6,416 research outputs found
Dark matter in galaxies
Dark matter in galaxies, its abundance, and its distribution remain a subject
of long-standing discussion, especially in view of the fact that neither dark
matter particles nor dark matter bodies have yet been found. Experts' opinions
range from a very large number of completely dark galaxies exist to nonbaryonic
dark matter does not exist at all in any significant amounts. We discuss
astronomical evidence for the existence of dark matter and its connection with
visible matter and examine attempts to estimate its mass and distribution in
galaxies from photometry, dynamics, gravitational lensing, and other
observations (the cosmological aspects of the existence of dark matter are not
considered in this review). In our view, the presence of dark matter in and
around galaxies is a well-established fact. We conclude with an overview of
mechanisms by which a dark halo can influence intragalactic processes.Comment: 82 pages, 35 figure
Searching for black holes in subways.
Abstract Current mobile agent algorithms for mapping faults in computer networks assume that the network is static. However, for large classes of highly dynamic networks (e.g., wireless mobile ad hoc networks, sensor networks, vehicular networks), the topology changes as a function of time. These networks, called delay-tolerant, challenged, opportunistic, etc., have never been investigated with regard to locating faults. We consider a subclass of these networks modelled on an urban subway system. We examine the problem of creating a map of such a subway. More precisely, we study the problem of a team of asynchronous computational entities (the mapping agents) determining the location of black holes in a highly dynamic graph, whose edges are defined by the asynchronous movements of mobile entities (the subway carriers). We determine necessary conditions for the problem to be solvable. We then present and analyze a solution protocol; we show that our algorithm solves the fault mapping problem in subway networks with the minimum number of agents possible, k = γ + 1, where γ is the number of carrier stops at black holes. The number of carrier moves between stations required by the algorithm in the worst case is , where n C is the number of subway trains, and l R is the length of the subway route with the most stops. We establish lower bounds showing that this bound is tight. Thus, our protocol is both agent-optimal and move-optimal
Dynamical Boson Stars
The idea of stable, localized bundles of energy has strong appeal as a model
for particles. In the 1950s John Wheeler envisioned such bundles as smooth
configurations of electromagnetic energy that he called {\em geons}, but none
were found. Instead, particle-like solutions were found in the late 1960s with
the addition of a scalar field, and these were given the name {\em boson
stars}. Since then, boson stars find use in a wide variety of models as sources
of dark matter, as black hole mimickers, in simple models of binary systems,
and as a tool in finding black holes in higher dimensions with only a single
killing vector. We discuss important varieties of boson stars, their dynamic
properties, and some of their uses, concentrating on recent efforts.Comment: 79 pages, 25 figures, invited review for Living Reviews in
Relativity; major revision in 201
Probing the Lorentz Symmetry Violation Using the First Image of Sagittarius A*: Constraints on Standard-Model Extension Coefficients
Thanks to unparalleled near-horizon images of the shadows of Messier 87*
(M87*) and Sagittarius A* (Sgr A*) delivered by the Event Horizon Telescope
(EHT), two amazing windows opened up to us for the strong-field test of the
gravity theories as well as fundamental physics. Information recently published
from EHT about the Sgr A*'s shadow lets us have a novel possibility of
exploration of Lorentz symmetry violation (LSV) within the Standard-Model
Extension (SME) framework. Despite the agreement between the shadow image of
Sgr A* and the prediction of the general theory of relativity, there is still a
slight difference which is expected to be fixed by taking some fundamental
corrections into account. We bring up the idea that the recent inferred shadow
image of Sgr A* is explicable by a minimal SME-inspired Schwarzschild metric
containing the Lorentz violating (LV) terms obtained from the post-Newtonian
approximation. The LV terms embedded in Schwarzschild metric are dimensionless
spatial coefficients associated with the field responsible for
LSV in the gravitational sector of the minimal SME theory. In this way, one can
control Lorentz invariance violation in the allowed sensitivity level of the
first shadow image of Sgr A*. Actually, using the bounds released within
uncertainty for the shadow size of Sgr A* and whose fractional
deviation from standard Schwarzschild, we set upper limits for the two
different combinations of spatial diagonal coefficients and the time-time
coefficient of the SME, as well. The best upper bound is at the
level, which should be interpreted differently from those constraints
previously extracted from well-known frameworks since unlike standard SME
studies it is not obtained from a Sun-centered celestial frame but comes from
probing the black hole horizon scale.Comment: 16 pages, 5 figures. v3:discussion improved, references added,
figures revised; matches the version accepted for publication in PR
Gathering in Dynamic Rings
The gathering problem requires a set of mobile agents, arbitrarily positioned
at different nodes of a network to group within finite time at the same
location, not fixed in advanced.
The extensive existing literature on this problem shares the same fundamental
assumption: the topological structure does not change during the rendezvous or
the gathering; this is true also for those investigations that consider faulty
nodes. In other words, they only consider static graphs. In this paper we start
the investigation of gathering in dynamic graphs, that is networks where the
topology changes continuously and at unpredictable locations.
We study the feasibility of gathering mobile agents, identical and without
explicit communication capabilities, in a dynamic ring of anonymous nodes; the
class of dynamics we consider is the classic 1-interval-connectivity.
We focus on the impact that factors such as chirality (i.e., a common sense
of orientation) and cross detection (i.e., the ability to detect, when
traversing an edge, whether some agent is traversing it in the other
direction), have on the solvability of the problem. We provide a complete
characterization of the classes of initial configurations from which the
gathering problem is solvable in presence and in absence of cross detection and
of chirality. The feasibility results of the characterization are all
constructive: we provide distributed algorithms that allow the agents to
gather. In particular, the protocols for gathering with cross detection are
time optimal. We also show that cross detection is a powerful computational
element.
We prove that, without chirality, knowledge of the ring size is strictly more
powerful than knowledge of the number of agents; on the other hand, with
chirality, knowledge of n can be substituted by knowledge of k, yielding the
same classes of feasible initial configurations
A Rotating Aperture Mask for Small Telescopes
Observing the dynamic interaction between stars and their close stellar neighbors is key to establishing the stars’ orbits, masses, and other properties. Our ability to visually discriminate nearby stars is limited by the power of our telescopes, posing a challenge to astronomers at small observatories that contribute to binary star surveys. Masks placed at the telescope aperture promise to augment the resolving power of telescopes of all sizes, but many of these masks must be manually and repetitively reoriented about the optical axis to achieve their full benefits. This paper introduces a design concept for a mask rotation mechanism that can be adapted to telescopes of different types and proportions, focusing on an implementation for a Celestron C11 Schmidt–Cassegrain optical tube assembly. Mask concepts were first evaluated using diffraction simulation programs, later manufactured, and finally tested on close double stars using a C11. An electronic rotation mechanism was designed, produced, and evaluated. Results show that applying a properly shaped and oriented mask to a C11 enhances contrast in images of double star systems relative to images captured with the unmasked telescope, and they show that the rotation mechanism accurately and repeatably places masks at target orientations with minimal manual effort. Detail drawings of the mask rotation mechanism and code for the software interface are included
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