55,965 research outputs found
Quantum geometry from 2+1 AdS quantum gravity on the torus
Wilson observables for 2+1 quantum gravity with negative cosmological
constant, when the spatial manifold is a torus, exhibit several novel features:
signed area phases relate the observables assigned to homotopic loops, and
their commutators describe loop intersections, with properties that are not yet
fully understood. We describe progress in our study of this bracket, which can
be interpreted as a q-deformed Goldman bracket, and provide a geometrical
interpretation in terms of a quantum version of Pick's formula for the area of
a polygon with integer vertices.Comment: 19 pages, 11 figures, revised with more explanations, improved
figures and extra figures. To appear GER
Mean Reversion in Equilibrium Asset Prices
Recent empirical studies have found that stock returns contain substantial negative serial correlation at long horizons. We examine this finding with a series of Monte Carlo simulations in order to demonstrate that it is consistent with an equilibrium model of asset pricing. When investors display only a moderate degree of risk aversion, commonly used measures of mean reversion in stock prices calculated from actual returns data nearly always lie within a 60 percent confidence interval of the median of the Monte Carlo distributions. From this evidence, we conclude that the degree of serial correlation in the data could plausibly have been generated by our model.
Price Level Convergence Among United States Cities: Lessons for the European Central Bank
We study the dynamics of price indices for major U.S. cities using panel econometric methods and find that relative price levels among cities mean revert at an exceptionally slow rate. In a panel of 19 cities from 1918 to 1995, we estimate the half-life of convergence to be approximately nine years. These estimates provide an upper bound on speed of convergence that participants in European Monetary Union are likely to experience. The surprisingly slow rate of convergence can be explained by a combination of the presence of transportation costs, differential speeds of adjustment to small and large shocks, and the inclusion of non-traded good prices in the overall price index.
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Biogeochemical Changes During Bio-cementation Mediated by Stimulated and Augmented Ureolytic Microorganisms.
Microbially Induced Calcite Precipitation (MICP) is a bio-mediated cementation process that can improve the engineering properties of granular soils through the precipitation of calcite. The process is made possible by soil microorganisms containing urease enzymes, which hydrolyze urea and enable carbonate ions to become available for precipitation. While most researchers have injected non-native ureolytic bacteria to complete bio-cementation, enrichment of native ureolytic microorganisms may enable reductions in process treatment costs and environmental impacts. In this study, a large-scale bio-cementation experiment involving two 1.7-meter diameter tanks and a complementary soil column experiment were performed to investigate biogeochemical differences between bio-cementation mediated by either native or augmented (Sporosarcina pasteurii) ureolytic microorganisms. Although post-treatment distributions of calcite and engineering properties were similar between approaches, the results of this study suggest that significant differences in ureolysis rates and related precipitation rates between native and augmented microbial communities may influence the temporal progression and spatial distribution of bio-cementation, solution biogeochemical changes, and precipitate microstructure. The role of urea hydrolysis in enabling calcite precipitation through sustained super-saturation following treatment injections is explored
Topological low-temperature limit of Z(2) spin-gauge theory in three dimensions
We study Z(2) lattice gauge theory on triangulations of a compact 3-manifold.
We reformulate the theory algebraically, describing it in terms of the
structure constants of a bidimensional vector space H equipped with algebra and
coalgebra structures, and prove that in the low-temperature limit H reduces to
a Hopf Algebra, in which case the theory becomes equivalent to a topological
field theory. The degeneracy of the ground state is shown to be a topological
invariant. This fact is used to compute the zeroth- and first-order terms in
the low-temperature expansion of Z for arbitrary triangulations. In finite
temperatures, the algebraic reformulation gives rise to new duality relations
among classical spin models, related to changes of basis of H.Comment: 10 pages, no figure
Three Numerical Puzzles and the Top Quark's Chiral Weak-Moment
Versus the standard model's t --> W b decay helicity amplitudes, three
numerical puzzles occur at the 0.1 % level when one considers the amplitudes in
the case of an additional (f_M + f_E) coupling of relative strength 53 GeV. The
puzzles are theoretical ones which involve the t --> W b decay helicity
amplitudes in the two cases, the relative strength of this additional coupling,
and the observed masses of these three particles. A deeper analytic realization
is obtained for two of them. Equivalent realizations are given for the
remaining one. An empirical consequence of these analytic realizations is that
it is important to search for effects of a large chiral weak-moment of the
top-quark, the effective mass-scale is about 53 GeV. A full theoretical
resolution would include relating the origin of such a chiral weak-moment and
the mass generation of the top-quark, the W-boson, and probably the b-quark.Comment: 18 pages, 1 postscript table (revised to better explain notation,
model #1, add a little material...
How Much of the Web Is Archived?
Although the Internet Archive's Wayback Machine is the largest and most
well-known web archive, there have been a number of public web archives that
have emerged in the last several years. With varying resources, audiences and
collection development policies, these archives have varying levels of overlap
with each other. While individual archives can be measured in terms of number
of URIs, number of copies per URI, and intersection with other archives, to
date there has been no answer to the question "How much of the Web is
archived?" We study the question by approximating the Web using sample URIs
from DMOZ, Delicious, Bitly, and search engine indexes; and, counting the
number of copies of the sample URIs exist in various public web archives. Each
sample set provides its own bias. The results from our sample sets indicate
that range from 35%-90% of the Web has at least one archived copy, 17%-49% has
between 2-5 copies, 1%-8% has 6-10 copies, and 8%-63% has more than 10 copies
in public web archives. The number of URI copies varies as a function of time,
but no more than 31.3% of URIs are archived more than once per month.Comment: This is the long version of the short paper by the same title
published at JCDL'11. 10 pages, 5 figures, 7 tables. Version 2 includes minor
typographical correction
First-principles calculation of DNA looping in tethered particle experiments
We calculate the probability of DNA loop formation mediated by regulatory
proteins such as Lac repressor (LacI), using a mathematical model of DNA
elasticity. Our model is adapted to calculating quantities directly observable
in Tethered Particle Motion (TPM) experiments, and it accounts for all the
entropic forces present in such experiments. Our model has no free parameters;
it characterizes DNA elasticity using information obtained in other kinds of
experiments. [...] We show how to compute both the "looping J factor" (or
equivalently, the looping free energy) for various DNA construct geometries and
LacI concentrations, as well as the detailed probability density function of
bead excursions. We also show how to extract the same quantities from recent
experimental data on tethered particle motion, and then compare to our model's
predictions. [...] Our model successfully reproduces the detailed distributions
of bead excursion, including their surprising three-peak structure, without any
fit parameters and without invoking any alternative conformation of the LacI
tetramer. Indeed, the model qualitatively reproduces the observed dependence of
these distributions on tether length (e.g., phasing) and on LacI concentration
(titration). However, for short DNA loops (around 95 basepairs) the experiments
show more looping than is predicted by the harmonic-elasticity model, echoing
other recent experimental results. Because the experiments we study are done in
vitro, this anomalously high looping cannot be rationalized as resulting from
the presence of DNA-bending proteins or other cellular machinery. We also show
that it is unlikely to be the result of a hypothetical "open" conformation of
the LacI tetramer.Comment: See the supplement at
http://www.physics.upenn.edu/~pcn/Ms/TowlesEtalSuppl.pdf . This revised
version accepted for publication at Physical Biolog
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