142 research outputs found
Einstein's cosmology review of 1933: a new perspective on the Einstein-de Sitter model of the cosmos
We present a first English translation and analysis of a little-known review
of relativistic cosmology written by Albert Einstein in late 1932. The article,
which was published in 1933 in a book of Einstein papers translated into
French, contains a substantial review of static and dynamic relativistic models
of the cosmos, culminating in a discussion of the Einstein-de Sitter model. The
article offers a valuable contemporaneous insight into Einstein's cosmology in
the 1930s and confirms that his interest lay in the development of the simplest
model of the cosmos that could account for observation, rather than an
exploration of all possible cosmic models. The article also confirms that
Einstein did not believe that simplistic relativistic models could give an
accurate description of the early universe.Comment: Accepted for publication in the European Physical Journal (H).
Includes an English translation of a little-known review of cosmology written
by Albert Einstein in 1933. 20 pages, 4 figure
One Hundred Years of the Cosmological Constant: from 'Superfluous Stunt' to Dark Energy
We present a centennial review of the history of the term known as the
cosmological constant. First introduced to the general theory of relativity by
Einstein in 1917 in order to describe a universe that was assumed to be static,
the term fell from favour in the wake of the discovery of the expanding
universe, only to make a dramatic return in recent times. We consider
historical and philosophical aspects of the cosmological constant over four
main epochs: (i) the use of the term in static cosmologies (both Newtonian and
relativistic); (ii) the marginalization of the term following the discovery of
cosmic expansion; (iii) the use of the term to address specific cosmic puzzles
such as the timespan of expansion, the formation of galaxies and the redshifts
of the quasars; (iv) the re-emergence of the term in today's Lamda-CDM
cosmology. We find that the cosmological constant was never truly banished from
theoretical models of the universe, but was sidelined by astronomers for
reasons of convenience. We also find that the return of the term to the
forefront of modern cosmology did not occur as an abrupt paradigm shift due to
one particular set of observations, but as the result of a number of empirical
advances such as the measurement of present cosmic expansion using the Hubble
Space Telescope, the measurement of past expansion using type SN 1a supernovae
as standard candles, and the measurement of perturbations in the cosmic
microwave background by balloon and satellite. We give a brief overview of
contemporary interpretations of the physics underlying the cosmic constant and
conclude with a synopsis of the famous cosmological constant problem.Comment: 60 pages, 6 figures. Some corrections, additions and extra
references. Accepted for publication the European Physical Journal (H
Simulating a Ground Truth for Transit Time Analysis of Indicator Dilution Curves
Transit times of a bolus through an organ can provide valuable information for researchers, technicians and clinicians. Therefore, an indicator is injected and the temporal propagation is monitored at two distinct locations. The transit time extracted from two indicator dilution curves can be used to calculate for example blood flow and thus provide the surgeon with important diagnostic information. However, the performance of methods to determine the transit time Δt cannot be assessed quantitatively due to the lack of a sufficient and trustworthy ground truth derived from in vivo measurements. Therefore, we propose a method to obtain an in silico generated dataset of differently subsampled indicator dilution curves with a ground truth of the transit time. This method allows variations on shape, sampling rate and noise while being accurate and easily configurable. COMSOL Multiphysics is used to simulate a laminar flow through a pipe containing blood analogue. The indicator is modelled as a rectangular function of concentration in a segment of the pipe. Afterwards, a flow is applied and the rectangular function will be diluted. Shape varying dilution curves are obtained by discrete-time measurement of the average dye concentration over different cross-sectional areas of the pipe. One dataset is obtained by duplicating one curve followed by subsampling, delaying and applying noise. Multiple indicator dilution curves were simulated, which are qualitatively matching in vivo measurements. The curves temporal resolution, delay and noise level can be chosen according to the requirements of the field of research. Various datasets, each containing two corresponding dilution curves with an existing ground truth transit time, are now available. With additional knowledge or assumptions regarding the detection-specific transfer function, realistic signal characteristics can be simulated. The accuracy of methods for the assessment of Δt can now be quantitatively compared and their sensitivity to noise evaluated
Transit Time Measurement in Indicator Dilution Curves: Overcoming the Missing Ground Truth and Quantifying the Error
The vascular function of a vessel can be qualitatively and intraoperatively checked by recording the blood dynamics inside the vessel via fluorescence angiography (FA). Although FA is the state of the art in proving the existence of blood flow during interventions such as bypass surgery, it still lacks a quantitative blood flow measurement that could decrease the recurrence rate and postsurgical mortality. Previous approaches show that the measured flow has a significant deviation compared to the gold standard reference (ultrasonic flow meter). In order to systematically address the possible sources of error, we investigated the error in transit time measurement of an indicator. Obtaining in vivo indicator dilution curves with a known ground truth is complex and often not possible. Further, the error in transit time measurement should be quantified and reduced. To tackle both issues, we first computed many diverse indicator dilution curves using an in silico simulation of the indicator\u27s flow. Second, we post-processed these curves to mimic measured signals. Finally, we fitted mathematical models (parabola, gamma variate, local density random walk, and mono-exponential model) to re-continualize the obtained discrete indicator dilution curves and calculate the time delay of two analytical functions. This re-continualization showed an increase in the temporal accuracy up to a sub-sample accuracy. Thereby, the Local Density Random Walk (LDRW) model performed best using the cross-correlation of the first derivative of both indicator curves with a cutting of the data at 40% of the peak intensity. The error in frames depends on the noise level and is for a signal-to-noise ratio (SNR) of 20 dB and a sampling rate of f = 60 Hz at f⋅0.25(±0.18), so this error is smaller than the distance between two consecutive samples. The accurate determination of the transit time and the quantification of the error allow the calculation of the error propagation onto the flow measurement. Both can assist surgeons as an intraoperative quality check and thereby reduce the recurrence rate and post-surgical mortality
The Importance of the Exceptional in Tackling Riddles of Consciousness and Unusual Episodes of Lucidity
The problem of how biochemical processes in the brain give rise to conscious experience is still unanswered. This paper aims at stimulating the debate surrounding this enigma by advocating the study of unusual and anomalous aspects of consciousness. For this purpose, the contents of this paper are organized in three parts. In the first part, I provide a brief overview on unsolved riddles of the mind. These include unusual episodes of lucidity that have been termed terminal lucidity and paradoxical lucidity. Because the use of these terms has sometimes been inappropriate in recent literature, I clarify the basic meanings of these two concepts in the second part. The third part contains suggestions for future research. Specifically, I argue that the field of studies into episodes of lucidity in dementias and the field of studies into end-of-life experiences, such as near-death visions, should engage in an active dialogue in order to build bridges between these disciplines. Such a dialogue will enable a better understanding of the whole spectrum, and thus, possible circumstances, causes and underpinnings of lucid episodes. In sum, this paper argues that the study of lucid episodes such as terminal lucidity, paradoxical lucidity, and related occurrences holds enormous significance for improving our understanding of brain functions and accompanying states of consciousness – from a practice-orientated perspective in the contexts of the dementias and dealing with end-of-life experiences, and from a theoretical perspective in the context of the scientific debate about the nature of consciousness
A note on the (1, 1,..., 1) monopole metric
Recently K. Lee, E.J. Weinberg and P. Yi in CU-TP-739, hep-th/9602167,
calculated the asymptotic metric on the moduli space of (1, 1, ..., 1) BPS
monopoles and conjectured that it was globally exact. I lend support to this
conjecture by showing that the metric on the corresponding space of Nahm data
is the same as the metric they calculate.Comment: 12 pages, latex, no figures, uses amsmath, amsthm, amsfont
Laplacian modes probing gauge fields
We show that low-lying eigenmodes of the Laplace operator are suitable to
represent properties of the underlying SU(2) lattice configurations. We study
this for the case of finite temperature background fields, yet in the
confinement phase. For calorons as classical solutions put on the lattice, the
lowest mode localizes one of the constituent monopoles by a maximum and the
other one by a minimum, respectively. We introduce adjustable phase boundary
conditions in the time direction, under which the role of the monopoles in the
mode localization is interchanged. Similar hopping phenomena are observed for
thermalized configurations. We also investigate periodic and antiperiodic modes
of the adjoint Laplacian for comparison.
In the second part we introduce a new Fourier-like low-pass filter method. It
provides link variables by truncating a sum involving the Laplacian eigenmodes.
The filter not only reproduces classical structures, but also preserves the
confining potential for thermalized ensembles. We give a first characterization
of the structures emerging from this procedure.Comment: 43 pages, 26 figure
Serendipity in Science
Serendipity plays an important role in scientific discovery. Indeed, many of
the most important breakthroughs, ranging from penicillin to the electric
battery, have been made by scientists who were stimulated by a chance exposure
to unsought but useful information. However, not all scientists are equally
likely to benefit from such serendipitous exposure. Although scholars generally
agree that scientists with a prepared mind are most likely to benefit from
serendipitous encounters, there is much less consensus over what precisely
constitutes a prepared mind, with some research suggesting the importance of
openness and others emphasizing the need for deep prior experience in a
particular domain. In this paper, we empirically investigate the role of
serendipity in science by leveraging a policy change that exogenously shifted
the shelving location of journals in university libraries and subsequently
exposed scientists to unsought scientific information. Using large-scale data
on 2.4 million papers published in 9,750 journals by 520,000 scientists at 115
North American research universities, we find that scientists with greater
openness are more likely to benefit from serendipitous encounters. Following
the policy change, these scientists tended to cite less familiar and newer
work, and ultimately published papers that were more innovative. By contrast,
we find little effect on innovativeness for scientists with greater depth of
experience, who, in our sample, tended to cite more familiar and older work
following the policy change
Nonmeromorphic operator product expansion and C_2-cofiniteness for a family of W-algebras
We prove the existence and associativity of the nonmeromorphic operator
product expansion for an infinite family of vertex operator algebras, the
triplet W-algebras, using results from P(z)-tensor product theory. While doing
this, we also show that all these vertex operator algebras are C_2-cofinite.Comment: 21 pages, to appear in J. Phys. A: Math. Gen.; the exposition is
improved and one reference is adde
A note on the index bundle over the moduli space of monopoles
Donaldson has shown that the moduli space of monopoles is diffeomorphic
to the space \Rat_k of based rational maps from the two-sphere to itself. We
use this diffeomorphism to give an explicit description of the bundle on
\Rat_k obtained by pushing out the index bundle from . This gives an
alternative and more explicit proof of some earlier results of Cohen and Jones.Comment: 9 page
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