294 research outputs found
Orbit equivalence rigidity for ergodic actions of the mapping class group
We establish orbit equivalence rigidity for any ergodic, essentially free and
measure-preserving action on a standard Borel space with a finite positive
measure of the mapping class group for a compact orientable surface with higher
complexity. We prove similar rigidity results for a finite direct product of
mapping class groups as well.Comment: 11 pages, title changed, a part of contents remove
Large-scale cosmic flows and moving dark energy
Large-scale matter bulk flows with respect to the cosmic microwave background
have very recently been detected on scales 100 Mpc/h and 300 Mpc/h by using two
different techniques showing an excellent agreement in the motion direction.
However, the unexpectedly large measured amplitudes are difficult to understand
within the context of standard LCDM cosmology. In this work we show that the
existence of such a flow could be signaling the presence of moving dark energy
at the time when photons decoupled from matter. We also comment on the relation
between the direction of the CMB dipole and the preferred axis observed in the
quadrupole in this scenario.Comment: 11 pages, 2 figures. New comments and references included. Final
version to appear in JCA
Nonlinear Effects in the Amplitude of Cosmological Density Fluctuations
The amplitude of cosmological density fluctuations, sigma_8, has been studied
and estimated by analysing many cosmological observations. The values of the
estimates vary considerably between the various probes. However, different
estimators probe the value of sigma_8 in different cosmological scales and do
not take into account the nonlinear evolution of the parameter at late times.
We show that estimates of the amplitude of cosmological density fluctuations
derived from cosmic flows are systematically higher than those inferred at
early epochs from the CMB because of nonlinear evolution at later times. We
discuss the past and future evolution of linear and nonlinear perturbations,
derive corrections to the value of sigma_8 and compare amplitudes after
accounting for these differences.Comment: 9 pages, 4 figures, 1 table. Accepted for publication in JCA
Rigorous proof of Luttinger liquid behavior in the 1d Hubbard model
We give the first rigorous (non perturbative) proof of Luttinger liquid
behavior in the one dimensional Hubbard model, for small repulsive interaction
and values of the density different from half filling. The analysis is based on
the combination of multiscale analysis with Ward identities bases on a hidden
and approximate local chiral gauge invariance. No use is done of exact
solutions or special integrability properties of the Hubbard model, and the
results can be in fact easily generalized to include non local interactions,
magnetic fields or interaction with external potential
Probing Primordial Non-Gaussianity with Large-Scale Structure
We consider primordial non-Gaussianity due to quadratic corrections in the
gravitational potential parametrized by a non-linear coupling parameter fnl. We
study constraints on fnl from measurements of the galaxy bispectrum in redshift
surveys. Using estimates for idealized survey geometries of the 2dF and SDSS
surveys and realistic ones from SDSS mock catalogs, we show that it is possible
to probe |fnl|~100, after marginalization over bias parameters. We apply our
methods to the galaxy bispectrum measured from the PSCz survey, and obtain a
2sigma-constraint |fnl|< 1800. We estimate that an all sky redshift survey up
to z~1 can probe |fnl|~1. We also consider the use of cluster abundance to
constrain fnl and find that in order to be sensitive to |fnl|~100, cluster
masses need to be determined with an accuracy of a few percent, assuming
perfect knowledge of the mass function and cosmological parameters.Comment: 15 pages, 7 figure
A cosmological concordance model with dynamical vacuum term
We demonstrate that creation of dark-matter particles at a constant rate
implies the existence of a cosmological term that decays linearly with the
Hubble rate. We discuss the cosmological model that arises in this context and
test it against observations of the first acoustic peak in the cosmic microwave
background (CMB) anisotropy spectrum, the Hubble diagram for supernovas of type
Ia (SNIa), the distance scale of baryonic acoustic oscillations (BAO) and the
distribution of large scale structures (LSS). We show that a good concordance
is obtained, albeit with a higher value of the present matter abundance than in
the \Lambda CDM model. We also comment on general features of the CMB
anisotropy spectrum and on the cosmic coincidence problem.Comment: Revised version. Accepted for publication in Physics Letters
The phase relation between sunspot numbers and soft X-ray flares
To better understand long-term flare activity, we present a statistical study
on soft X-ray flares from May 1976 to May 2008. It is found that the smoothed
monthly peak fluxes of C-class, M-class, and X-class flares have a very
noticeable time lag of 13, 8, and 8 months in cycle 21 respectively with
respect to the smoothed monthly sunspot numbers. There is no time lag between
the sunspot numbers and M-class flares in cycle 22. However, there is a
one-month time lag for C-class flares and a one-month time lead for X-class
flares with regard to sunspot numbers in cycle 22. For cycle 23, the smoothed
monthly peak fluxes of C-class, M-class, and X-class flares have a very
noticeable time lag of one month, 5 months, and 21 months respectively with
respect to sunspot numbers. If we take the three types of flares together, the
smoothed monthly peak fluxes of soft X-ray flares have a time lag of 9 months
in cycle 21, no time lag in cycle 22 and a characteristic time lag of 5 months
in cycle 23 with respect to the smoothed monthly sunspot numbers. Furthermore,
the correlation coefficients of the smoothed monthly peak fluxes of M-class and
X-class flares and the smoothed monthly sunspot numbers are higher in cycle 22
than those in cycles 21 and 23. The correlation coefficients between the three
kinds of soft X-ray flares in cycle 22 are higher than those in cycles 21 and
23. These findings may be instructive in predicting C-class, M-class, and
X-class flares regarding sunspot numbers in the next cycle and the physical
processes of energy storage and dissipation in the corona.Comment: 8 pages, 3 figures, Accepted for publication in Astrophysics & Space
Scienc
Charged hydrogenic problem in a magnetic field: Non-commutative translations, unitary transformations, and coherent states
An operator formalism is developed for a description of charged electron-hole
complexes in magnetic fields. A novel unitary transformation of the Hamiltonian
that allows one to partially separate the center-of-mass and internal motions
is proposed. We study the operator algebra that leads to the appearance of new
effective particles, electrons and holes with modified interparticle
interactions, and their coherent states in magnetic fields. The developed
formalism is used for studying a two-dimensional negatively charged
magnetoexciton . It is shown that Fano-resonances are present in the
spectra of internal transitions, indicating the existence of
three-particle quasi-bound states embedded in the continuum of higher Landau
levels.Comment: 9 pages + 2 figures, accepted in PRB, a couple of typos correcte
The Coherent State Representation of Quantum Fluctuations in the Early Universe
Using the squeezed state formalism the coherent state representation of
quantum fluctuations in an expanding universe is derived. It is shown that this
provides a useful alternative to the Wigner function as a phase space
representation of quantum fluctuations. The quantum to classical transition of
fluctuations is naturally implemented by decohering the density matrix in this
representation. The entropy of the decohered vacua is derived. It is shown that
the decoherence process breaks the physical equivalence between vacua that
differ by a coordinate dependent phase generated by a surface term in the
Lagrangian. In particular, scale invariant power spectra are only obtained for
a special choice of surface term.Comment: 25 pages in revtex 3. This version is completely revised with
corrections and significant new calculation
- …