32 research outputs found
Measuring the Cosmic Web
A quantitative study of the clustering properties of the cosmic web as a
function of absolute magnitude and colour is presented using the SDSS Data
Release 7 galaxy survey. Mark correlations are included in the analysis. We
compare our results with mock galaxy samples obtained with four different
semi-analytical models of galaxy formation imposed on the merger trees of the
Millenium simulation. The clustering of both red and blue galaxies is studied
separately.Comment: 8 pages, 5 figures, to be published in Baltic Astronom
Vacuum driven accelerated expansion
It has been shown that an improved estimation of quantum vacuum energy can
yield not only acceptable but also experimentally sensible results. The very
idea consists in a straightforward extraction of gravitationally interacting
part of the full quantum vacuum energy by means of gauge transformations. The
implementation of the idea has been performed in the formalism of effective
action, in the language of Schwinger's proper time and the Seeley-DeWitt heat
kernel expansion, in the background of the Friedmann-Robertson-Walker geometry.Comment: 11 pages, 2 figures, minor improvements, final preprint version,
published version:
http://www3.interscience.wiley.com/journal/120847180/abstract, devoted to the
memory of professor Ryszard Raczka on the occasion of the 11th anniversary of
his deat
Analysis of the possibility of analog detectors calibration by exploiting Stimulated Parametric Down Conversion
Spontaneous parametric down conversion (SPDC) has been largely exploited as a
tool for absolute calibration of photon-counting detectors, i.e detectors
registering very small photon fluxes. In [J. Opt. Soc. Am. B 23, 2185 (2006)]
we derived a method for absolute calibration of analog detectors using SPDC
emission at higher photon fluxes, where the beam is seen as a continuum by the
detector. Nevertheless intrinsic limitations appear when high-gain regime of
SPDC is required to reach even larger photon fluxes. Here we show that
stimulated parametric down conversion allow one to avoid this limitation, since
stimulated photon fluxes are increased by the presence of the seed beam.Comment: 9 pages, 1 figur
Moment instabilities in multidimensional systems with noise
We present a systematic study of moment evolution in multidimensional
stochastic difference systems, focusing on characterizing systems whose
low-order moments diverge in the neighborhood of a stable fixed point. We
consider systems with a simple, dominant eigenvalue and stationary, white
noise. When the noise is small, we obtain general expressions for the
approximate asymptotic distribution and moment Lyapunov exponents. In the case
of larger noise, the second moment is calculated using a different approach,
which gives an exact result for some types of noise. We analyze the dependence
of the moments on the system's dimension, relevant system properties, the form
of the noise, and the magnitude of the noise. We determine a critical value for
noise strength, as a function of the unperturbed system's convergence rate,
above which the second moment diverges and large fluctuations are likely.
Analytical results are validated by numerical simulations. We show that our
results cannot be extended to the continuous time limit except in certain
special cases.Comment: 21 pages, 15 figure
A Wasserstein approach to the one-dimensional sticky particle system
We present a simple approach to study the one-dimensional pressureless Euler
system via adhesion dynamics in the Wasserstein space of probability measures
with finite quadratic moments.
Starting from a discrete system of a finite number of "sticky" particles, we
obtain new explicit estimates of the solution in terms of the initial mass and
momentum and we are able to construct an evolution semigroup in a
measure-theoretic phase space, allowing mass distributions with finite
quadratic moment and corresponding L^2-velocity fields. We investigate various
interesting properties of this semigroup, in particular its link with the
gradient flow of the (opposite) squared Wasserstein distance.
Our arguments rely on an equivalent formulation of the evolution as a
gradient flow in the convex cone of nondecreasing functions in the Hilbert
space L^2(0,1), which corresponds to the Lagrangian system of coordinates given
by the canonical monotone rearrangement of the measures.Comment: Added reference
Graviton production in the scaling of a long-cosmic-string network
In a previous paper [1] we considered the possibility that (within the
early-radiation epoch) there has been (also) a short period of a significant
presence of cosmic strings. During this radiation-plus-strings stage the
Universe matter-energy content can be modelled by a two-component fluid,
consisting of radiation (dominant) and a cosmic-string fluid (subdominant). It
was found that, during this stage, the cosmological gravitational waves (CGWs)
- that had been produced in an earlier (inflationary) epoch - with comoving
wave-numbers below a critical value (which depends on the physics of the
cosmic-string network) were filtered, leading to a distorsion in the expected
(scale-invariant) CGW power spectrum. In any case, the cosmological evolution
gradually results in the scaling of any long-cosmic-string network and, hence,
after a short time-interval, the Universe enters into the late-radiation era.
However, along the transition from an early-radiation epoch to the
late-radiation era through the radiation-plus-strings stage, the
time-dependence of the cosmological scale factor is modified, something that
leads to a discontinuous change of the corresponding scalar curvature, which,
in turn, triggers the quantum-mechanical creation of gravitons. In this paper
we discuss several aspects of such a process, and, in particular, the
observational consequences on the expected gravitational-wave (GW) power
spectrum.Comment: 12 pages, 2 figures, accepted for publication in Physical Review
Nonlinear evolution of dark matter and dark energy in the Chaplygin-gas cosmology
The hypothesis that dark matter and dark energy are unified through the
Chaplygin gas is reexamined. Using generalizations of the spherical model which
incorporate effects of the acoustic horizon we show that an initially
perturbative Chaplygin gas evolves into a mixed system containing cold dark
matter-like gravitational condensate.Comment: 11 pages, 3 figures, substantial revision, title changed, content
changed, added references, to appear in JCA
Dynamics of a thin shell in the Reissner-Nordstrom metric
We describe the dynamics of a thin spherically symmetric gravitating shell in
the Reissner-Nordstrom metric of the electrically charged black hole. The
energy-momentum tensor of electrically neutral shell is modelled by the perfect
fluid with a polytropic equation of state. The motion of a shell is described
fully analytically in the particular case of the dust equation of state. We
construct the Carter-Penrose diagrams for the global geometry of the eternal
black hole, which illustrate all possible types of solutions for moving shell.
It is shown that for some specific range of initial parameters there are
possible the stable oscillating motion of the shell transferring it
consecutively in infinite series of internal universes. We demonstrate also
that this oscillating type of motion is possible for an arbitrary polytropic
equation of state on the shell.Comment: 17 pages, 7 figure