743 research outputs found
On the possible role of massive neutrinos in cosmological structure formation
In addition to the problem of galaxy formation, one of the greatest open
questions of cosmology is represented by the existence of an asymmetry between
matter and antimatter in the baryonic component of the Universe. We believe
that a net lepton number for the three neutrino species can be used to
understand this asymmetry. This also implies an asymmetry in the
matter-antimatter component of the leptons. The existence of a nonnull lepton
number for the neutrinos can easily explain a cosmological abundance of
neutrinos consistent with the one needed to explain both the rotation curves of
galaxies and the flatness of the Universe. Some propedeutic results are
presented in order to attack this problem.Comment: RevTeX4, 25 pages, 5 figures, to appear in the "Proceedings of the
Xth Brazilian School of Cosmology and Gravitation", M. Novello, editor, AIP,
in pres
Algorithmic statistics revisited
The mission of statistics is to provide adequate statistical hypotheses
(models) for observed data. But what is an "adequate" model? To answer this
question, one needs to use the notions of algorithmic information theory. It
turns out that for every data string one can naturally define
"stochasticity profile", a curve that represents a trade-off between complexity
of a model and its adequacy. This curve has four different equivalent
definitions in terms of (1)~randomness deficiency, (2)~minimal description
length, (3)~position in the lists of simple strings and (4)~Kolmogorov
complexity with decompression time bounded by busy beaver function. We present
a survey of the corresponding definitions and results relating them to each
other
Non-Singular Bouncing Universes in Loop Quantum Cosmology
Non-perturbative quantum geometric effects in Loop Quantum Cosmology predict
a modification to the Friedmann equation at high energies. The
quadratic term is negative definite and can lead to generic bounces when the
matter energy density becomes equal to a critical value of the order of the
Planck density. The non-singular bounce is achieved for arbitrary matter
without violation of positive energy conditions. By performing a qualitative
analysis we explore the nature of the bounce for inflationary and Cyclic model
potentials. For the former we show that inflationary trajectories are
attractors of the dynamics after the bounce implying that inflation can be
harmoniously embedded in LQC. For the latter difficulties associated with
singularities in cyclic models can be overcome. We show that non-singular
cyclic models can be constructed with a small variation in the original Cyclic
model potential by making it slightly positive in the regime where scalar field
is negative.Comment: Minor changes and one figure added to improve presentation.
References added. To appear in Physical Review
Physical constants and the Gurzadyan-Xue formula for the dark energy
We consider cosmological implications of the formula for the dark energy
density derived by Gurzadyan and Xue which predicts a value fitting the
observational one. Cosmological models with varying by time physical constants,
namely, speed of light and gravitational constant and/or their combinations,
are considered. In one of the models, for example, vacuum energy density
induces effective negative curvature, while another one has an unusual
asymptotic. This analysis also explicitely rises the issue of the meaning and
content of physical units and constants in cosmological context.Comment: version corrected to match the one to appear in Modern Physics
Letters
Algorithmic statistics: forty years later
Algorithmic statistics has two different (and almost orthogonal) motivations.
From the philosophical point of view, it tries to formalize how the statistics
works and why some statistical models are better than others. After this notion
of a "good model" is introduced, a natural question arises: it is possible that
for some piece of data there is no good model? If yes, how often these bad
("non-stochastic") data appear "in real life"?
Another, more technical motivation comes from algorithmic information theory.
In this theory a notion of complexity of a finite object (=amount of
information in this object) is introduced; it assigns to every object some
number, called its algorithmic complexity (or Kolmogorov complexity).
Algorithmic statistic provides a more fine-grained classification: for each
finite object some curve is defined that characterizes its behavior. It turns
out that several different definitions give (approximately) the same curve.
In this survey we try to provide an exposition of the main results in the
field (including full proofs for the most important ones), as well as some
historical comments. We assume that the reader is familiar with the main
notions of algorithmic information (Kolmogorov complexity) theory.Comment: Missing proofs adde
Quantum Geometry and its Implications for Black Holes
General relativity successfully describes space-times at scales that we can
observe and probe today, but it cannot be complete as a consequence of
singularity theorems. For a long time there have been indications that quantum
gravity will provide a more complete, non-singular extension which, however,
was difficult to verify in the absence of a quantum theory of gravity. By now
there are several candidates which show essential hints as to what a quantum
theory of gravity may look like. In particular, loop quantum gravity is a
non-perturbative formulation which is background independent, two properties
which are essential close to a classical singularity with strong fields and a
degenerate metric. In cosmological and black hole settings one can indeed see
explicitly how classical singularities are removed by quantum geometry: there
is a well-defined evolution all the way down to, and across, the smallest
scales. As for black holes, their horizon dynamics can be studied showing
characteristic modifications to the classical behavior. Conceptual and physical
issues can also be addressed in this context, providing lessons for quantum
gravity in general. Here, we conclude with some comments on the uniqueness
issue often linked to quantum gravity in some form or another.Comment: 16 pages, Plenary talk at ``Einstein's Legacy in the New Millenium,''
Puri, India, December 200
GRBs and the thermalization process of electron-positron plasmas
We discuss the temporal evolution of the pair plasma created in Gamma-Ray
Burst sources. A particular attention is paid to the relaxation of the plasma
into thermal equilibrium. We also discuss the connection between the dynamics
of expansion and the spatial geometry of the plasma. The role of the baryonic
loading parameter is emphasized.Comment: 4 pages, 3 figures, in the Proceedings of the "Gamma Ray Bursts 2007"
meeting, November 5-9, 2007, Santa Fe, New Mexico, US
Coordinate time dependence in Quantum Gravity
The intuitive classical space-time picture breaks down in quantum gravity,
which makes a comparison and the development of semiclassical techniques quite
complicated. Using ingredients of the group averaging method to solve
constraints one can nevertheless introduce a classical coordinate time into the
quantum theory, and use it to investigate the way a semiclassical continuous
description emerges from discrete quantum evolution. Applying this technique to
test effective classical equations of loop cosmology and their implications for
inflation and bounces, we show that the effective semiclassical theory is in
good agreement with the quantum description even at short scales.Comment: 35 pages, 17 figure. Revised version. To appear in Phys. Rev.
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