1,060 research outputs found
Optimal Galaxy Distance Estimators
The statistical properties of galaxy distance estimators are studied and a
rigorous framework is developed for identifying and removing the effects of
Malmquist bias due to obsevational selection. The prescription of Schechter
(1980) for defining unbiased distance estimators is extended to more general --
and more realistic -- cases. The derivation of `optimal' unbiased distance
estimators of minimum dispersion, by utilising information from additional --
suitably correlated -- observables, is discussed and the results applied to a
calibrating sample from the Fornax cluster, as used in the Mathewson spiral
galaxy redshift survey. The optimal distance estimator derived from I-band
magnitude, diameter and 21cm line width has an intrinsic scatter which is 25 \%
smaller than that of the Tully-Fisher relation quoted for this calibrating
sample. (Figures are available on request).Comment: Plain Latex, 19 pages, Sussex-AST-93/9-
Gamma Ray Bursts: Cosmic Rulers for the High-Redshift Universe?
The desire to extend the Hubble Diagram to higher redshifts than the range of
current Type Ia Supernovae observations has prompted investigation into
spectral correlations in Gamma Ray Bursts, in the hope that standard
candle-like properties can be identified. In this paper we discuss the
potential of these new `cosmic rulers' and highlight their limitations by
investigating the constraints that current data can place on an alternative
Cosmological model in the form of Conformal Gravity. By fitting current Type 1a
Supernovae and Gamma Ray Burst (GRB) data to the predicted luminosity distance
redshift relation of both the standard Concordance Model and Conformal Gravity,
we show that currently \emph{neither} model is strongly favoured at high
redshift. The scatter in the current GRB data testifies to the further work
required if GRBs are to cement their place as effective probes of the
cosmological distance scale.Comment: 2 pages, 1 figure (black & white, colour available). To be published
in "Phil. Trans. of the Royal Society" as proceedings from Discussion Meeting
on Gamma Ray Burst
A Cautionary Note on Cosmological Magnetic Fields
This note is concerned with potentially misleading concepts in the treatment
of cosmological magnetic fields by magnetohydrodynamical (MHD) modelling. It is
not a criticism of MHD itself but rather a cautionary comment on the validity
of its use in cosmology. Now that cosmological data are greatly improved
compared with a few decades ago, and even better data are imminent, it makes
sense to revisit original modelling assumptions and examine critically their
shortcomings in respect of modern science. Specifically this article argues
that ideal MHD is a poor approximation around recombination, since it
inherently restricts evolutionary timescales, and is often misapplied in the
existing literature.Comment: 5 page
How far is it to a sudden future singularity of pressure?
We discuss the constraints coming from current observations of type Ia
supernovae on cosmological models which allow sudden future singularities of
pressure (with the scale factor and the energy density regular). We show that
such a sudden singularity may happen in the very near future (e.g. within ten
million years) and its prediction at the present moment of cosmic evolution
cannot be distinguished, with current observational data, from the prediction
given by the standard quintessence scenario of future evolution. Fortunately,
sudden future singularities are characterized by a momentary peak of infinite
tidal forces only; there is no geodesic incompletness which means that the
evolution of the universe may eventually be continued throughout until another
``more serious'' singularity such as Big-Crunch or Big-Rip.Comment: REVTEX4, 4 pages, 2 figures, references change
A New Method for Estimating the Distance to Young Open Clusters
We present a new technique for estimating the distance to young open
clusters. The method requires accurate measurement of the axial rotation period
of late-type members of the cluster: rotation periods are first combined with
projected rotation velocities and an estimate of the angular diameter for each
star -- obtained using the Barnes-Evans relation between colour and surface
brightness. A 'best' cluster distance estimate is then determined using
standard techniques from the theory of order statistics which are in common use
in the general statistics literature. It is hoped that this new method will
prove a useful adjoint to more traditional distance methods, and will provide a
better %determination of the distance scale within the solar neighbourhood.
(Figures / preprint available on request)Comment: Plain Latex version 3.1, 14 pages, Sussex-AST-93/8-1, to appear in
MNRA
Cosmological tests of sudden future singularities
We discuss combined constraints, coming from the cosmic microwave background
shift parameter , baryon acoustic oscillations (BAO) distance
parameter , and from the latest type Ia supernovae data, imposed
on cosmological models which allow sudden future singularities of pressure. We
show that due to their weakness such sudden singularities may happen in the
very near future and that at present they can mimic standard dark energy
models.Comment: 8 pages, 2 references adde
Reducing the Impact of Weak-lensing Errors on Gravitational-wave Standard Sirens
The mergers of supermassive black hole binaries (SMBHBs) can serve as
standard sirens: the gravitational wave (GW) analog of standard candles. The
upcoming space-borne GW detectors will be able to discover such systems and
estimate their luminosity distances precisely. Unfortunately, weak
gravitational lensing can induce significant errors in the measured distance of
these standard sirens at high redshift, severely limiting their usefulness as
precise distance probes. The uncertainty due to weak lensing can be reduced if
the lensing magnification of the siren can be estimated independently, a
procedure called 'delensing'. With the help of up-to-date numerical
simulations, here we investigate how much the weak-lensing errors can be
reduced using convergence maps reconstructed from shear measurements. We also
evaluate the impact of delensing on cosmological parameter estimation with
bright standard sirens. We find that the weak-lensing errors for sirens at can be reduced by about a factor of two on average, but to achieve this
would require expensive ultra-deep field observations for every siren. Such an
approach is likely to be practical in only limited cases, and the reduction in
the weak-lensing error is therefore likely to be insufficient to significantly
improve the cosmological parameter estimation. We conclude that performing
delensing corrections is unlikely to be worthwhile, in contrast to the more
positive expectations presented in previous studies. For delensing to become
more practicable and useful in the future will require significant improvements
in the resolution/depth of the weak-lensing surveys themselves and/or the
accuracy of the methods to reconstruct convergence maps from these surveys.Comment: 19 pages, 22 figures, preparing for submitting to MNRA
Characterization of a succession of small insect viruses in a wild South African population of Nudaurelia cytherea capensis (Lepidoptera: Saturniidae)
The Tetraviridae are a family of small insect RNA viruses first discovered in South Africa some 40 years ago. They consist of one or two single-stranded (+) RNAs encapsidated in an icosahedral capsid of approximately 40 nm in diameter, with T = 4 symmetry. The type members of the two genera within this family, Nudaurelia β virus (NβV) and Nudaurelia ω virus (NωV), infect Nudaurelia cytherea capensis (pine emperor moth) larvae. The absence of N. capensis laboratory colonies and tissue culture cell lines susceptible to virus infection have limited research on the biology of NβV and NωV because the availability of infectious virus is dependent upon sporadic outbreaks in the wild N. capensis populations. In September 2002, dead and dying N. capensis larvae exhibiting symptoms similar to those reported previously in other tetravirus infections were observed in a wild population in a pine forest in the Western Cape province of South Africa. We report here the isolation of three small insect viruses from this population over a period of three years. Transmission electron microscopy and serological characterization indicate that all three are tetra-like virus isolates. One isolate was shown by cDNA sequence analysis to be NβV, which was thought to have been extinct since 1985. The two other isolates are likely new tetraviruses, designated Nudaurelia ψ virus (NψV) and Nudaurelia ζ virus (NζV), which are morphologically and serologically related to NωV and NβV, respectively
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