2,736 research outputs found
Dark spinor inflation -- theory primer and dynamics
Inflation driven by a single dark spinor field is discussed. We define the
notion of a dark spinor field and derive the cosmological field equations for
such a matter source. The conditions for inflation are determined and an
exactly solvable model is presented. We find the power spectrum of the quantum
fluctuation of this field and compare the results with scalar field inflation.Comment: 13 pages; typo in Eq. (12) corrected, minor improvement
Limitations of Bayesian Evidence Applied to Cosmology
There has been increasing interest by cosmologists in applying Bayesian
techniques, such as Bayesian Evidence, for model selection. A typical example
is in assessing whether observational data favour a cosmological constant over
evolving dark energy. In this paper, the example of dark energy is used to
illustrate limitations in the application of Bayesian Evidence associated with
subjective judgements concerning the choice of model and priors. An analysis of
recent cosmological data shows a statistically insignificant preference for a
cosmological constant over simple dynamical models of dark energy. It is argued
that for nested problems, as considered here, Bayesian parameter estimation can
be more informative than computing Bayesian Evidence for poorly motivated
physical models.Comment: 8 pages 4 figures MNRAS accepted. Substantially revised and extende
Inflation and the cosmic microwave background
I give a status report and outlook concerning the use of the cosmic microwave
background anisotropies to constrain the inflationary cosmology, and stress its
crucial role as an underlying paradigm for the estimation of cosmological
parameters.Comment: 8 pages LaTeX file, with two figures incorporated using epsf. To
appear, proceedings of `The non-sleeping universe', Porto (Astrophysics and
Space Science
Future fuels and engines for railroad locomotives. Volume 1: Summary
The potential for reducing the dependence of railroads on petroleum fuel, particularly Diesel No. 2 was investigated. Two approaches are studied: (1) to determine how the use of Diesel No. 2 can be reduced through increased efficiency and conservation, and (2) to use fuels other than Diesel No. 2 both in Diesel and other types of engines. Because synthetic hydrocarbon fuels are particularly suited to medium speed diesel engines, the first commercial application of these fuels may be by the railroad industry
Leptogenesis from Spin-Gravity Coupling Following Inflation
The energy levels of the left and the right handed neutrinos is split in the
background of gravitational waves generated during inflation which in presence
of lepton number violating interactions gives rise to a net lepton asymmetry at
equilibrium. Lepton number violation is achieved by the same dimension five
operator which gives rise to neutrino masses after electro-weak symmetry
breaking. A net baryon asymmetry of the same magnitude can be generated from
this lepton asymmetry by electroweak sphaleron processes.Comment: Journal version (accepted for publication in Phys. Rev. Lett.
Two Component Model of Dark Energy
We consider the possibility that the dark energy is made up of two or more
independent components, each having a different equation of state. We fit the
model with supernova and gamma-ray burst (GRB) data from resent observations,
and use the Markov Chain Monte Carlo (MCMC) technique to estimate the allowed
parameter regions. We also use various model selection criteria to compare the
two component model with the LCDM, one component dark energy model with static
or variable w(XCDM), and with other multi-component models. We find that the
two component models can give reasonably good fit to the current data. For some
data sets, and depending somewhat on the model selection criteria, the two
component model can give better fit to the data than XCDM with static w and
XCDM with variable w parameterized by w = w_0 + w_az/(1+z).Comment: 10 pages, 8 figures, 3 tables; Version accepted by PR
Apparent and actual galaxy cluster temperatures
The redshift evolution of the galaxy cluster temperature function is a
powerful probe of cosmology. However, its determination requires the
measurement of redshifts for all clusters in a catalogue, which is likely to
prove challenging for large catalogues expected from XMM--Newton, which may
contain of order 2000 clusters with measurable temperatures distributed around
the sky. In this paper we study the apparent cluster temperature, which can be
obtained without cluster redshifts. We show that the apparent temperature
function itself is of limited use in constraining cosmology, and so concentrate
our focus on studying how apparent temperatures can be combined with other
X-ray information to constrain the redshift. We also briefly study the
circumstances in which non-thermal spectral features can give redshift
information.Comment: 7 pages LaTeX file with 13 figures incorporated (uses mn.sty and
epsf). Minor changes to match MNRAS accepted versio
Curvature perturbation spectrum from false vacuum inflation
In the inflationary cosmology it occurs frequently that the inflaton field is
trapped in a local, transient minimum with non-zero vacuum energy. The
difficulty regarding the curvature perturbation produced during such a stage is
that classically the inflaton does not move so that the comoving hypersurfaces
are not well defined at linear order in the scalar field perturbation. In this
paper, assuming a mechanism of trapping which resembles a high temperature
correction to the potential, we explicitly calculate for the first time the
resulting power spectrum of the curvature perturbation by evaluating the
quantum two-point correlation function directly. The spectrum is steeply blue
with the spectral index n_R = 4.Comment: (v1) 17 pages, no figure; (v2) 18 pages, more clarifications and
discussions added, to appear in JCA
Present and future evidence for evolving dark energy
We compute the Bayesian evidences for one- and two-parameter models of
evolving dark energy, and compare them to the evidence for a cosmological
constant, using current data from Type Ia supernova, baryon acoustic
oscillations, and the cosmic microwave background. We use only distance
information, ignoring dark energy perturbations. We find that, under various
priors on the dark energy parameters, LambdaCDM is currently favoured as
compared to the dark energy models. We consider the parameter constraints that
arise under Bayesian model averaging, and discuss the implication of our
results for future dark energy projects seeking to detect dark energy
evolution. The model selection approach complements and extends the
figure-of-merit approach of the Dark Energy Task Force in assessing future
experiments, and suggests a significantly-modified interpretation of that
statistic.Comment: 10 pages RevTex4, 3 figures included. Minor changes to match version
accepted by PR
Hybrid vehicle assessment. Phase 1: Petroleum savings analysis
The results of a comprehensive analysis of near term electric hybrid vehicles are presented, with emphasis on their potential to save significant amounts of petroleum on a national scale in the 1990s. Performance requirements and expected annual usage patterns of these vehicles are first modeled. The projected U.S. fleet composition is estimated, and conceptual hybrid vehicle designs are conceived and analyzed for petroleum use when driven in the expected annual patterns. These petroleum consumption estimates are then compared to similar estimates for projected 1990 conventional vehicles having the same performance and driven in the same patterns. Results are presented in the form of three utility functions and comparisons of sevral conceptual designs are made. The Hybrid Vehicle (HV) design and assessment techniques are discussed and a general method is explained for selecting the optimum energy management strategy for any vehicle mission battery combination. Conclusions and recommendations are presented, and development recommendations are identified
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