3,621 research outputs found
Near-optimal asymmetric binary matrix partitions
We study the asymmetric binary matrix partition problem that was recently
introduced by Alon et al. (WINE 2013) to model the impact of asymmetric
information on the revenue of the seller in take-it-or-leave-it sales.
Instances of the problem consist of an binary matrix and a
probability distribution over its columns. A partition scheme
consists of a partition for each row of . The partition acts
as a smoothing operator on row that distributes the expected value of each
partition subset proportionally to all its entries. Given a scheme that
induces a smooth matrix , the partition value is the expected maximum
column entry of . The objective is to find a partition scheme such that
the resulting partition value is maximized. We present a -approximation
algorithm for the case where the probability distribution is uniform and a
-approximation algorithm for non-uniform distributions, significantly
improving results of Alon et al. Although our first algorithm is combinatorial
(and very simple), the analysis is based on linear programming and duality
arguments. In our second result we exploit a nice relation of the problem to
submodular welfare maximization.Comment: 17 page
The Bright Side of Dark Matter
We show that it is not possible in the absence of dark matter to construct a
four-dimensional metric that explains galactic observations. In particular, by
working with an effective potential it is shown that a metric which is
constructed to fit flat rotation curves in spiral galaxies leads to the wrong
sign for the bending of light i.e. repulsion instead of attraction. Hence,
without dark matter the motion of particles on galactic scales cannot be
explained in terms of geodesic motion on a four- dimensional metric. This
reveals a new bright side to dark matter: it is indispensable if we wish to
retain the cherished equivalence principle.Comment: 7 pages, latex, no figures. Received an honorable mention in the 1999
Gravity research Foundation Essay Competition. Submitted to Phys. Rev. Let
Stability of disk galaxies in the modified dynamics
General analytic arguments lead us to expect that in the modified dynamics
(MOND) self-gravitating disks are more stable than their like in Newtonian
dynamics. We study this question numerically, using a particle-mesh code based
on a multi-grid solver for the (nonlinear) MOND field equation. We start with
equilibrium distribution functions for MOND disk models having a smoothly
truncated, exponential surface-density profiles and a constant Toomre
parameter. We find that, indeed, disks of a given ``temperature'' are locally
more stable in MOND than in Newtonian dynamics. As regards global instability
to bar formation, we find that as the mean acceleration in the disk is lowered,
the stability of the disk is increased as we cross from the Newtonian to the
MOND regime. The degree of stability levels off deep in the MOND regime, as
expected from scaling laws in MOND. For the disk model we use, this maximum
degree of stability is similar to the one imparted to a Newtonian disk by a
halo three times as massive at five disk scale lengths.Comment: 20 pages, Latex, 8 embedded figures, version to be published in The
Astrophys.
Modified gravity without dark matter
On an empirical level, the most successful alternative to dark matter in
bound gravitational systems is the modified Newtonian dynamics, or MOND,
proposed by Milgrom. Here I discuss the attempts to formulate MOND as a
modification of General Relativity. I begin with a summary of the
phenomenological successes of MOND and then discuss the various covariant
theories that have been proposed as a basis for the idea. I show why these
proposals have led inevitably to a multi-field theory. I describe in some
detail TeVeS, the tensor-vector-scalar theory proposed by Bekenstein, and
discuss its successes and shortcomings. This lecture is primarily pedagogical
and directed to those with some, but not a deep, background in General
RelativityComment: 28 pages, 10 figures, lecture given at Third Aegean Summer School,
The Invisible Universe: Dark Matter and Dark Energy, minor errors corrected,
references update
On the Possibility of Quantum Gravity Effects at Astrophysical Scales
The nonperturbative renormalization group flow of Quantum Einstein Gravity
(QEG) is reviewed. It is argued that at large distances there could be strong
renormalization effects, including a scale dependence of Newton's constant,
which mimic the presence of dark matter at galactic and cosmological scales.Comment: LaTeX, 18 pages, 4 figures. Invited contribution to the Int. J. Mod.
Phys. D special issue on dark matter and dark energ
Cosmological extrapolation of MOND
Regime of MOND, which is used in astronomy to describe the gravitating
systems of island type without the need to postulate the existence of a
hypothetical dark matter, is generalized to the case of homogeneous
distribution of usual matter by introducing a linear dependence of the critical
acceleration on the size of region under consideration. We show that such the
extrapolation of MOND in cosmology is consistent with both the observed
dependence of brightness on the redshift for type Ia supernovae and the
parameters of large-scale structure of Universe in the evolution, that is
determined by the presence of a cosmological constant, the ordinary matter of
baryons and electrons as well as the photon and neutrino radiation without any
dark matter.Comment: 20 pages, 5 figures, comments adde
Phenomenological covariant approach to gravity
We covariantly modify the Einstein-Hilbert action such that the modified
action perturbatively resolves the flat rotational velocity curve of the spiral
galaxies and gives rise to the Tully-Fisher relation, and dynamically generates
the cosmological constant. This modification requires introducing just a single
new universal parameter.Comment: v6: a mistake in deriving the equation of the cosmological constant
corrected, refs adde
Testing Modified Newtonian Dynamics with Rotation Curves of Dwarf and Low Surface Brightness Galaxies
Dwarf and low surface brightness galaxies are ideal objects to test modified
Newtonian dynamics (MOND), because in most of these galaxies the accelerations
fall below the threshold below where MOND supposedly applies. We have selected
from the literature a sample of 27 dwarf and low surface brightness galaxies.
MOND is successful in explaining the general shape of the observed rotation
curves for roughly three quarters of the galaxies in the sample presented here.
However, for the remaining quarter, MOND does not adequately explain the
observed rotation curves. Considering the uncertainties in distances and
inclinations for the galaxies in our sample, a small fraction of poor MOND
predictions is expected and is not necessarily a problem for MOND. We have also
made fits taking the MOND acceleration constant, a_0, as a free parameter in
order to identify any systematic trends. We find that there appears to be a
correlation between central surface brightness and the best-fit value of a_0,
in the sense that lower surface brightness galaxies tend to have lower a_0.
However, this correlation depends strongly on a small number of galaxies whose
rotation curves might be uncertain due to either bars or warps. Without these
galaxies, there is less evidence of a trend, but the average value we find for
a_0 ~ 0.7*10^-8 cm s^-2 is somewhat lower than derived from previous studies.
Such lower fitted values of a_0 could occur if external gravitational fields
are important.Comment: 12 pages, accepted for publication in Ap
The Mass of the Compact Object in the X-Ray Binary Her X-1/HZ Her
We have obtained the first estimates of the masses of the components of the
Her X-1/HZ Her X-ray binary system taking into account non-LTE effects in the
formation of the H_gamma absorption line: mx=1.8Msun and mv=2.5Msun. These mass
estimates were made in a Roche model based on the observed radial-velocity
curve of the optical star, HZ Her. The masses for the X-ray pulsar and optical
star obtained for an LTE model lie are mx=0.85\pm0.15Msun and
mv=1.87\pm0.13Msun. These mass estimates for the components of Her X-1/HZ Her
derived from the radial-velocity curve should be considered tentative. Further
mass estimates from high-precision observations of the orbital variability of
the absorption profiles in a non-LTE model for the atmosphere of the optical
component should be made.Comment: 20 pages, 4 tables, 8 figure
Inclination Effects and Beaming in Black Hole X-ray Binaries
We investigate the dependence of observational properties of black hole X-ray
binaries on the inclination angle i of their orbits. We find the following: (1)
Transient black hole binaries show no trend in their quiescent X-ray
luminosities as a function of i, suggesting that the radiation is not
significantly beamed. This is consistent with emission from an accretion disk.
If the X-rays are from a jet, then the Lorentz factor gamma of the jet is less
than 1.24 at the 90% confidence level. (2) The X-ray binary 4U1543-47 with i of
order 21 degrees has a surprisingly strong fluorescent iron line in the high
soft state. Quantifying an earlier argument by Park et al. (2004), we conclude
that if the continuum X-ray emission in this source is from a jet, then gamma <
1.04. (3) None of the known binaries has cos i 75 degrees. This
fact, plus the lack of eclipses among the 20 black hole binaries in our sample,
strongly suggests at the 99.5% confidence level that systems with large
inclination angles are hidden from view. The obscuration could be the result of
disk flaring, as suggested by Milgrom (1978) for neutron star X-ray binaries.
(4) Transient black hole binaries with i ~ 70-75 degrees have significantly
more complex X-ray light curves than systems with i < 65 degrees. This may be
the result of variable obscuration and/or variable height above the disk of the
radiating gas.Comment: 26 pages, to appear in The Astrophysical Journal, vol. 624, May 1,
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