1,686 research outputs found
Fermion absorption cross section of a Schwarzschild black hole
We study the absorption of massive spin-half particles by a small
Schwarzschild black hole by numerically solving the single-particle Dirac
equation in Painleve-Gullstrand coordinates. We calculate the absorption cross
section for a range of gravitational couplings Mm/m_P^2 and incident particle
energies E. At high couplings, where the Schwarzschild radius R_S is much
greater than the wavelength lambda, we find that the cross section approaches
the classical result for a point particle. At intermediate couplings we find
oscillations around the classical limit whose precise form depends on the
particle mass. These oscillations give quantum violations of the equivalence
principle. At high energies the cross section converges on the geometric-optics
value of 27 \pi R_S^2/4, and at low energies we find agreement with an
approximation derived by Unruh. When the hole is much smaller than the particle
wavelength we confirm that the minimum possible cross section approaches \pi
R_S^2/2.Comment: 11 pages, 3 figure
Reflection Symmetries for Multiqubit Density Operators
For multiqubit density operators in a suitable tensorial basis, we show that
a number of nonunitary operations used in the detection and synthesis of
entanglement are classifiable as reflection symmetries, i.e., orientation
changing rotations. While one-qubit reflections correspond to antiunitary
symmetries, as is known for example from the partial transposition criterion,
reflections on the joint density of two or more qubits are not accounted for by
the Wigner Theorem and are well-posed only for sufficiently mixed states. One
example of such nonlocal reflections is the unconditional NOT operation on a
multiparty density, i.e., an operation yelding another density and such that
the sum of the two is the identity operator. This nonphysical operation is
admissible only for sufficiently mixed states.Comment: 9 page
New Techniques for Analysing Axisymmetric Gravitational Systems. 1. Vacuum Fields
A new framework for analysing the gravitational fields in a stationary,
axisymmetric configuration is introduced. The method is used to construct a
complete set of field equations for the vacuum region outside a rotating
source. These equations are under-determined. Restricting the Weyl tensor to
type D produces a set of equations which can be solved, and a range of new
techniques are introduced to simplify the problem. Imposing the further
condition that the solution is asymptotically flat yields the Kerr solution
uniquely. The implications of this result for the no-hair theorem are
discussed. The techniques developed here have many other applications, which
are described in the conclusions.Comment: 30 pages, no figure
Lightlike infinity in GCA models of Spacetime
This paper discusses a 7 dimensional conformal geometric algebra model for
spacetime based on the notion that spacelike and timelike infinities are
distinct. I show how naturally of the dimensions represents the lightlike
infinity and appears redundant in computations, yet usefull in interpretationComment: 12 page
The emergence and development of behavioral individuality in clonal fish
Behavioral individuality is a ubiquitous phenomenon in animal populations, yet the origins and developmental trajectories of individuality, especially very early in life, are still a black box. Using a high-resolution tracking system, we mapped the behavioral trajectories of genetically identical fish (Poecilia formosa), separated immediately after birth into identical environments, over the first 10 weeks of their life at 3 s resolution. We find that (i) strong behavioral individuality is present at the very first day after birth, (ii) behavioral differences at day 1 of life predict behavior up to at least 10 weeks later, and (iii) patterns of individuality strengthen gradually over developmental time. Our results establish a null model for how behavioral individuality can develop in the absence of genetic and environmental variation and provide experimental evidence that later-in-life individuality can be strongly shaped by factors predating birth like maternal provisioning, epigenetics and pre-birth developmental stochasticity
Early Dark Energy Cosmologies
We propose a novel parameterization of the dark energy density. It is
particularly well suited to describe a non-negligible contribution of dark
energy at early times and contains only three parameters, which are all
physically meaningful: the fractional dark energy density today, the equation
of state today and the fractional dark energy density at early times. As we
parameterize Omega_d(a) directly instead of the equation of state, we can give
analytic expressions for the Hubble parameter, the conformal horizon today and
at last scattering, the sound horizon at last scattering, the acoustic scale as
well as the luminosity distance. For an equation of state today w_0 < -1, our
model crosses the cosmological constant boundary. We perform numerical studies
to constrain the parameters of our model by using Cosmic Microwave Background,
Large Scale Structure and Supernovae Ia data. At 95% confidence, we find that
the fractional dark energy density at early times Omega_early < 0.06. This
bound tightens considerably to Omega_early < 0.04 when the latest Boomerang
data is included. We find that both the gold sample of Riess et. al. and the
SNLS data by Astier et. al. when combined with CMB and LSS data mildly prefer
w_0 < -1, but are well compatible with a cosmological constant.Comment: 6 pages, 3 figures; references added, matches published versio
Constraints on early dark energy from CMB lensing and weak lensing tomography
Dark energy can be studied by its influence on the expansion of the Universe
as well as on the growth history of the large-scale structure. In this paper,
we follow the growth of the cosmic density field in early dark energy
cosmologies by combining observations of the primary CMB temperature and
polarisation power spectra at high redshift, of the CMB lensing deflection
field at intermediate redshift and of weak cosmic shear at low redshifts for
constraining the allowed amount of early dark energy. We present these
forecasts using the Fisher-matrix formalism and consider the combination of
Planck-data with the weak lensing survey of Euclid. We find that combining
these data sets gives powerful constraints on early dark energy and is able to
break degeneracies in the parameter set inherent to the various observational
channels. The derived statistical 1-sigma-bound on the early dark energy
density parameter is sigma(Omega_d^e)=0.0022 which suggests that early dark
energy models can be well examined in our approach. In addition, we derive the
dark energy figure of merit for the considered dark energy parameterisation and
comment on the applicability of the growth index to early dark energy
cosmologies.Comment: 25 pages, 14 figures, 3 tables; v2: very minor additions, updated to
match version to be published in JCA
Conjugate (solid/fluid) computational fluid dynamics analysis of the space shuttle solid rocket motor nozzle/case and case field joints
Three-dimensional, conjugate (solid/fluid) heat transfer analyses of new designs of the Solid Rocket Motor (SRM) nozzle/case and case field joints are described. The main focus was to predict the consequences of multiple rips (or debonds) in the ambient cure adhesive packed between the nozzle/case joint surfaces and the bond line between the mating field joint surfaces. The models calculate the transient temperature responses of the various materials neighboring postulated flow/leakpaths into, past, and out from the nozzle/case primary O-ring cavity and case field capture O-ring cavity. These results were used to assess if the design was failsafe (i.e., no potential O-ring erosion) and reusable (i.e., no excessive steel temperatures). The models are adaptions and extensions of the general purpose PHOENICS fluid dynamics code. A non-orthogonal coordinate system was employed and 11,592 control cells for the nozzle/case and 20,088 for the case field joints are used with non-uniform distribution. Physical properties of both fluid and solids are temperature dependent. A number of parametric studies were run for both joints with results showing temperature limits for reuse for the steel case on the nozzle joint being exceeded while the steel case temperatures for the field joint were not. O-ring temperatures for the nozzle joint predicted erosion while for the field joint they did not
Effective Symmetries of the Minimal Supermultiplet of N = 8 Extended Worldline Supersymmetry
A minimal representation of the N = 8 extended worldline supersymmetry, known
as the `ultra-multiplet', is closely related to a family of supermultiplets
with the same, E(8) chromotopology. We catalogue their effective symmetries and
find a Spin(4) x Z(2) subgroup common to them all, which explains the
particular basis used in the original construction. We specify a constrained
superfield representation of the supermultiplets in the ultra-multiplet family,
and show that such a superfield representation in fact exists for all adinkraic
supermultiplets. We also exhibit the correspondences between these
supermultiplets, their Adinkras and the E(8) root lattice bases. Finally, we
construct quadratic Lagrangians that provide the standard kinetic terms and
afford a mixing of an even number of such supermultiplets controlled by a
coupling to an external 2-form of fluxes.Comment: 13 Figure
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