7,897 research outputs found
The Einstein static universe with torsion and the sign problem of the cosmological constant
In the field equations of Einstein-Cartan theory with cosmological constant a
static spherically symmetric perfect fluid with spin density satisfying the
Weyssenhoff restriction is considered. This serves as a rough model of space
filled with (fermionic) dark matter. From this the Einstein static universe
with constant torsion is constructed, generalising the Einstein Cosmos to
Einstein-Cartan theory.
The interplay between torsion and the cosmological constant is discussed. A
possible way out of the cosmological constant's sign problem is suggested.Comment: 8 pages, LaTeX; minor layout changes, typos corrected, one new
equation, new reference [5], completed reference [13], two references adde
Spinless Matter in Transposed-Equi-Affine Theory of Gravity
We derive and discus the equations of motion for spinless matter:
relativistic spinless scalar fields, particles and fluids in the recently
proposed by A. Saa model of gravity with covariantly constant volume with
respect to the transposed connection in Einstein-Cartan spaces.
A new interpretation of this theory as a theory with variable Plank
"constant" is suggested.
We show that the consistency of the semiclassical limit of the wave equation
and classical motion dictates a new definite universal interaction of torsion
with massive fields.Comment: 29 pages, latex, no figures. New Section on semiclassical limit of
wave equation added; old references rearranged; new references, remarks,
comments, and acknowledgments added; typos correcte
Self-healing diffusion quantum Monte Carlo algorithms: methods for direct reduction of the fermion sign error in electronic structure calculations
We develop a formalism and present an algorithm for optimization of the trial
wave-function used in fixed-node diffusion quantum Monte Carlo (DMC) methods.
We take advantage of a basic property of the walker configuration distribution
generated in a DMC calculation, to (i) project-out a multi-determinant
expansion of the fixed-node ground-state wave function and (ii) to define a
cost function that relates the fixed-node ground-state and the non-interacting
trial wave functions. We show that (a) locally smoothing out the kink of the
fixed-node ground-state wave function at the node generates a new trial
wave-function with better nodal structure and (b) we argue that the noise in
the fixed-node wave-function resulting from finite sampling plays a beneficial
role, allowing the nodes to adjust towards the ones of the exact many-body
ground state in a simulated annealing-like process. We propose a method to
improve both single determinant and multi-determinant expansions of the trial
wave-function. We test the method in a model system where benchmark
configuration interaction calculations can be performed. Comparing the DMC
calculations with the exact solutions, we find that the trial wave-function is
systematically improved. The overlap of the optimized trial wave function and
the exact ground state converges to 100% even starting from wave-functions
orthogonal to the exact ground state. In the optimization process we find an
optimal non-interacting nodal potential of density-functional-like form whose
existence was predicted earlier[Phys.Rev. B {\bf 77}, 245110 (2008)]. We obtain
the exact Kohn-Sham effective potential from the DMC data.Comment: Final version of the paper accepted in Physical Review B. The review
reports and replies are included in the sourc
Torsion and the Gravitational Interaction
By using a nonholonomous-frame formulation of the general covariance
principle, seen as an active version of the strong equivalence principle, an
analysis of the gravitational coupling prescription in the presence of
curvature and torsion is made. The coupling prescription implied by this
principle is found to be always equivalent with that of general relativity, a
result that reinforces the completeness of this theory, as well as the
teleparallel point of view according to which torsion does not represent
additional degrees of freedom for gravity, but simply an alternative way of
representing the gravitational field.Comment: Version 2: minor presentation changes, a reference added, 11 pages
(IOP style
Conformal Couplings in Induced Gravity
It is found that the induced gravity with conformal couplings requires the
conformal invariance in both classical and quantum levels for consistency. This
is also true for the induced gravity with an extended conformal coupling
interacting with torsion.Comment: 10 pages, Revtex3.0, to appear in General Relativity and Gravitatio
The free surface of superfluid 4He at zero temperature
The structure and energetics of the free surface of superfluid He are
studied using the diffusion Monte Carlo method. Extending a previous
calculation by Vall\'es and Schmidt, which used the Green's function Monte
Carlo method, we study the surface of liquid He within a slab geometry
using a larger number of particles in the slab and an updated interatomic
potential. The surface tension is accurately estimated from the energy of slabs
of increasing surface density and its value is close to one of the two existing
experimental values. Results for the density profiles allow for the calculation
of the surface width which shows an overall agreement with recent experimental
data. The dependence on the transverse direction to the surface of other
properties such as the two-body radial distribution function, structure factor,
and one-body density matrix is also studied. The condensate fraction, extracted
from the asymptotic behavior of the one-body density matrix, shows an
unambiguous enhancement when approaching the surface.Comment: RevTex, 11 pages, accepted in Phys. Rev.
Investigation of mechanical losses of thin silicon flexures at low temperatures
The investigation of the mechanical loss of different silicon flexures in a
temperature region from 5 to 300 K is presented. The flexures have been
prepared by different fabrication techniques. A lowest mechanical loss of
was observed for a 130 m thick flexure at around 10 K.
While the mechanical loss follows the thermoelastic predictions down to 50 K a
difference can be observed at lower temperatures for different surface
treatments. This surface loss will be limiting for all applications using
silicon based oscillators at low temperatures. The extraction of a surface loss
parameter using different results from our measurements and other references is
presented. We focused on structures that are relevant for gravitational wave
detectors. The surface loss parameter = 0.5 pm was obtained. This
reveals that the surface loss of silicon is significantly lower than the
surface loss of fused silica.Comment: 16 pages, 7 figure
Progress in Monte Carlo calculations of Fermi systems: normal liquid 3He
The application of the diffusion Monte Carlo method to a strongly interacting
Fermi system as normal liquid He is explored. We show that the fixed-node
method together with the released-node technique and a systematic method to
analytically improve the nodal surface constitute an efficient strategy to
improve the calculation up to a desired accuracy. This methodology shows
unambiguously that backflow correlations, when properly optimized, are enough
to generate an equation of state of liquid He in excellent agreement with
experimental data from equilibrium up to freezing.Comment: 14 pages, 3 eps figure
Examining Neanderthal and carnivore occupations of Teixoneres Cave (Moià, Barcelona, Spain) using archaeostratigraphic and intra-site spatial analysis
Teixoneres Cave (Moia, Barcelona, Spain) is a reference site for Middle Palaeolithic studies of the Iberian Peninsula. The cave preserves an extensive stratigraphic sequence made up of eight units, which is presented in depth in this work. The main goal of this study is to undertake an initial spatial examination of Unit III, formed during Marine Isotope Stage 3, with the aim of understanding spatial organization and past activities developed by Neanderthals and carnivores (bears, hyenas and smaller carnivores). The total sample analysed includes 38,244 archaeological items and 5888 limestone blocks. The application of GIS tools allows us to clearly distinguish three geologically-defined stratigraphic subunits. Unit III has been previously interpreted as a palimpsest resulting from alternating occupation of the cave by human groups and carnivores. The distribution study shows that faunal specimens, lithic artefacts, hearths and charcoal fragments are significantly concentrated at the entrance of the cave where, it is inferred, hominins carried out different activities, while carnivores preferred the sheltered zones in the inner areas of the cave. The results obtained reveal a spatial pattern characterized by fire use related zones, and show that the site was occupied by Neanderthals in a similar and consistent way throughout the (>)7000 years range covered by the analysed subunits. This spatial pattern is interpreted as resulting from repeated short-term human occupations
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