2,046 research outputs found
The Near-Linear Regime of Gravitational Waves in Numerical Relativity
We report on a systematic study of the dynamics of gravitational waves in
full 3D numerical relativity. We find that there exists an interesting regime
in the parameter space of the wave configurations: a near-linear regime in
which the amplitude of the wave is low enough that one expects the geometric
deviation from flat spacetime to be negligible, but nevertheless where
nonlinearities can excite unstable modes of the Einstein evolution equations
causing the metric functions to evolve out of control. The implications of this
for numerical relativity are discussed.Comment: 10 pages, 2 postscript figures, revised tex
Grazing Collisions of Black Holes via the Excision of Singularities
We present the first simulations of non-headon (grazing) collisions of binary
black holes in which the black hole singularities have been excised from the
computational domain. Initially two equal mass black holes are separated a
distance and with impact parameter . Initial data are
based on superposed, boosted (velocity ) solutions of single black
holes in Kerr-Schild coordinates. Both rotating and non-rotating black holes
are considered. The excised regions containing the singularities are specified
by following the dynamics of apparent horizons. Evolutions of up to are obtained in which two initially separate apparent horizons are present
for . At that time a single enveloping apparent horizon forms,
indicating that the holes have merged. Apparent horizon area estimates suggest
gravitational radiation of about 2.6% of the total mass. The evolutions end
after a moderate amount of time because of instabilities.Comment: 2 References corrected, reference to figure update
An Axisymmetric Gravitational Collapse Code
We present a new numerical code designed to solve the Einstein field
equations for axisymmetric spacetimes. The long term goal of this project is to
construct a code that will be capable of studying many problems of interest in
axisymmetry, including gravitational collapse, critical phenomena,
investigations of cosmic censorship, and head-on black hole collisions. Our
objective here is to detail the (2+1)+1 formalism we use to arrive at the
corresponding system of equations and the numerical methods we use to solve
them. We are able to obtain stable evolution, despite the singular nature of
the coordinate system on the axis, by enforcing appropriate regularity
conditions on all variables and by adding numerical dissipation to hyperbolic
equations.Comment: 19 pages, 9 figure
Constraint-preserving boundary treatment for a harmonic formulation of the Einstein equations
We present a set of well-posed constraint-preserving boundary conditions for
a first-order in time, second-order in space, harmonic formulation of the
Einstein equations. The boundary conditions are tested using robust stability,
linear and nonlinear waves, and are found to be both less reflective and
constraint preserving than standard Sommerfeld-type boundary conditions.Comment: 18 pages, 7 figures, accepted in CQ
Sinking properties of some phytoplankton shapes and the relation of form resistance to morphological diversity of plankton â an experimental study
Form resistance (Phi) is a dimensionless number expressing how much slower or faster a particle of any form sinks in a fluid medium than the sphere of equivalent volume. Form resistance factors of PVC models of phytoplankton sinking in glycerin were measured in a large aquarium (0.6 x 0.6 x 0.95 m). For cylindrical forms, a positive relationship was found between Phi and length/ width ratio. Coiling decreased Phi in filamentous forms. Form resistance of Asterionella colonies increased from single cells up to 6-celled colonies than remained nearly constant. For Fragilaria crotonensis chains, no such upper limit to Phi was observed in chains of up to 20 cells ( longer ones were not measured). The effect of symmetry on Phi was tested in 1 - 6-celled Asterionella colonies, having variable angles between the cells, and in Tetrastrum staurogeniaeforme coenobia, having different spine arrangements. In all cases, symmetric forms had considerably higher form resistance than asymmetric ones. However, for Pediastrum coenobia with symmetric/asymmetric fenestration, no difference was observed with respect to symmetry. Increasing number and length of spines on Tetrastrum coenobia substantially increased Phi. For a series of Staurastrum forms, a significant positive correlation was found between arm-length/cell-width ratio and Phi: protuberances increased form resistance. Flagellates (Rhodomonas, Gymnodinium) had a Phi 1. The highest value ( Phi = 8.1) was established for a 20-celled Fragilaria crotonensis chain. Possible origin of the so-called 'vital component' ( a factor that shows how much slower viable populations sink than morphologically similar senescent or dead ones) is discussed, as is the role of form resistance in evolution of high diversity of plankton morphologies
Ammonium regeneration: Its contribution to phytoplankton nitrogen requirements in a eutrophic environment
Ammonium regeneration, nutrient uptake, bacterial activity and primary production were measured from March to August 1980 in Bedford Basin, Nova Scotia, Canada, a eutrophic environment. Rates of regeneration and nutrient uptake were determined using 15N isotope dilution and tracer methodology. Although primary production, nutrient uptake and ammonium regeneration were significantly intercorrelated, no relationship was detected between these parameters and heterotrophic activity. The average contribution of ammonium to total nitrogen (ammonium+nitrate) uptake was similar in the spring and in the summer (approximately 60%). On a seasonal average basis, 36% of the phytoplankton ammonium uptake could be supplied by rapid remineralization processes. In spite of the high average contribution of NH4 regeneration to phytoplankton ammonia uptake, there is indirect evidence suggesting that other NH4 sources may occasionally be important
Dynamics of Gravitational Waves in 3D: Formulations, Methods, and Tests
The dynamics of gravitational waves is investigated in full 3+1 dimensional
numerical relativity, emphasizing the difficulties that one might encounter in
numerical evolutions, particularly those arising from non-linearities and gauge
degrees of freedom. Using gravitational waves with amplitudes low enough that
one has a good understanding of the physics involved, but large enough to
enable non-linear effects to emerge, we study the coupling between numerical
errors, coordinate effects, and the nonlinearities of the theory. We discuss
the various strategies used in identifying specific features of the evolution.
We show the importance of the flexibility of being able to use different
numerical schemes, different slicing conditions, different formulations of the
Einstein equations (standard ADM vs. first order hyperbolic), and different
sets of equations (linearized vs. full Einstein equations). A non-linear scalar
field equation is presented which captures some properties of the full Einstein
equations, and has been useful in our understanding of the coupling between
finite differencing errors and non-linearites. We present a set of monitoring
devices which have been crucial in our studying of the waves, including Riemann
invariants, pseudo-energy momentum tensor, hamiltonian constraint violation,
and fourier spectrum analysis.Comment: 34 pages, 14 figure
Variability approaching the thermal limits can drive diatom community dynamics
Organismal distributions are largely mediated by temperature, suggesting thermal trait variability plays a key role in defining species\u27 niches. We employed a traitâbased approach to better understand how interâ and intraspecific thermal trait variability could explain diatom community dynamics using 24 strains from 5 species in the diatom genusSkeletonema, isolated from Narragansett Bay (NBay), where this genus can comprise up to 99% of the microplankton. Strainâspecific thermal reaction norms were generated using growth rates obtained at temperatures ranging from â2°C to 36°C. Comparison of thermal reaction norms revealed interâ and intraspecific similarities in the thermal optima, but significant differences approaching the thermal limits. Cellular elemental composition was determined for two thermally differentiated species and again, the most variation occurred approaching the thermal limits. To determine the potential impact of interspecific variability on community composition, a species succession model was formulated utilizing each species\u27 empirically determined thermal reaction norm and historical temperature data from NBay. Seasonal succession in the modeled community resembled the timing of species occurrence in the field, but not species\u27 relative abundance. The model correctly predicted the timing of the dominant winterâspring species, Skeletonema marinoi, within 0â14âd of its observed peak occurrence in the field. Interspecific variability approaching the thermal limits provides an alternative mechanism for temporal diatom succession, leads to altered cellular elemental composition, and thus has the potential to influence carbon flux and nutrient cycling, suggesting that growth approaching the thermal limits be incorporated into both empirical and modeling efforts in the future
Nonquantum Gravity
One of the great challenges for 21st century physics is to quantize gravity
and generate a theory that will unify gravity with the other three fundamental
forces of nature. This paper takes the (heretical) point of view that gravity
may be an inherently classical, i.e., nonquantum, phenomenon and investigates
the experimental consequences of such a model. At present there is no
experimental evidence of the quantum nature of gravity and the liklihood of
definitive tests in the future is not at all certain. If gravity is, indeed, a
nonquantum phenomenon, then it is suggested that evidence will most likely
appear at mesoscopic scales.Comment: essentially the same as the version that appears in Foundations of
Physics, 39, 331 (2009
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