2,295 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
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
Investigations of the Non-Linear LMC Cepheid Period-Luminosity Relation with Testimator and Schwarz Information Criterion Methods
In this paper, we investigate the linearity versus non-linearity of the Large
Magellanic Cloud (LMC) Cepheid period-luminosity (P-L) relation using two
statistical approaches not previously applied to this problem: the testimator
method and the Schwarz Information Criterion (SIC). The testimator method is
extended to multiple stages for the first time, shown to be unbiased and the
variance of the estimated slope can be proved to be smaller than the standard
slope estimated from linear regression theory. The Schwarz Information
Criterion (also known as the Bayesian Information Criterion) is more
conservative than the Akaike Information Criterion and tends to choose lower
order models. By using simulated data sets, we verify that these statistical
techniques can be used to detect intrinsically linear and/or non-linear P-L
relations. These methods are then applied to independent LMC Cepheid data sets
from the OGLE project and the MACHO project, respectively. Our results imply
that there is a change of slope in longer period ranges for all of the data
sets. This strongly supports previous results, obtained from independent
statistical tests, that the observed LMC P-L relation is non-linear with a
break period at/around 10 days.Comment: 9 pages, 5 figures and 3 tables, PASP accepte
Phenomenological Constraints on Axion Models of Dynamical Dark Matter
In two recent papers (arXiv:1106.4546, arXiv:1107.0721), we introduced
"dynamical dark matter" (DDM), a new framework for dark-matter physics in which
the requirement of stability is replaced by a delicate balancing between
lifetimes and cosmological abundances across a vast ensemble of individual
dark-matter components whose collective behavior transcends that normally
associated with traditional dark-matter candidates. We also presented an
explicit model involving axions in large extra spacetime dimensions, and
demonstrated that this model has all of the features necessary to constitute a
viable realization of the general DDM framework. In this paper, we complete our
study by performing a general analysis of all phenomenological constraints
which are relevant to this bulk-axion DDM model. Although the analysis in this
paper is primarily aimed at our specific DDM model, many of our findings have
important implications for bulk axion theories in general. Our analysis can
also serve as a prototype for phenomenological studies of theories in which
there exist large numbers of interacting and decaying particles.Comment: 48 pages, LaTeX, 13 figures, 1 tabl
Can Inflation be Falsified?
Despite its central role in modern cosmology, doubts are often expressed as
to whether cosmological inflation is really a falsifiable theory. We
distinguish two facets of inflation, one as a theory of initial conditions for
the hot big bang and the other as a model for the origin of structure in the
Universe. We argue that the latter can readily be excluded by observations, and
that there are also a number of ways in which the former can find itself in
conflict with observational data. Both aspects of the theory are indeed
falsifiable.Comment: 7 pages LaTeX file with two figures incorporated by epsf. Fifth Prize
in Gravity Research Foundation Essay Competition. To appear, General
Relativity and Gravitatio
Can We See Lorentz-Violating Vector Fields in the CMB?
We investigate the perturbation theory of a fixed-norm, timelike
Lorentz-violating vector field. After consistently quantizing the vector field
to put constraints on its parameters, we compute the primordial spectra of
perturbations generated by inflation in the presence of this vector field. We
find that its perturbations are sourced by the perturbations of the inflaton;
without the inflaton perturbation the vector field perturbations decay away
leaving no primordial spectra of perturbations. Since the inflaton perturbation
does not have a spin-1 component, the vector field generically does not
generate any spin-1 ``vector-type'' perturbations. Nevertheless, it will modify
the amplitude of both the spin-0 ``scalar-type'' and spin-2 ``tensor-type''
perturbation spectra, leading to violations of the inflationary consistency
relationship.Comment: 36 pages, 1 fig, RevTex4, Submitted to PR
String-inspired cosmology
I discuss cosmological models either derived from, or inspired by, string
theory or M-theory. In particular I discuss solutions in the low-energy
effective theory and the role of the dilaton, moduli and antisymmetric form
fields in the dimensionally reduced effective action. The pre big bang model is
an attempt to use cosmological solutions to make observational predictions. I
then discuss the effective theory of gravity found in recent brane-world models
where we live on a 3-brane embedded in a five-dimensional spacetime and how the
study of cosmological perturbations may enable us to test these ideas.Comment: 15 pages, 5 figures, latex with iopart, invited talk at `The Early
Universe and Cosmological Observations: a Critical Review', Cape Town, July
200
Inflation by non-minimal coupling
Inflationary scenarios based on simple non-minimal coupling and its
generalizations are studied. Generalizing the form of non-minimal coupling to
"K(phi)R" with an arbitrary function K(phi), we show that the flat potential
still is obtainable when V(phi)/K^2(phi) is asymptotically constant. Very
interestingly, if the ratio of the dimensionless self-coupling constant of the
inflaton field and the non-minimal coupling constant is small the cosmological
observables for general monomial cases are in good agreement with recent
observational data.Comment: 9 pages, 1 figur
Chirality of tensor perturbations for complex values of the Immirzi parameter
In this paper we generalise previous work on tensor perturbations in a de
Sitter background in terms of Ashtekar variables to cover all complex values of
the Immirzi parameter gamma (previous work was restricted to imaginary gamma).
Particular attention is paid to the case of real gamma. Following the same
approach as in the imaginary case, we can obtain physical graviton states by
invoking reality and torsion free conditions. The Hamiltonian in terms of
graviton states has the same form whether gamma has a real part or not; however
changes occur for the vacuum energy and fluctuations. Specifically, we observe
a gamma dependent chiral asymmetry in the vacuum fluctuations only if gamma has
an imaginary part. Ordering prescriptions also change this asymmetry. We thus
present a measurable result for CMB polarisation experiments that could shed
light on the workings of quantum gravity.Comment: 6 pages, 1 figur
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