886 research outputs found
Information and Timing in Repeated Partnerships
In a repeated partnership game with imperfect monitoring, we distinguish among the effects of (1) shortening the period over which actions are held fixed, (2) increasing the frequency with which accumulated information is reported, and (3) reducing the amount of discounting of payoffs between successive periods. While reducing the amount of discounting generally improves incentives for cooperation, the other two changes can have the reverse effect. When the game is specified in the customary way with information reported at the end of each period of fixed action, the net effect of shortening the period length can be to destroy all incentives for cooperation, reversing the usual conclusion associated with the Folk Theorem for repeated games. Moreover, when interest rates are low, reducing the frequency of information reporting can greatly enhance the efficiency of equilibrium
New constraints on MOND from galaxy clusters
We revisit the application of Modified Newtonian Dynamics (MOND) to galaxy
clusters. We confront the high quality X-ray data for eight clusters of
galaxies observed by the \xmm satellite with the predictions of MOND. We obtain
a ratio of the MOND dynamical mass to the baryonic mass of in the outer parts (i.e ~ R), in the concordance
cosmological model where the predicted asymptotic ratio, if any baryons are
present, is (at ~ R). We confirm that
the MOND paradigm lowers the discrepancy between the binding mass and the
baryonic mass in clusters by a factor of at about half the virial
radius. However, at this radius about 80% of the mass is still missing, and as
pointed out by \citet{sanders03}, this necessitates a component of dark baryons
or neutrinos in the cluster core. Concerning the neutrino hypothesis,
application of the new data requires a minimum neutrino mass of eV to fill this gap. The corresponding 2 lower limit of
eV is marginally inconsistent with the current constraints from
the cluster number counts, and from the CMB and large scale structure data.
MOND must invoke neutrinos to represent the main component that account for the
missing mass problem in clusters.Comment: 5 pages, 1 figure, accepted for publication in MNRAS. A section on
the neutrino mass in the MOND framework was added to the discussio
The High Energy Budget Allocations in Shocks and GRB
The statistical distribution of energies among particles responsible for long
Gamma Ray Burst (GRB) emission is analyzed in light of recent results of the
Fermi Observatory. The allsky flux, , recorded by the Gamma Ray
Burst Monitor (GBM) is shown, despite its larger energy range, to be not
significantly larger than that reported by the Burst and Transient Explorer
(BATSE), suggesting a relatively small flux in the 3 - 30 MeV energy range. The
present-day energy input rate in -rays recorded by the GBM from long
GRB is found, assuming star-formation rates in the literature, to be . The Large Area
Telescope (LAT) fluence, when observed, is about 5-10\% per decade of the
total, in good agreement with the predictions of saturated, non-linear shock
acceleration.
The high-energy component of long GRBs, as measured by Fermi, is found to
contain only of the energy needed to produce ultrahigh-energy
cosmic rays (UHECR) above 4 Eev, assuming the latter to be extragalactic, when
various numerical factors are carefully included, if the cosmic ray source
spectrum has a spectral index of -2. The observed -ray fraction of the
required UHECR energy is even smaller if the source spectrum is softer than
.
The AMANDA II limits rule out such a GRB origin for UHECR if much more than
of the cosmic ray energy goes into neutrinos that are within, and
simultaneous with, the -ray beam.
It is suggested that "orphan" neutrinos out of the -ray beam might be
identifiable via orphan afterglow { or other wide angle signatures of GRB in
lieu of coincidence with prompt -rays}, and it is recommended that
feasible single neutrino trigger criteria be established to search for such
coincidences.Comment: to appear in The Astrophysical Journa
Reducing Oral Health Disparities: A Focus on Social and Cultural Determinants
Oral health is essential to the general health and well-being of individuals and the population. Yet significant oral health disparities persist in the U.S. population because of a web of influences that include complex cultural and social processes that affect both oral health and access to effective dental health care
Properties of Dark Matter Haloes
An overview is presented of the main properties of dark matter haloes, as we
know them from observations, essentially from rotation curves around spiral and
dwarf galaxies. Detailed rotation curves are now known for more than a thousand
galaxies, revealing that they are not so flat in the outer parts, but rising
for late-types, and falling for early-types. A well established result now is
that most bright galaxies are not dominated by dark matter inside their optical
disks. Only for dwarfs and LSB (Low Surface Brightness galaxies) dark matter
plays a dominant role in the visible regions. The 3D-shape of haloes are
investigated through several methods, that will be discussed: polar rings,
flaring of HI planes, X-ray isophotes.
It is not yet possible with rotation curves to know how far haloes extend,
but tentatives have been made. It will be shown that the dark matter appears to
be coupled to the gas in spirals and dwarfs, suggesting that dark baryons could
play the major role in rotation curves. Theories proposing to replace the
non-baryonic dark matter by a different dynamical or gravity law, such as MOND,
have to take into account the dark baryons, especially since their spatial
distribution is likely to be quite different from the visible matter.Comment: 20 pages, 8 figures, submitted to NewA Reviews, review given at MOND
workshop, Paris, October 200
Relativistic Modified Newtonian Dynamics from String Theory?
We argue that TeVeS-like vector fields appear naturally in certain string
theory backgrounds involving D0-branes, as a result of the recoil velocity
field, expressing the interaction of neutrino string matter with point-like
branes. However, the similarity with TeVeS models is restricted only to the
bi-metric properties of space time, namely the difference of the background
metric from the one felt by (some) matter fields interacting, in a
topologically non-trivial manner, with the D0-brane defects. In our approach,
neutrinos appear as dark matter candidates that could be ``captured'' by the D0
branes, as a result of stringy properties, and thus couple with the
recoil-vector fields. Moreover, we argue in support of a possibly preferential
role of neutrinos in inducing novel non-perturbative contributions to
``vacuum'' (dark) energy, in addition to their ordinary dark matter
contribution. In fact, the role of neutrinos as providing substantial
contributions to dark matter and dark energy components of the Universe,
suggested by our approach, appears also to be necessitated by the need to
reproduce the peaks in the Cosmic Microwave Background radiation spectrum, as
claimed recently in the literature. Thus, our framework may be viewed as
providing a microscopic explanation of such phenomenological conclusions
concerning TeVeS-like, Lorentz-violating models.Comment: 6 pages; slightly amended version to appear in Physics Letters
Information and Timing in Repeated Partnerships
In a repeated partnership game with imperfect monitoring, we distinguish among the effects of (1) shortening the period over which actions are held fixed, (2) increasing the frequency with which accumulated information is reported, and (3) reducing the amount of discounting of payoffs between successive periods. While reducing the amount of discounting generally improves incentives for cooperation, the other two changes can have the reverse effect. When the game is specified in the customary way with information reported at the end of each period of fixed action, the net effect of shortening the period length can be to destroy all incentives for cooperation, reversing the usual conclusion associated with the Folk Theorem for repeated games. Moreover, when interest rates are low, reducing the frequency of information reporting can greatly enhance the efficiency of equilibrium.Monitoring, repeated games, partnership, incentives, folk theorems
Discovery of Damped Lyman-Alpha Systems at Redshifts Less Than 1.65 and Results on their Incidence and Cosmological Mass Density
We report results on the incidence and cosmological mass density of damped
Lyman-alpha (DLA) systems at redshifts less that 1.65. We used HST and an
efficient non-traditional (but unbiased) survey technique to discover DLA
systems at redshifts z<1.65, where we observe the Lyman-alpha line in known
MgII absorption-line systems. We uncovered 14 DLA lines including 2
serendipitously. We find that (1) The DLA absorbers are drawn almost
exclusively from the population of MgII absorbers which have rest equivalent
widths W(2796)>0.6A. (2) The incidence of DLA systems per unit redshift,
n(DLA), is observed to decrease with decreasing redshift. (3) On the other
hand, the cosmological mass density of neutral gas in low-redshift DLA
absorbers, Omega(DLA), is observed to be comparable to that observed at high
redshift. (4) The low-redshift DLA absorbers exhibit a significantly larger
fraction of very high column density systems in comparison to determinations at
both high redshift and locally.Comment: 47 pages in LaTeX - emulateapj style with included tables and
encapsulated postscript figures. Accepted for Publication in Astrophysical
Journal Supplements. Results unchanged, text revise
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