31,253 research outputs found
Information, information processing and gravity
I discuss fundamental limits placed on information and information processing
by gravity. Such limits arise because both information and its processing
require energy, while gravitational collapse (formation of a horizon or black
hole) restricts the amount of energy allowed in a finite region. Specifically,
I use a criterion for gravitational collapse called the hoop conjecture. Once
the hoop conjecture is assumed a number of results can be obtained directly:
the existence of a fundamental uncertainty in spatial distance of order the
Planck length, bounds on information (entropy) in a finite region, and a bound
on the rate of information processing in a finite region. In the final section
I discuss some cosmological issues related to the total amount of information
in the universe, and note that almost all detailed aspects of the late universe
are determined by the randomness of quantum outcomes. This paper is based on a
talk presented at a 2007 Bellairs Research Institute (McGill University)
workshop on black holes and quantum information.Comment: 7 pages, 5 figures, revte
A high-speed bi-polar outflow from the archetypical pulsating star Mira A
Optical images and high-dispersion spectra have been obtained of the ejected
material surrounding the pulsating AGB star Mira A. The two streams of knots on
either side of the star, found in far ultra-viollet (FUV) GALEX images, have
now been imaged clearly in the light of Halpha. Spatially resolved profiles of
the same line reveal that the bulk of these knots form a bi-polar outflow with
radial velocity extremes of +- 150 km/s with respect to the central star. The
South stream is approaching and the North stream receding from the observer. A
displacement away from Mira A between the position of one of the South stream
knots in the new Halpha image and its position in the previous Palomar
Observatory Sky Survey (POSS I) red plate has been noted. If interpreted as a
consequence of expansion proper motions the bipolar outflow is tilted at 69deg
+- 2deg to the plane of the sky, has an outflow velocity of 160 +- 10 km/s and
is ~1000 y old.Comment: 8 pages, 5 figures. Accepted for pubication by A&
VLT observations of the asymmetric Etched Hourglass Nebula, MyCn 18
Context. The mechanisms that form extreme bipolar planetary nebulae remain
unclear. Aims. The physical properties, structure, and dynamics of the bipolar
planetary nebula, MyCn 18, are investigated in detail with the aim of
understanding the shaping mechanism and evolutionary history of this object.
Methods. VLT infrared images, VLT ISAAC infrared spectra, and long-slit optical
Echelle spectra are used to investigate MyCn 18. Morpho-kinematic modelling was
used to firmly constrain the structure and kinematics of the source. A
timescale analysis was used to determine the kinematical age of the nebula and
its main components. Results. A spectroscopic study of MyCn 18's central and
offset region reveals the detailed make-up of its nebular composition.
Molecular hydrogen, atomic helium, and Bracket gamma emission are detected from
the central regions of MyCn 18. ISAAC spectra from a slit position along the
narrow waist of the nebula demonstrate that the ionised gas resides closer to
the centre of the nebula than the molecular emission. A kinematical age of the
nebula and its components were obtained by the P-V arrays and timescale
analysis. Conclusions. The structure and kinematics of MyCn 18 are better
understood using an interactive 3-D modelling tool called shape. A dimensional
and timescale analysis of MyCn 18's major components provides a possible
mechanism for the nebula's asymmetry. The putative central star is somewhat
offset from the geometric centre of the nebula, which is thought to be the
result of a binary system. We speculate that the engulfing and destruction of
an exoplanet during the AGB phase may have been a key event in shaping MyCn 18
and generating of its hypersonic knotty outflow.Comment: 15 pages, 3 tables, 13 figures. Accepted for publication by A&
Count Me In: The dimensions of social inclusion through culture and sport
This study was set up to examine claims made for the ability of cultural projects to promote social inclusion (cultural projects are here taken to include those incorporating sport, the arts, media, heritage and outdoor adventure). This was to be achieved primarily by collecting evidence from a sample of 14 projects selected from some 200 that had volunteered their services. The report to the governmentâs Social Exclusion Unit (SEU) from the Policy Action Team (PAT10) (1999)2 noted the potential. In his foreword, Chris Smith (then Secretary of State for the Department for Culture Media and Sport (DCMS)) wrote: â⊠art and sport can not only make a valuable contribution to delivering key outcomes of lower long term unemployment, less crime, better health and better qualifications, but can also help to develop the individual pride, community spirit and capacity for responsibility that enable communities to run regeneration programmes themselvesâ. Similar statements have followed from other politicians, particularly in the recent Commons debate on sport and social exclusion (22/11/01), and again in the public health debate (13/12/01). However, the PAT 10 report also came to the same conclusion as previous commentators (e.g. Glyptis, 19893; Allison & Coalter, 19964; Long & Sanderson, 19985) that there is little âhardâ evidence of the social benefits that accrue
Distribution of Spectral Characteristics and the Cosmological Evolution of GRBs
We investigate the cosmological evolution of GRBs, using the total gamma ray
fluence as a measure of the burst strength. This involves an understanding of
the distributions of the spectral parameters of GRBs as well as the total
fluence distribution - both of which are subject to detector selection effects.
We present new non-parametric statistical techniques to account for these
effects, and use these methods to estimate the true distribution of the peak of
the nu F_nu spectrum, E_p, from the raw distribution. The distributions are
obtained from four channel data and therefore are rough estimates. Here, we
emphasize the methods and present qualitative results. Given its spectral
parameters, we then calculate the total fluence for each burst, and compute its
cumulative and differential distributions. We use these distributions to
estimate the cosmological rate evolution of GRBs, for three cosmological
models. Our two main conclusions are the following: 1) Given our estimates of
the spectral parameters, we find that there may exist a significant population
of high E_p bursts that are not detected by BATSE, 2) We find a GRB co-moving
rate density quite different from that of other extragalactic objects; in
particular, it is different from the recently determined star formation rate.Comment: 20 pages, including 10 postscript figures. Submitted to Ap
The Birmingham-CfA cluster scaling project - I: gas fraction and the M-T relation
We have assembled a large sample of virialized systems, comprising 66 galaxy
clusters, groups and elliptical galaxies with high quality X-ray data. To each
system we have fitted analytical profiles describing the gas density and
temperature variation with radius, corrected for the effects of central gas
cooling. We present an analysis of the scaling properties of these systems and
focus in this paper on the gas distribution and M-T relation. In addition to
clusters and groups, our sample includes two early-type galaxies, carefully
selected to avoid contamination from group or cluster X-ray emission. We
compare the properties of these objects with those of more massive systems and
find evidence for a systematic difference between galaxy-sized haloes and
groups of a similar temperature. We derive a mean logarithmic slope of the M-T
relation within R_200 of 1.84+/-0.06, although there is some evidence of a
gradual steepening in the M-T relation, with decreasing mass. We recover a
similar slope using two additional methods of calculating the mean temperature.
Repeating the analysis with the assumption of isothermality, we find the slope
changes only slightly, to 1.89+/-0.04, but the normalization is increased by
30%. Correspondingly, the mean gas fraction within R_200 changes from
(0.13+/-0.01)h70^-1.5 to (0.11+/-0.01)h70^-1.5, for the isothermal case, with
the smaller fractional change reflecting different behaviour between hot and
cool systems. There is a strong correlation between the gas fraction within
0.3*R_200 and temperature. This reflects the strong (5.8 sigma) trend between
the gas density slope parameter, beta, and temperature, which has been found in
previous work. (abridged)Comment: 27 pages, accepted for publication in MNRAS; uses longtable.sty &
lscape.st
Ground-layer wavefront reconstruction from multiple natural guide stars
Observational tests of ground layer wavefront recovery have been made in open
loop using a constellation of four natural guide stars at the 1.55 m Kuiper
telescope in Arizona. Such tests explore the effectiveness of wide-field seeing
improvement by correction of low-lying atmospheric turbulence with ground-layer
adaptive optics (GLAO). The wavefronts from the four stars were measured
simultaneously on a Shack-Hartmann wavefront sensor (WFS). The WFS placed a 5 x
5 array of square subapertures across the pupil of the telescope, allowing for
wavefront reconstruction up to the fifth radial Zernike order. We find that the
wavefront aberration in each star can be roughly halved by subtracting the
average of the wavefronts from the other three stars. Wavefront correction on
this basis leads to a reduction in width of the seeing-limited stellar image by
up to a factor of 3, with image sharpening effective from the visible to near
infrared wavelengths over a field of at least 2 arc minutes. We conclude that
GLAO correction will be a valuable tool that can increase resolution and
spectrographic throughput across a broad range of seeing-limited observations.Comment: 25 pages, 8 figures, to be published in Astrophys.
Entanglement in a Valence-Bond-Solid State
We study entanglement in Valence-Bond-Solid state. It describes the ground
state of Affleck, Kennedy, Lieb and Tasaki quantum spin chain. The AKLT model
has a gap and open boundary conditions. We calculate an entropy of a subsystem
(continuous block of spins). It quantifies the entanglement of this block with
the rest of the ground state. We prove that the entanglement approaches a
constant value exponentially fast as the size of the subsystem increases.
Actually we proved that the density matrix of the continuous block of spins
depends only on the length of the block, but not on the total size of the chain
[distance to the ends also not essential]. We also study reduced density
matrices of two spins both in the bulk and on the boundary. We evaluated
concurrencies.Comment: 4pages, no figure
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