657 research outputs found
Rigid motions in Einstein spaces
Rigid motion in Einstein space-time using dyadic formulation of general relativit
The implications of precise timekeeping of Doppler gravitational wave observations
Gravitational radiation from galactic and extragalactic astrophysical sources will induce spatial strains in the solar system, strains which can be measured directly by the Doppler radio link to distant spacecraft. Current noise sources in Pioneer and Voyager Doppler data are delineated and a comparison is made with expected signal levels from gravitational wave sources. The main conclusion is that it is possible to detect gravitational radiation with current DSN hydrogen maser systems stable in fractional frequency + or - 2 x 10 to the minus 14th power over 1000 sec. In the future, however, a serious Doppler observational program in gravitational wave astronomy will require frequency systems stable to at least 10 to the minus 16th power, but at the same time the current single frequency S-band uplink transmission will have to be replaced by a dual frequency capability
Numerical Ecology
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/119126/1/ecy1982632593.pd
Vicariance or dispersal: the use of natural historical data to test competing hypotheses of disjunction on the Tyrrhenian coast
To illustrate the use of natural historical data to evaluate vicariance and dispersal as hypotheses competing to explain disjunct populations. Location Nine disjunct areas on the margin of the Tyrrhenian basin of the Mediterranean Sea. Methods First describe how each hypothesized mechanism might explain the observed morphological variation in the model species complex, Genista ephedroides (Fabaceae); then confront the hypotheses with natural historical data including geology, oxygen isotopes, palynology, macro-, micro- and nano-fossils, and sea level changes, and with the ecological tolerances of the model species complex. Results Dispersal seems the more credible explanation. Main conclusion Patterns of morphological (or other) variation among related disjunct taxa can fit both vicariance and dispersal hypotheses. However they can possibly be distinguished by considering natural historical data.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/72200/1/j.1365-2699.2001.00533.x.pd
SyZyGy: A Straight Interferometric Spacecraft System for Gravity Wave Observations
We apply TDI, unfolding the general triangular configuration, to the special
case of a linear array of three spacecraft. We show that such an array
("SyZyGy") has, compared with an equilateral triangle GW detector of the same
scale, degraded (but non-zero) sensitivity at low-frequencies (f<<c/(arrany
size)) but similar peak and high-frequency sensitivities to GWs. Sensitivity
curves are presented for SyZyGys having various arm-lengths. A number of
technical simplifications result from the linear configuration. These include
only one faceted (e.g., cubical) proof mass per spacecraft, intra-spacecraft
laser metrology needed only at the central spacecraft, placement in a single
appropriate orbit can reduce Doppler drifts so that no laser beam modulation is
required for ultra-stable oscillator noise calibration, and little or no
time-dependent articulation of the telescopes to maintain pointing. Because
SyZyGy's sensitivity falls off more sharply at low frequency than that of an
equilateral triangular array, it may be more useful for GW observations in the
band between those of ground-based interferometers (10-2000 Hz) and LISA (.1
mHz-.1 Hz). A SyZyGy with ~1 light- second scale could, for the same
instrumental assumptions as LISA, make obseervations in this intermediate
frequency GW band with 5 sigma sensitivity to sinusoidal waves of ~2.5 x 10^-23
in a year's integration.Comment: 13 pages, 6 figures; typos corrected, figure modified, references
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A Relativistic Description of Gentry's New Redshift Interpretation
We obtain a new expression of the Friedmann-Robertson-Walker metric, which is
an analogue of a static chart of the de Sitter space-time. The reduced metric
contains two functions, and , which are interpreted as,
respectively, the mass function and the gravitational potential. We find that,
near the coordinate origin, the reduced metric can be approximated in a static
form and that the approximated metric function, , satisfies the
Poisson equation. Moreover, when the model parameters of the
Friedmann-Robertson-Walker metric are suitably chosen, the approximated metric
coincides with exact solutions of the Einstein equation with the perfect fluid
matter. We then solve the radial geodesics on the approximated space-time to
obtain the distance-redshift relation of geodesic sources observed by the
comoving observer at the origin. We find that the redshift is expressed in
terms of a peculiar velocity of the source and the metric function, ,
evaluated at the source position, and one may think that this is a new
interpretation of {\it Gentry's new redshift interpretation}.Comment: 11 pages. Submitted to Modern Physics Letters
Hyperbolic Equations for Vacuum Gravity Using Special Orthonormal Frames
By adopting Nester's higher dimensional special orthonormal frames (HSOF) the
tetrad equations for vacuum gravity are put into first order symmetric
hyperbolic (FOSH) form with constant coefficients, independent of any time
slicing or coordinate specialization.Comment: 14 pages, 3 figures, LaTeX, 13 macros. CQG 14 (1997) 1237-1247 has
algebraic errors. +/- signs in Equations (2), (4) and (5) are here corrected,
and factors of 2 added to Eqs. (18) and (19
A Test Of Several Hypotheses For The Determination Of Seed Number In Amelanchier Arborea, Using Simulated Probability Distributions To Evaluate Data
Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/141472/1/ajb208792.pd
Trait-based diversification shifts reflect differential extinction among fossil taxa
Evolution provides many cases of apparent shifts in diversification associated with particular anatomical traits. Three general models connect these patterns to anatomical evolution: (i) elevated net extinction of taxa bearing particular traits, (ii) elevated net speciation of taxa bearing particular traits, and (iii) elevated evolvability expanding the range of anatomies available to some species. Traitbased diversification shifts predict elevated hierarchical stratigraphic compatibility (i.e., primitive→derived→highly derived sequences) among pairs of anatomical characters. The three specific models further predict (i) early loss of diversity for taxa retaining primitive conditions (elevated net extinction), (ii) increased diversification among later members of a clade (elevated net speciation), and (iii) increased disparity among later members in a clade (elevated evolvability). Analyses of 319 anatomical and stratigraphic datasets for fossil species and genera show that hierarchical stratigraphic compatibility exceeds the expectations of trait-independent diversification in the vast majority of cases, which was expected if traitdependent diversification shifts are common. Excess hierarchical stratigraphic compatibility correlates with early loss of diversity for groups retaining primitive conditions rather than delayed bursts of diversity or disparity across entire clades. Cambrian clades (predominantly trilobites) alone fit null expectations well. However, it is not clearwhether evolution was unusual among Cambrian taxa or only early trilobites. At least among post-Cambrian taxa, these results implicate models, such as competition and extinction selectivity/resistance, as major drivers of trait-based diversification shifts at the species and genus levels while contradicting the predictions of elevated net speciation and elevated evolvability models
Trait-based diversification shifts reflect differential extinction among fossil taxa
Evolution provides many cases of apparent shifts in diversification associated with particular anatomical traits. Three general models connect these patterns to anatomical evolution: (i) elevated net extinction of taxa bearing particular traits, (ii) elevated net speciation of taxa bearing particular traits, and (iii) elevated evolvability expanding the range of anatomies available to some species. Traitbased diversification shifts predict elevated hierarchical stratigraphic compatibility (i.e., primitive→derived→highly derived sequences) among pairs of anatomical characters. The three specific models further predict (i) early loss of diversity for taxa retaining primitive conditions (elevated net extinction), (ii) increased diversification among later members of a clade (elevated net speciation), and (iii) increased disparity among later members in a clade (elevated evolvability). Analyses of 319 anatomical and stratigraphic datasets for fossil species and genera show that hierarchical stratigraphic compatibility exceeds the expectations of trait-independent diversification in the vast majority of cases, which was expected if traitdependent diversification shifts are common. Excess hierarchical stratigraphic compatibility correlates with early loss of diversity for groups retaining primitive conditions rather than delayed bursts of diversity or disparity across entire clades. Cambrian clades (predominantly trilobites) alone fit null expectations well. However, it is not clearwhether evolution was unusual among Cambrian taxa or only early trilobites. At least among post-Cambrian taxa, these results implicate models, such as competition and extinction selectivity/resistance, as major drivers of trait-based diversification shifts at the species and genus levels while contradicting the predictions of elevated net speciation and elevated evolvability models
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