71,793 research outputs found
The COBE Normalization for Standard CDM
The COBE detection of CMB anisotropies provides the best way of fixing the
amplitude of fluctuations on the largest scales. This normalization is usually
given for an n=1 spectrum, including only the anisotropy caused by the Sachs-
Wolfe effect. This is certainly not a good approximation for a model containing
any reasonable amount of baryonic matter. In fact, even tilted S-W spectra are
not a good fit to models like CDM. Here we normalize standard CDM (sCDM) to the
2-year COBE data, and quote the best amplitude in terms of the conventionally
used measures of power. We also give normalizations for some specific variants
of this standard model, and we indicate how the normalization depends on the
assumed values of n, Omega_B and H_0. For sCDM we find =19.9\pm1.5uK,
corresponding to sigma_8=1.34\pm0.10, with the normalization at large scales
being B=(8.16\pm1.04)\times10^5 (Mpc/h)^4, and other numbers given in the
Table. The measured rms temperature fluctuation smoothed on 10deg is a little
low relative to this normalization. This is mainly due to the low quadrupole in
the data: when the quadrupole is removed, the measured value of sigma(10) is
quite consistent with the best-fitting . The use of should be preferred
over sigma(10), when its value can be determined for a particular theory, since
it makes full use of the data.Comment: 4 pages compressed uuencoded postscript. We have corrected an error
in our analysi
Red Sequence Cluster Finding in the Millennium Simulation
We investigate halo mass selection properties of red-sequence cluster finders
using galaxy populations of the Millennium Simulation (MS). A clear red
sequence exists for MS galaxies in massive halos at redshifts z < 1, and we use
this knowledge to inform a cluster-finding algorithm applied to 500 Mpc/h
projections of the simulated volume. At low redshift (z=0.4), we find that 90%
of the clusters found have galaxy membership dominated by a single, real-space
halo, and that 10% are blended systems for which no single halo contributes a
majority of a cluster's membership. At z=1, the fraction of blends increases to
22%, as weaker redshift evolution in observed color extends the comoving length
probed by a fixed range of color. Other factors contributing to the increased
blending at high-z include broadening of the red sequence and confusion from a
larger number of intermediate mass halos hosting bright red galaxies of
magnitude similar to those in higher mass halos. Our method produces catalogs
of cluster candidates whose halo mass selection function, p(M|\Ngal,z), is
characterized by a bimodal log-normal model with a dominant component that
reproduces well the real-space distribution, and a redshift-dependent tail that
is broader and displaced by a factor ~2 lower in mass. We discuss implications
for X-ray properties of optically selected clusters and offer ideas for
improving both mock catalogs and cluster-finding in future surveys.Comment: final version to appear in MNRAS. Appendix added on purity and
completeness, small shift in red sequence due to correcting an error in
finding i
Development of a semi-autonomous service robot with telerobotic capabilities
The importance to the United States of semi-autonomous systems for application to a large number of manufacturing and service processes is very clear. Two principal reasons emerge as the primary driving forces for development of such systems: enhanced national productivity and operation in environments whch are hazardous to humans. Completely autonomous systems may not currently be economically feasible. However, autonomous systems that operate in a limited operation domain or that are supervised by humans are within the technology capability of this decade and will likely provide reasonable return on investment. The two research and development efforts of autonomy and telerobotics are distinctly different, yet interconnected. The first addresses the communication of an intelligent electronic system with a robot while the second requires human communication and ergonomic consideration. Discussed here are work in robotic control, human/robot team implementation, expert system robot operation, and sensor development by the American Welding Institute, MTS Systems Corporation, and the Colorado School of Mines--Center for Welding Research
Cooper-pair transport through a Hubbard chain sandwiched between two superconductors: Density matrix renormalization group calculations
We present a numerical approach to study the coherent transport of Cooper
pairs through a Hubbard chain, and study the role of the contacts in achieving
perfect Andreev reflection. We calculate the pair transport using the Density
Matrix Renormalization Group by measuring the response of the system to quantum
pair fields with complex phases on the two ends of an open system. This
approach gives an effective superfluid weight which is in close agreement with
the Bethe Ansatz results for the superfluid weight for closed Hubbard rings.Comment: 5 pages, 6 figure
PHYLOGENY OF SOME MIDDLE AMERICAN PITVIPERS BASED ON A CLADISTIC ANALYSIS OF MITOCHONDRIAL 12S AND 16S DNA SEQUENCE INFORMATION
The cladistic relationships of several Middle American pitvipers representing the genera Bothrops (sensu stricto), Bothriechis, Cerrophidion, Lachesis and Porthidium were determined using mitochondrial 12S and 16S DNA sequence information. Maximum parsimony analyses were performed using PAUP on aligned sequences that included published information for related taxa. Two sets of analyses were conducted: one disregarding gaps in the aligned matrix, and another with gaps treated as a fifth base. When gaps were excluded resolution declined, although the general arrangement of the taxa changed little. A consistent relationship was the grouping of ((Porthidium, Bothriechis) Lachesis). The placement of Lachesis, as nested within other bothropoid genera, is only partially supported by results of other authors. The arrangement of Crotalus, Bothrops and Cerrophidion was ambiguous when gaps were discounted. In both trees, Agkistrodon was basal to the New World forms. The remaining genera, Trimeresurus (Protobothrops), Vipera, Azemiops, and Coluber, were uniformly distant to the former taxa. Also of interest is the lack of close relationship, based on the DNA data here and elsewhere, between Bothrops and Porthidium. This is in striking contrast to results based on morphologic and allozymic analyses of previous studies. It is concluded that additional DNA sequence information from a larger sample of taxa will be necessary to better assess the phylogenetic relationships among Middle American and related pitvipers
Is Cosmology Solved?
We have fossil evidence from the thermal background radiation that our
universe expanded from a considerably hotter denser state. We have a well
defined and testable description of the expansion, the relativistic
Friedmann-Lemaitre model. Its observational successes are impressive but I
think hardly enough for a convincing scientific case. The lists of
observational constraints and free hypotheses within the model have similar
lengths. The scorecard on the search for concordant measures of the mass
density parameter and the cosmological constant shows that the high density
Einstein-de Sitter model is challenged, but that we cannot choose between low
density models with and without a cosmological constant. That is, the
relativistic model is not strongly overconstrained, the usual test of a mature
theory. Work in progress will greatly improve the situation and may at last
yield a compelling test. If so, and the relativistic model survives, it will
close one line of research in cosmology: we will know the outlines of what
happened as our universe expanded and cooled from high density. It will not end
research: some of us will occupy ourselves with the details of how galaxies and
other large-scale structures came to be the way they are, others with the issue
of what our universe was doing before it was expanding. The former is being
driven by rapid observational advances. The latter is being driven mainly by
theory, but there are hints of observational guidance.Comment: 13 pages, 3 figures. To be published in PASP as part of the
proceedings of the Smithsonian debate, Is Cosmology Solved
Decaying dark matter: a stacking analysis of galaxy clusters to improve on current limits
We show that a stacking approach to galaxy clusters can improve current
limits on decaying dark matter by a factor , with respect to a
single source analysis, for all-sky instruments such as Fermi-LAT. Based on the
largest sample of X-ray-selected galaxy clusters available to date (the MCXC
meta-catalogue), we provide all the astrophysical information, in particular
the astrophysical term for decaying dark matter, required to perform an
analysis with current instruments.Comment: 6 pages, 3 figures, supplementary file available on demand, accepted
for publication in PR
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