2,930 research outputs found
The galaxy halo formation rate
The rate at which galaxy halos form is thought to play a key role in
explaining many observable cosmological phenomena such as the initial epoch at
which luminous matter forms and the distribution of active galaxies. Here we
show how Press-Schechter theory can be used to provide a simple, completely
analytic model of the halo formation rate. This model shows good agreement with
both Monte-Carlo and N-body simulation results.Comment: 2 pages, 1 figure, to appear in proceedings of the Xth Recontres de
Blois, "The Birth of Galaxies," LaTeX style file include
Cosmological evolution and hierarchical galaxy formation
We provide a new multi-waveband compilation of the data describing the
cosmological evolution of quasars, and discuss a model that attributes the
evolution to variation in the rate of merging between dark halos in a
hierarchical universe. We present a new Press-Schechter calculation of the
expected merger rate and show that this can reproduce the principal features of
the evolution. We also show that the evolution in the star-formation history of
the universe is well-described by this model.Comment: 4 pages, 1 figure. Presented at Xth Rencontres de Blois, "The Birth
of Galaxies", June 199
First phase testing of solar thermal engine at United Stirling
The objective of the program is to demonstrate that the Stirling engine is a practical efficient and reliable energy converter when integrated with a parabolic dish concentrator, and that it has the potential of being cost competitive with fossil fueled electric generating systems of today. The engine, with its receiver (solar heat exchanger), alternator and control system, is described
The Halo Formation Rate and its link to the Global Star Formation Rate
The star formation history of the universe shows strong evolution with
cosmological epoch. Although we know mergers between galaxies can cause
luminous bursts of star formation, the relative importance of such mergers to
the global star formation rate (SFR) is unknown. We present a simple analytic
formula for the rate at which halos merge to form higher-mass systems, derived
from Press-Schechter theory and confirmed by numerical simulations (for high
halo masses). A comparison of the evolution in halo formation rate with the
observed evolution in the global SFR indicates that the latter is largely
driven by halo mergers at z>1. Recent numerical simulations by Kolatt et al.
(1999) and Knebe & Muller (1999) show how merging systems are strongly biased
tracers of mass fluctuations, thereby explaining the strong clustering observed
for Lyman-break galaxies without any need to assume that Lyman-break galaxies
are associated only with the most massive systems at z~3.Comment: 4 pages, 2 figures. To appear in `The Hy-redshift universe: Galaxy
formation and evolution at high redshift' eds. A.J. Bunker and W.J.M. van
Breuge
Decoherence of quantum wavepackets due to interaction with conformal spacetime fluctuations
One of the biggest problems faced by those attempting to combine quantum
theory and general relativity is the experimental inaccessibility of the
unification scale. In this paper we show how incoherent conformal waves in the
gravitational field, which may be produced by quantum mechanical zero-point
fluctuations, interact with the wavepackets of massive particles. The result of
this interaction is to produce decoherence within the wavepackets which could
be accessible in experiments at the atomic scale.
Using a simple model for the coherence properties of the gravitational field
we derive an equation for the evolution of the density matrix of such a
wavepacket. Following the primary state diffusion programme, the most promising
source of spacetime fluctuations for detection are the above zero-point energy
fluctuations. According to our model, the absence of intrinsic irremoveable
decoherence in matter interferometry experiments puts bounds on some of the
parameters of quantum gravity theories. Current experiments give \lambda > 18.
, where \lambda t_{Planck} is an effective cut-off for the validity of
low-energy quantum gravity theories.Comment: REVTeX forma
Age constraints on the evolution of the Quetico belt, Superior Province, Ontario
Much attention has been focused on the nature of Archean tectonic processes and the extent to which they were different from modern rigid-plate tectonics. The Archean Superior Province has linear metavolcanic and metasediment-dominated subprovinces of similar scale to cenozoic island arc-trench systems of the western Pacific, suggesting an origin by accreting arcs. Models of the evolution of metavolcanic belts in parts of the Superior Province suggest an arc setting but the tectonic environment and evolution of the intervening metasedimentary belts are poorly understood. In addition to explaining the setting giving rise to a linear sedimentary basin, models must account for subsequent shortening and high-temperature, low-pressure metamorphism. Correlation of rock units and events in adjacent metavolcanic and metasedimentary belts is a first step toward understanding large-scale crustal interactions. To this end, zircon geochronology has been applied to metavolcanic belts of the western Superior Province; new age data for the Quetico metasedimentary belt is reported, permitting correlation with the adjacent Wabigoon and Wawa metavolcanic subprovinces
An analytic model for the epoch of halo creation
In this paper we describe the Bayesian link between the cosmological mass
function and the distribution of times at which isolated halos of a given mass
exist. By assuming that clumps of dark matter undergo monotonic growth on the
time-scales of interest, this distribution of times is also the distribution of
`creation' times of the halos. This monotonic growth is an inevitable aspect of
gravitational instability. The spherical top-hat collapse model is used to
estimate the rate at which clumps of dark matter collapse. This gives the prior
for the creation time given no information about halo mass. Applying Bayes'
theorem then allows any mass function to be converted into a distribution of
times at which halos of a given mass are created. This general result covers
both Gaussian and non-Gaussian models. We also demonstrate how the mass
function and the creation time distribution can be combined to give a joint
density function, and discuss the relation between the time distribution of
major merger events and the formula calculated. Finally, we determine the
creation time of halos within three N-body simulations, and compare the link
between the mass function and creation rate with the analytic theory.Comment: 7 pages, 2 figures, submitted to MNRA
Roman Jakobson and the birth of linguistic structuralism
The term “structuralism” was introduced into linguistics by Roman Jakobson in the early days of the Linguistic Circle of Prague, founded in 1926. The cluster of ideas defended by Jakobson and his colleagues can be specified but differ considerably from the concept of structuralism as it has come to be understood more recently. That took place because from the 1930s on it became customary to equate structuralism with the ideas of Ferdinand de Saussure, as expounded in his posthumous Cours de linguistique générale (1916). It can be shown, however, that Jakobson’s group rejected Saussure’s theory for ideological reasons. As the term “structuralism” became more widely used it came to be associated with positivist approaches to linguistics rather than with the original phenomenological orientation that had characterized the Linguistic Circle of Prague. The purpose of this paper is to clarify these different approaches and to suggest that because of its extreme porosity the word “structuralism” is an example of a “terminological pandemic”. More research on the varied uses to which the key terms “structure” and “structuralism” were put will undoubtedly further elucidate this important episode in 20th-century intellectual history
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