490 research outputs found
Finding a Spherically Symmetric Cosmology from Observations in Observational Coordinates -- Advantages and Challenges
One of the continuing challenges in cosmology has been to determine the
large-scale space-time metric from observations with a minimum of assumptions
-- without, for instance, assuming that the universe is almost
Friedmann-Lema\^{i}tre-Robertson-Walker (FLRW). If we are lucky enough this
would be a way of demonstrating that our universe is FLRW, instead of
presupposing it or simply showing that the observations are consistent with
FLRW. Showing how to do this within the more general spherically symmetric,
inhomogeneous space-time framework takes us a long way towards fulfilling this
goal. In recent work researchers have shown how this can be done both in the
traditional Lema\^{i}tre-Tolman-Bondi (LTB) 3 + 1 coordinate framework, and in
the observational coordinate (OC) framework. In this paper we investigate the
stability of solutions, and the use of data in the OC field equations including
their time evolution and compare both approaches with respect to the
singularity problem at the maximum of the angular-diameter distance, the
stability of solutions, and the use of data in the field equations. This allows
a more detailed account and assessment of the OC integration procedure, and
enables a comparison of the relative advantages of the two equivalent solution
frameworks. Both formulations and integration procedures should, in principle,
lead to the same results. However, as we show in this paper, the OC procedure
manifests certain advantages, particularly in the avoidance of coordinate
singularities at the maximum of the angular-diameter distance, and in the
stability of the solutions obtained. This particular feature is what allows us
to do the best fitting of the data to smooth data functions and the possibility
of constructing analytic solutions to the field equations.Comment: 31 page
Recommended from our members
Fusion reactor applications of the high-density linear theta pinch
On perfect fluid models in non-comoving observational spherical coordinates
We use null spherical (observational) coordinates to describe a class of
inhomogeneous cosmological models. The proposed cosmological construction is
based on the observer past null cone. A known difficulty in using inhomogeneous
models is that the null geodesic equation is not integrable in general. Our
choice of null coordinates solves the radial ingoing null geodesic by
construction. Furthermore, we use an approach where the velocity field is
uniquely calculated from the metric rather than put in by hand. Conveniently,
this allows us to explore models in a non-comoving frame of reference. In this
frame, we find that the velocity field has shear, acceleration and expansion
rate in general. We show that a comoving frame is not compatible with expanding
perfect fluid models in the coordinates proposed and dust models are simply not
possible. We describe the models in a non-comoving frame. We use the dust
models in a non-comoving frame to outline a fitting procedure.Comment: 8 pages, 1 figure. To appear in Phys.Rev.
An anisotropic cosmological model with isotropic background radiation
We present an exact solution of Einstein equations that describes a Bianchi
type III spacetime with conformal expansion. The matter content is given by an
anisotropic scalar field and two perfect fluids representing dust and isotropic
radiation. Based on this solution, we construct a cosmological model that
respects the evolution of the scale factor predicted in standard cosmology.Comment: 4 pages; contribution to the Proceedings of the 24th Spanish
Relativity Meeting (ERE2001
The Lemaitre Model and the Generalisation of the Cosmic Mass
We consider the spherically symmetric metric with a comoving perfect fluid
and non-zero pressure -- the Lemaitre metric -- and present it in the form of a
calculational algorithm. We use it to review the definition of mass, and to
look at the apparent horizon relations on the observer's past null cone. We
show that the introduction of pressure makes it difficult to separate the mass
from other physical parameters in an invariant way. Under the usual mass
definition, the apparent horizon relation, that relates the diameter distance
to the cosmic mass, remains the same as in the Lemaitre-Tolman case.Comment: latex, 16 pages, Revision has minor changes due to referee's
comments
Conditions for spontaneous homogenization of the Universe
The present-day Universe appears to be homogeneous on very large scales. Yet
when the casual structure of the early Universe is considered, it becomes
apparent that the early Universe must have been highly inhomogeneous. The
current paradigm attempts to answer this problem by postulating the inflation
mechanism However, inflation in order to start requires a homogeneous patch of
at least the horizon size. This paper examines if dynamical processes of the
early Universe could lead to homogenization. In the past similar studies seem
to imply that the set of initial conditions that leads to homogenization is of
measure zero. This essay proves contrary: a set of initial conditions for
spontaneous homogenization of cosmological models can form a set of non-zero
measure.Comment: 7 pages. Fifth Award in the 2010 Gravity Research Foundation essay
competitio
Large Deviations for Random Trees
We consider large random trees under Gibbs distributions and prove a Large
Deviation Principle (LDP) for the distribution of degrees of vertices of the
tree. The LDP rate function is given explicitly. An immediate consequence is a
Law of Large Numbers for the distribution of vertex degrees in a large random
tree. Our motivation for this study comes from the analysis of RNA secondary
structures.Comment: 10 page
Supersymmetry Without Prejudice
We begin an exploration of the physics associated with the general
CP-conserving MSSM with Minimal Flavor Violation, the pMSSM. The 19 soft SUSY
breaking parameters in this scenario are chosen so as to satisfy all existing
experimental and theoretical constraints assuming that the WIMP is a
conventional thermal relic, ie, the lightest neutralino. We scan this parameter
space twice using both flat and log priors for the soft SUSY breaking mass
parameters and compare the results which yield similar conclusions. Detailed
constraints from both LEP and the Tevatron searches play a particularly
important role in obtaining our final model samples. We find that the pMSSM
leads to a much broader set of predictions for the properties of the SUSY
partners as well as for a number of experimental observables than those found
in any of the conventional SUSY breaking scenarios such as mSUGRA. This set of
models can easily lead to atypical expectations for SUSY signals at the LHC.Comment: 61 pages, 24 figs. Refs., figs, and text added, typos fixed; This
version has reduced/bitmapped figs. For a version with better figs please go
to http://www.slac.stanford.edu/~rizz
Agnesi Weighting for the Measure Problem of Cosmology
The measure problem of cosmology is how to assign normalized probabilities to
observations in a universe so large that it may have many observations
occurring at many different spacetime locations. I have previously shown how
the Boltzmann brain problem (that observations arising from thermal or quantum
fluctuations may dominate over ordinary observations if the universe expands
sufficiently and/or lasts long enough) may be ameliorated by volume averaging,
but that still leaves problems if the universe lasts too long. Here a solution
is proposed for that residual problem by a simple weighting factor 1/(1+t^2) to
make the time integral convergent. The resulting Agnesi measure appears to
avoid problems other measures may have with vacua of zero or negative
cosmological constant.Comment: 26 pages, LaTeX; discussion is added of how Agnesi weighting appears
better than other recent measure
Further investigation of a relic neutralino as a possible origin of an annual-modulation effect in WIMP direct search
We analyze the annual-modulation effect, measured by the DAMA Collaboration
with the new implementation of a further two-years running, in the context of a
possible interpretation in terms of relic neutralinos. We impose over the set
of supersymmetric configurations, selected by the annual-modulation data, the
constraints derived from WIMP indirect measurements, and discuss the features
of the ensuing relic neutralinos. We critically discuss the sources of the main
theoretical uncertainties in the analysis of event rates for direct and
indirect WIMP searches.Comment: 29 pages, 12 figures, typeset with ReVTeX. In order to reduce size,
the version on the archive has low resolution figures. A full version of the
paper can be found at http://www.to.infn.it/~fornengo/papers
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