21,501 research outputs found
Model-Independent Measurement of the Primordial Power Spectrum
In inflationary models with minimal amount of gravity waves, the primordial
power spectrum of density fluctuations, , together with the basic
cosmological parameters, completely specify the predictions for the cosmic
microwave background (CMB) anisotropy and large scale structure. Here we show
how we can strongly constrain both and the cosmological parameters
by combining the data from the Microwave Anisotropy Probe (MAP) and the galaxy
redshift survey from the Sloan Digital Sky Survey (SDSS). We allow
to be a free function, and thus probe features in the primordial power spectrum
on all scales. MAP and SDSS have scale-dependent measurement errors that
decrease in opposite directions on astrophysically interesting scales; they
complement each other and allow the measurement of the primordial power
spectrum independent of inflationary models, giving us valuable information on
physics in the early Universe, and providing clues to the correct inflationary
model.Comment: 4 pages including 4 figures. To appear in "Particle Physics and the
Early Universe (COSMO-98)", editor David O. Caldwell (American Institute of
Physics
Dynamics of chromatin and transcription during transient depletion of the RSC chromatin remodeling complex
Nucleosome organization has a key role in transcriptional regulation, yet the precise mechanisms establishing nucleosome locations and their effect on transcription are unclear. Here, we use an induced degradation system to screen all yeast ATP-dependent chromatin remodelers. We characterize how rapid clearance of the remodeler affects nucleosome locations. Specifically, depletion of Sth1, the catalytic subunit of the RSC (remodel the structure of chromatin) complex, leads to rapid fill-in of nucleosome-free regions at gene promoters. These changes are reversible upon reintroduction of Sth1 and do not depend on DNA replication. RSC-dependent nucleosome positioning is pivotal in maintaining promoters of lowly expressed genes free from nucleosomes. In contrast, we observe that upon acute stress, the RSC is not necessary for the transcriptional response. Moreover, RSC-dependent nucleosome positions are tightly related to usage of specific transcription start sites. Our results suggest organizational principles that determine nucleosome positions with and without RSC and how these interact with the transcriptional process
Determination of the Baryon Density from Large Scale Galaxy Redshift Surveys
We estimate the degree to which the baryon density, , can be
determined from the galaxy power spectrum measured from large scale galaxy
redshift surveys, and in particular, the Sloan Digital Sky Survey. A high
baryon density will cause wiggles to appear in the power spectrum, which should
be observable at the current epoch. We assume linear theory on scales and do not include the effects of redshift distortions, evolution,
or biasing. With an optimum estimate of to ,
the uncertainties in are roughly 0.07 and 0.016 in flat
and open () cosmological models, respectively. This result
suggests that it should be possible to test for consistency with big bang
nucleosynthesis estimates of if we live in an open universe.Comment: 23 Pages, 10 Postscript figure
Turbulent Magnetic Reconnection in Two Dimensions
Two-dimensional numerical simulations of the effect of background turbulence
on 2D resistive magnetic reconnection are presented. For sufficiently small
values of the resistivity () and moderate values of the turbulent power
(), the reconnection rate is found to have a much weaker dependence
on than the Sweet-Parker scaling of and is even consistent
with an independent value. For a given value of , the dependence
of the reconnection rate on the turbulent power exhibits a critical threshold
in above which the reconnection rate is significantly enhanced.Comment: Accepted to MNRA
Dynamics in binary cluster crystals
As a result of the application of coarse-graining procedures to describe
complex fluids, the study of systems consisting of particles interacting
through bounded, repulsive pair potentials has become of increasing interest in
the last years. A well known example is the so-called Generalized Exponential
Model (GEM-), for which the interaction between particles is described by
the potential . Interactions with
lead to the formation of a novel phase of soft matter consisting of cluster
crystals. Recent studies on the phase behavior of binary mixtures of GEM-
particles have provided evidence for the formation of novel kinds of alloys,
depending on the cross interactions between the two species. This work aims to
study the dynamic behavior of such binary mixtures by means of extensive
molecular dynamics simulations, and in particular to investigate the effect of
the addition of non-clustering particles on the dynamic scenario of
one-component cluster crystals. Analogies and differences with the
one-component case are revealed and discussed by analyzing self- and collective
dynamic correlators.Comment: 17 pages, 8 figures, submitted to JSTA
The X-ray Spectral Properties and Variability of Luminous High-Redshift Active Galactic Nuclei
We perform a detailed investigation of moderate-to-high quality X-ray spectra
of ten of the most luminous active galactic nuclei (AGNs) known at z>4 (up to
z~6.28). This study includes five new XMM observations and five archived X-ray
observations (four by XMM and one by Chandra). We find that the X-ray power-law
photon indices of our sample, composed of eight radio-quiet sources and two
that are moderately radio loud, are not significantly different from those of
lower redshift AGNs. The upper limits obtained on intrinsic neutral hydrogen
column densities, N_H<~10^{22}-10^{23} cm^{-2}, indicate that these AGNs are
not significantly absorbed. A joint fit performed on our eight radio-quiet
sources, with a total of ~7000 photons, constrains the mean photon index of z>4
radio-quiet AGNs to Gamma=1.97^{+0.06}_{-0.04}, with no detectable intrinsic
dispersion from source to source. We also obtain a strong constraint on the
mean intrinsic column density, N_H<~3x10^{21} cm^{-2}, showing that optically
selected radio-quiet AGNs at z>4 are, on average, not more absorbed than their
lower-redshift counterparts. All this suggests that the X-ray production
mechanism and the central environment in radio-quiet AGNs have not
significantly evolved over cosmic time. The mean equivalent width of a putative
neutral narrow Fe Ka line is constrained to be <~190 eV, and similarly we place
constraints on the mean Compton reflection component (R<~1.2). None of the AGNs
varied on short (~1 hr) timescales, but on longer timescales (months-to-years)
strong variability is observed in four of the sources. In particular, the X-ray
flux of the z=5.41 radio-quiet AGN SDSS 0231-0728 dropped by a factor of ~4
over a rest-frame period of 73 d. This is the most extreme X-ray variation
observed in a luminous z>4 radio-quiet AGN.Comment: 10 pages (emulateapj), 5 figures. Accepted by Ap
Representation of Quantum Mechanical Resonances in the Lax-Phillips Hilbert Space
We discuss the quantum Lax-Phillips theory of scattering and unstable
systems. In this framework, the decay of an unstable system is described by a
semigroup. The spectrum of the generator of the semigroup corresponds to the
singularities of the Lax-Phillips -matrix. In the case of discrete (complex)
spectrum of the generator of the semigroup, associated with resonances, the
decay law is exactly exponential. The states corresponding to these resonances
(eigenfunctions of the generator of the semigroup) lie in the Lax-Phillips
Hilbert space, and therefore all physical properties of the resonant states can
be computed.
We show that the Lax-Phillips -matrix is unitarily related to the
-matrix of standard scattering theory by a unitary transformation
parametrized by the spectral variable of the Lax-Phillips theory.
Analytic continuation in has some of the properties of a method
developed some time ago for application to dilation analytic potentials.
We work out an illustrative example using a Lee-Friedrichs model for the
underlying dynamical system.Comment: Plain TeX, 26 pages. Minor revision
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