304 research outputs found
Does the Fine Structure Constant Really Vary in Time?
We discuss how laboratory experiments can be used to place constraints on
possible variations of the fine structure constant alpha in the observationally
relevant redshift interval z ~= 0 - 5, within a rather general theoretical
framework. We find a worst case upper limit for Delta alpha / alpha of 8 x
10^-6 for z <= 5 and Delta alpha / alpha of 0.9 x 10^-6 for z <= 1.6. The
derived limits are at variance with the recent findings by Webb et al. (1998),
who claim an observed variation of Delta alpha/alpha = -2.6 +- 0.4 x 10^-5 at
1<z<1.6.Comment: 11 pages, 2 figures, ApJL in pres
On the Ionisation of Warm Opaque Interstellar Clouds and the Intercloud Medium
In this paper we use a number of observations to construct an integrated
picture of the ionisation in the interiors of quiescent warm opaque
interstellar clouds and in the intercloud medium (ICM) outside dense HII
regions and hot dilute bubbles. Our main conclusion is that within 1kpc
of the sun the ionisation rate of hydrogen per unit volume in both the
interiors of such clouds and in the ICM is independent of the local density of
neutral hydrogen, and varies with position by less than 20 per cent.
These conclusions strongly favour the decaying neutrino hypothesis for the
ionisation of the interstellar medium in these regions.
Our analysis is based on a variety of observations, of which the most
remarkable is the discovery by Spitzer and Fitzpatrick (1993) that, in the four
slowly moving clouds along the line of sight to the halo star HD93521, the
column densities of both SII and CII, which individually range over a
factor 4, are proportional to the column density of HI to within 20
per cent. This proportionality is used to show that the free electrons exciting
the CII to CII are located mainly in the interiors of the clouds, rather
than in their skins, despite the large opacity of the clouds to Lyman continuum
radiation. The same conclusion also follows more unambiguously from the low
value of the H flux in this direction which was found by Reynolds
(1996) in unpublished observations.
These results are then used, in conjunction with observations of three pulsar
parallaxes and dispersion measures, and with data on HeI, NII and OI line
emissions, to constrain the ionisation of H, He, N and O and the flux of Lyman
continuum photons from O stars in the ICM.Comment: 16 pages, no figures, Latex fil
On the Interaction Between Cosmic Rays and Dark Matter Molecular Clouds - II. The Age Distribution of Cosmic Ray Electrons
We explore further the proposal in paper I of this series that the
confinement time of cosmic ray nuclei in the Milky Way is determined by their
interaction with dark matter molecular clouds rather than by their escape from
the halo, as is assumed in conventional models of cosmic ray propagation. The
same proposal can be made for cosmic ray electrons. This proposal leads to a
specific age distribution for the electrons which is in agreement with Tang's
(1984) observations of the electron spectrum at high energies but not with
Nishimura et al's (1980) earlier data, which lead to a flatter spectrum.
However, the simplest leaky box and diffusion models disagree with both sets of
data so that our trapping model is supported if Tang's data are correct.Comment: 7 pages, no figures, using MNRAS latex styl
Sonoluminescence and the QED vacuum
In this talk I shall describe an extension of the quantum-vacuum approach to
sonoluminescence proposed several years ago by J.Schwinger. We shall first
consider a model calculation based on Bogolubov coefficients relating the QED
vacuum in the presence of an expanded bubble to that in the presence of a
collapsed bubble. In this way we shall derive an estimate for the spectrum and
total energy emitted. This latter will be shown to be proportional to the
volume of space over which the refractive index changes, as Schwinger
predicted. After this preliminary check we shall deal with the physical
constraints that any viable dynamical model for SL has to satisfy in order to
fit the experimental data. We shall emphasize the importance of the timescale
of the change in refractive index. This discussion will led us to propose a
somewhat different version of dynamical Casimir effect in which the change in
volume of the bubble is no longer the only source for the change in the
refractive index.Comment: 15 pages, 1 figure, uses sprocl.sty. Talk at the 4th Workshop on
Quantum Field Theory Under the Influence of External Conditions, Leipzig,
14-18 September, 199
Evidence Against the Sciama Model of Radiative Decay of Massive Neutrinos
We report on spectral observations of the night sky in the band around 900
angstroms where the emission line in the Sciama model of radiatively decaying
massive neutrinos would be present. The data were obtained with a high
resolution, high sensitivity spectrometer flown on the Spanish MINISAT
satellite. The observed emission is far less intense than that expected in the
Sciama model.Comment: 9 pages, accepted to Ap
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