220 research outputs found
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
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
Acrylamide formation in potato products
End of Project ReportAcrylamide, a substance classified as a potential carcinogen, occurs in heated
starchy foods at concentrations many times in excess of levels permitted in
drinking water. Early surveys indicated that levels of acrylamide in potato
products such as French fries and potato crisps were the highest of the
foodstuffs investigated. The present project addressed this issue by
determining levels of acrylamide precursors (asparagine and reducing sugars)
in raw potatoes and levels of acrylamide in (i) potato products from different
storage regimes, (ii) spot-sampled potatoes purchased from a local
supermarket, (iii) samples that received pre-treatments and were fried at
different temperatures and (iv) French fries reheated in different ovens.A risk
assessment of the estimated acrylamide intake from potato products for
various cohorts of the Irish population was also conducted
Historical and philosophical reflections on the Einstein-de Sitter model
We present some historical and philosophical reflections on the paper "On the
Relation Between the Expansion and the Mean Density of the Universe", published
by Albert Einstein and Willem de Sitter in 1932. In this famous work, Einstein
and de Sitter considered a relativistic model of the expanding universe with
both the cosmological constant and the curvature of space set to zero. Although
the Einstein-deSitter model went on to serve as a standard model in 'big bang'
cosmology for many years, we note that the authors do not explicitly consider
the evolution of the cosmos in the paper. Indeed, the mathematics of the
article are quite puzzling to modern eyes. We consider claims that the paper
was neither original nor important; we find that, by providing the first
specific analysis of the case of a dynamic cosmology without a cosmological
constant or spatial curvature, the authors delivered a unique, simple model
with a straightforward relation between cosmic expansion and the mean density
of matter that set an important benchmark for both theorists and observers. We
consider some philosophical aspects of the model and provide a brief review of
its use as a prototype 'big bang' model over much of the 20th century.Comment: Some revisions and corrections to original MS. Accepted for
publication in the European Physical Journal (H
Threeâperiodic nets and tilings: semiregular nets
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/115982/1/S0108767303017100.pd
Threeâperiodic nets and tilings: regular and quasiregular nets
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/115935/1/S0108767302018494.pd
Minimal nets and minimal minimal surfaces
The 3-periodic nets of genus 3 ('minimal nets') are reviewed and their symmetries re-examined. Although they are all crystallographic, seven of the 15 only have maximum-symmetry embeddings if some links are allowed to have zero length. The connection bet
Threeâperiodic nets and tilings: minimal nets
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/116005/1/S0108767304015442.pd
Secondary building units, nets and bonding in the chemistry of metalâorganic frameworks
This critical review presents a comprehensive study of transition-metal carboxylate clusters which may serve as secondary building units (SBUs) towards construction and synthesis of metalâorganic frameworks (MOFs). We describe the geometries of 131 SBUs, their connectivity and composition. This contribution presents a comprehensive list of the wide variety of transition-metal carboxylate clusters which may serve as secondary building units (SBUs) in the construction and synthesis of metalâorganic frameworks. The SBUs discussed here were obtained from a search of molecules and extended structures archived in the Cambridge Structure Database (CSD, version 5.28, January 2007) which included only crystals containing metal carboxylate linkages (241 references)
- âŠ