
solution is broken leading to an inflationary period. We also establish
constraints for the different parameters appearing in our model.Comment: 18 pages, 4 figures. Accepted for publication in JCA
Probability for Primordial Black Holes Pair in 1/R Gravity
The probability for quantum creation of an inflationary universe with a pair
of black holes in 1/R - gravitational theory has been studied. Considering a
gravitational action which includes a cosmological constant () in
addition to term, the probability has been evaluated in a
semiclassical approximation with Hartle-Hawking boundary condition. We obtain
instanton solutions determined by the parameters and
satisfying the constraint . However, we
note that two different classes of instanton solutions exists in the region . The probabilities of creation of such
configurations are evaluated. It is found that the probability of creation of a
universe with a pair of black holes is strongly suppressed with a positive
cosmological constant except in one case when . It is
also found that gravitational instanton solution is permitted even with
but one has to consider . However, in the later case
a universe with a pair of black holes is less probable.Comment: 15 pages, no figure. submitted to Phys. Rev.
The cosmic gravitational wave background in a cyclic universe
Inflation predicts a primordial gravitational wave spectrum that is slightly
``red,'' i.e., nearly scale-invariant with slowly increasing power at longer
wavelengths. In this paper, we compute both the amplitude and spectral form of
the primordial tensor spectrum predicted by cyclic/ekpyrotic models. The
spectrum is blue and exponentially suppressed compared to inflation on long
wavelengths. The strongest observational constraint emerges from the
requirement that the energy density in gravitational waves should not exceed
around 10 per cent of the energy density at the time of nucleosynthesis.Comment: 4 pages, 3 figuer
Maximum likelihood analysis of systematic errors in interferometric observations of the cosmic microwave background
We investigate the impact of instrumental systematic errors in
interferometric measurements of the cosmic microwave background (CMB)
temperature and polarization power spectra. We simulate interferometric CMB
observations to generate mock visibilities and estimate power spectra using the
statistically optimal maximum likelihood technique. We define a quadratic error
measure to determine allowable levels of systematic error that do not induce
power spectrum errors beyond a given tolerance. As an example, in this study we
focus on differential pointing errors. The effects of other systematics can be
simulated by this pipeline in a straightforward manner. We find that, in order
to accurately recover the underlying B-modes for r=0.01 at 28<l<384,
Gaussian-distributed pointing errors must be controlled to 0.7^\circ rms for an
interferometer with an antenna configuration similar to QUBIC, in agreement
with analytical estimates. Only the statistical uncertainty for 28<l<88 would
be changed at ~10% level. With the same instrumental configuration, we find the
pointing errors would slightly bias the 2-\sigma upper limit of the
tensor-to-scalar ratio r by ~10%. We also show that the impact of pointing
errors on the TB and EB measurements is negligibly small.Comment: 10 pages, 4 figures, accepted for publication in ApJS. Includes
improvements in clarity of presentation and Fig.4 added, in response to
refere
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