25,810 research outputs found
Gravitational energy of rotating black holes
In the teleparallel equivalent of general relativity the energy density of
asymptotically flat gravitational fields can be naturaly defined as a scalar
density restricted to a three-dimensional spacelike hypersurface .
Integration over the whole yields the standard ADM energy. After
establishing the reference space with zero gravitational energy we obtain the
expression of the localized energy for a Kerr black hole. The expression of the
energy inside a surface of constant radius can be explicitly calculated in the
limit of small , the specific angular momentum. Such expression turns out to
be exactly the same as the one obtained by means of the method preposed
recently by Brown and York. We also calculate the energy contained within the
outer horizon of the black hole for {\it any} value of . The result is
practically indistinguishable from , where is the
irreducible mass of the black hole.Comment: 18 pages, LaTex file, one figur
Monte--Carlo Thermodynamic Bethe Ansatz
We introduce a Monte--Carlo simulation approach to thermodynamic Bethe ansatz
(TBA). We exemplify the method on one particle integrable models, which include
a free boson and a free fermions systems along with the scaling Lee--Yang model
(SLYM). It is confirmed that the central charges and energies are correct to a
very good precision, typically 0.1% or so. The advantage of the method is that
it enables the calculation of all the dimensions and even the particular
partition function.Comment: 22 pages. Added a footnote and realizations for the minimal models.
Fortran program, mont-s.f90, available from the source lin
Scaling properties of the Penna model
We investigate the scaling properties of the Penna model, which has become a
popular tool for the study of population dynamics and evolutionary problems in
recent years. We find that the model generates a normalised age distribution
for which a simple scaling rule is proposed, that is able to reproduce
qualitative features for all genome sizes.Comment: 4 pages, 4 figure
String Evolution with Friction
We study the effects of friction on the scaling evolution of string networks
in condensed matter and cosmological contexts. We derive a generalized
`one-scale' model with the string correlation length and velocity as
dynamical variables. In non-relativistic systems, we obtain a well-known
law, showing that loop production is important. For
electroweak cosmic strings, we show transient damped epoch scaling with
(or, in the matter era, ). A low initial
density implies an earlier period with . For GUT strings, the
approach to linear scaling is faster than previously estimated.Comment: 8 pages, uuencoded gziped .ps file. Paper submitted to Phys. Rev.
Let
Using gamma regression for photometric redshifts of survey galaxies
Machine learning techniques offer a plethora of opportunities in tackling big
data within the astronomical community. We present the set of Generalized
Linear Models as a fast alternative for determining photometric redshifts of
galaxies, a set of tools not commonly applied within astronomy, despite being
widely used in other professions. With this technique, we achieve catastrophic
outlier rates of the order of ~1%, that can be achieved in a matter of seconds
on large datasets of size ~1,000,000. To make these techniques easily
accessible to the astronomical community, we developed a set of libraries and
tools that are publicly available.Comment: Refereed Proceeding of "The Universe of Digital Sky Surveys"
conference held at the INAF - Observatory of Capodimonte, Naples, on
25th-28th November 2014, to be published in the Astrophysics and Space
Science Proceedings, edited by Longo, Napolitano, Marconi, Paolillo, Iodice,
6 pages, and 1 figur
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