64,000 research outputs found
Gravitational collapse: A case for thermal relaxation
Two relativistic models for collapsing spheres at different stages of
evolution, which include pre-relaxation processes, are presented. The influence
of relaxation time on the outcome of evolution in both cases is exhibited and
established. It is shown that relaxation processes can drastically change the
final state of the collapsing system. In particular, there are cases in which
the value of the relaxation time determines the bounce or the collapse of the
sphere.Comment: 33 pages, LaTex 2.09, 11 Postscript figures. To be published in
General Relativity and Gravitatio
A Conversation with Dorothy Gilford
In 1946, Public Law 588 of the 79th Congress established the Office of Naval
Research (ONR). Its mission was to plan, foster and encourage scientific
research in support of Naval problems. The establishment of ONR predates the
National Science Foundation and initiated the refocusing of scientific
infrastructure in the United States following World War II. At the time, ONR
was the only source for federal support of basic research in the United States.
Dorothy Gilford was one of the first Heads of the Probability and Statistics
program at the Office of Naval Research (1955 to 1962), and she went on to
serve as Director of the Mathematical Sciences Division (1962 to 1968). During
her time at ONR, Dorothy influenced many areas of statistics and mathematics
and was ahead of her time in promoting interdisciplinary projects. Dorothy
continued her career at the National Center for Education Statistics (1969 to
1974). She was active in starting international comparisons of education
outcomes in different countries, which has influenced educational policy in the
United States. Dorothy went on to serve in many capacities at the National
Academy of Sciences, including Director of Human Resources Studies (1975 to
1978), Senior Statistician on the Committee on National Statistics (1978 to
1988) and Director of the Board on International Comparative Studies in
Education (1988 to 1994). The following is a conversation we had with Dorothy
Gilford in March of 2004. We found her to be an interesting person and a
remarkable statistician. We hope you agree.Comment: Published in at http://dx.doi.org/10.1214/088342307000000023 the
Statistical Science (http://www.imstat.org/sts/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Possible cosmological implications in electrodynamics due to variations of the fine structure constant
Astronomical observations are suggesting that the fine structure constant
varies cosmologically. We present an analysis on the consequences that these
variations might induce on the electromagnetic field as a whole. We show that
under these circumstances the electrodynamics in vacuum could be described by
two fields, the ``standard'' Maxwell's field and a new scalar field. We provide
a generalised Lorentz force which can be used to test our results
experimentally.Comment: 7 pages, no figures. Accepted for publication in Rev. Mex. Fis. (Some
extra information included, references added and small corrections made to
the original version
Constraining our Universe with X-ray & Optical Cluster Data
We have used recent X-ray and optical data in order to impose some
constraints on the cosmology and cluster scaling relations. Generically two
kind of hypotheses define our model. First we consider that the cluster
population is well described by the standard Press-Schechter (PS) formalism,
and second, these clusters are supposed to follow scaling relations with mass:
Temperature-Mass (T-M) and X-ray Luminosity-Mass (L_x - M). As a difference
with many other authors we do not assume specific scaling relations to model
cluster properties such as the usual virial relation or one observational
determination of the relation. Instead we consider general free
parameter scaling relations. With the previous model (PS plus scalings) we fit
our free parameters to several X-ray and optical data with the advantage over
many other works that we consider all the data sets at the same time. This
prevents us from being inconsistent with some of the available observations.
Among other interesting conclusions, we find that only low-density universes
are compatible with all the data considered and that the degeneracy between
and is broken. Also we obtain interesting limits on the
parameters characterizing the scaling relations.Comment: 11 pages, 7 figures. MNRAS accepted versio
- …