135 research outputs found
Power law correlations in galaxy distribution and finite volume effects from the Sloan Digital Sky Survey Data Release Four
We discuss the estimation of galaxy correlation properties in several volume
limited samples, in different sky regions, obtained from the Fourth Data
Release of the Sloan Digital Sky Survey. The small scale properties are
characterized through the determination of the nearest neighbor probability
distribution. By using a very conservative statistical analysis, in the range
of scales [0.5,~30] Mpc/h we detect power-law correlations in the conditional
density in redshift space, with an exponent \gamma=1.0 \pm 0.1. This behavior
is stable in all different samples we considered thus it does not depend on
galaxy luminosity. In the range of scales [~30,~100] Mpc/h we find evidences
for systematic unaveraged fluctuations and we discuss in detail the problems
induced by finite volume effects on the determination of the conditional
density. We conclude that in such range of scales there is an evidence for a
smaller power-law index of the conditional density. However we cannot
distinguish between two possibilities: (i) that a crossover to homogeneity
(corresponding to \gamma=0 in the conditional density) occurs before 100 Mpc/h,
(ii) that correlations extend to scales of order 100 Mpc/h (with a smaller
exponent 0 < \gamma <1). We emphasize that galaxy distributions in these
samples present large fluctuations at the largest scales probed, corresponding
to the presence of large scale structures extending up to the boundaries of the
present survey. Finally we discuss several differences between the behavior of
the conditional density in mock galaxy catalogs built from cosmological N-body
simulations and real data. We discuss some theoretical implications of such a
fact considering also the super-homogeneous features of primordial density
fields.Comment: 13 pages, 19 figures, to be publsihed in Astronomy and Astrophysic
Classical evolution of fractal measures on the lattice
We consider the classical evolution of a lattice of non-linear coupled
oscillators for a special case of initial conditions resembling the equilibrium
state of a macroscopic thermal system at the critical point. The displacements
of the oscillators define initially a fractal measure on the lattice associated
with the scaling properties of the order parameter fluctuations in the
corresponding critical system. Assuming a sudden symmetry breaking (quench),
leading to a change in the equilibrium position of each oscillator, we
investigate in some detail the deformation of the initial fractal geometry as
time evolves. In particular we show that traces of the critical fractal measure
can sustain for large times and we extract the properties of the chain which
determine the associated time-scales. Our analysis applies generally to
critical systems for which, after a slow developing phase where equilibrium
conditions are justified, a rapid evolution, induced by a sudden symmetry
breaking, emerges in time scales much shorter than the corresponding relaxation
or observation time. In particular, it can be used in the fireball evolution in
a heavy-ion collision experiment, where the QCD critical point emerges, or in
the study of evolving fractals of astrophysical and cosmological scales, and
may lead to determination of the initial critical properties of the Universe
through observations in the symmetry broken phase.Comment: 15 pages, 15 figures, version publiced at Physical Review
Scale Dependence of Dark Energy Antigravity
We investigate the effects of negative pressure induced by dark energy
(cosmological constant or quintessence) on the dynamics at various
astrophysical scales. Negative pressure induces a repulsive term (antigravity)
in Newton's law which dominates on large scales. Assuming a value of the
cosmological constant consistent with the recent SnIa data we determine the
critical scale beyond which antigravity dominates the dynamics () and discuss some of the dynamical effects implied. We show that
dynamically induced mass estimates on the scale of the Local Group and beyond
are significantly modified due to negative pressure. We also briefly discuss
possible dynamical tests (eg effects on local Hubble flow) that can be applied
on relatively small scales (a few ) to determine the density and equation
of state of dark energy.Comment: Contributed talk at the 2nd Hellenic Cosmology Workshop at NOA
(Athens) Jan. 2001.To appear in the proceedings. Based on work done in
collaboration with M. Axenides and E. Florato
Galaxy distribution and extreme value statistics
We consider the conditional galaxy density around each galaxy, and study its
fluctuations in the newest samples of the Sloan Digital Sky Survey Data Release
7. Over a large range of scales, both the average conditional density and its
variance show a nontrivial scaling behavior, which resembles to criticality.
The density depends, for 10 < r < 80 Mpc/h, only weakly (logarithmically) on
the system size. Correspondingly, we find that the density fluctuations follow
the Gumbel distribution of extreme value statistics. This distribution is
clearly distinguishable from a Gaussian distribution, which would arise for a
homogeneous spatial galaxy configuration. We also point out similarities
between the galaxy distribution and critical systems of statistical physics
Exact isotropic cosmologies with local fractal number counts
We construct an exact relativistic cosmology in which an inhomogeneous but
isotropic local region has fractal number counts and matches to a homogeneous
background at a scale of the order of Mpc. We show that Einstein's
equations and the matching conditions imply either a nonlinear Hubble law or a
very low large-scale density.Comment: revised version, to appear Class. Q. Grav.; minor corrections
following eqn 16, additional comments on relation to other work, some new
reference
Breaking the self-averaging properties of spatial galaxy fluctuations in the Sloan Digital Sky Survey - Data Release Six
Statistical analyses of finite sample distributions usually assume that
fluctuations are self-averaging, i.e. that they are statistically similar in
different regions of the given sample volume. By using the scale-length method,
we test whether this assumption is satisfied in several samples of the Sloan
Digital Sky Survey Data Release Six. We find that the probability density
function (PDF) of conditional fluctuations, filtered on large enough spatial
scales (i.e., r>30 Mpc/h), shows relevant systematic variations in different
sub-volumes of the survey. Instead for scales r<30 Mpc/h the PDF is
statistically stable, and its first moment presents scaling behavior with a
negative exponent around one. Thus while up to 30 Mpc/h galaxy structures have
well-defined power-law correlations, on larger scales it is not possible to
consider whole sample average quantities as meaningful and useful statistical
descriptors. This situation is due to the fact that galaxy structures
correspond to density fluctuations which are too large in amplitude and too
extended in space to be self-averaging on such large scales inside the sample
volumes: galaxy distribution is inhomogeneous up to the largest scales, i.e. r
~ 100 Mpc/h, probed by the SDSS samples. We show that cosmological corrections,
as K-corrections and standard evolutionary corrections, do not qualitatively
change the relevant behaviors. Finally we show that the large amplitude galaxy
fluctuations observed in the SDSS samples are at odds with the predictions of
the standard LCDM model of structure formation.(Abridged version).Comment: 32 pages, 28 figures, accepted for publication in Astronomy and
Astrophysics. A higher resolution version is available at
http://pil.phys.uniroma1.it/~sylos/fsl_highlights.html . Version v2 has been
corrected to match the published on
Symmetry properties of the metric energy-momentum tensor in classical field theories and gravity
We derive a generic identity which holds for the metric (i.e. variational)
energy-momentum tensor under any field transformation in any generally
covariant classical Lagrangian field theory. The identity determines the
conditions under which a symmetry of the Lagrangian is also a symmetry of the
energy-momentum tensor. It turns out that the stress tensor acquires the
symmetry if the Lagrangian has the symmetry in a generic curved spacetime. In
this sense a field theory in flat spacetime is not self-contained. When the
identity is applied to the gauge invariant spin-two field in Minkowski space,
we obtain an alternative and direct derivation of a known no-go theorem: a
linear gauge invariant spin-2 field, which is dynamically equivalent to
linearized General Relativity, cannot have a gauge invariant metric
energy-momentum tensor. This implies that attempts to define the notion of
gravitational energy density in terms of the metric energy--momentum tensor in
a field-theoretical formulation of gravity must fail.Comment: Revised version to match the published version in Class. Quantum Gra
High time resolution multi-band photo-polarimetric observations of the binary millisecond redback pulsar J1023+0038 with the BTA
© Published under licence by IOP Publishing Ltd. We briefly report first results of high time resolution optical multi-band panoramic photo-polarimetric observations of the eclipsing binary millisecond redback pulsar J1023+0038 obtained in February 2017 with the 6 m BTA telescope. The time resolution was varied from 10 to 120 ms depending on observational mode. Our data show that the pulsar still remained in the low-mass X-ray binary stage, characterised by rapid flaring at time scales of 10-100 s with amplitudes of 0.2-0.5 mag. We resolved a fine structure of the flares at time scales of 0.1-10 s. The polarimetry at the time scale of 0.1 s shows no polarization with an upper limit of 2%-4% for the linear polarisation degree in flaring and quiet stages, while at a 10 minute scale averaging it is about 1.5% at 3σ significance. We shortly outline implications of the results
- …