arXiv subjects

D. W. Sciama

Publications and source records attributed to D. W. Sciama.

15 recordsLinked to original sources

arXiv1999

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.

astro-ph↗

arXiv1999

Sonoluminescence as a QED vacuum effect: Probing Schwinger's proposal

Several years ago Schwinger proposed a physical mechanism for sonoluminescence in terms of photon production due to changes in the properties of the quantum-electrodynamic (QED) vacuum arising from a collapsing dielectric bubble. This mechanism can be re-phrased in terms of the Casimir effect and has recently been the subject of considerable controversy. The present paper probes Schwinger's suggestion in detail: Using the sudden approximation we calculate Bogolubov coefficients relating the QED vacuum in the presence of the expanded bubble to that in the presence of the collapsed bubble. In this way we derive an estimate for the spectrum and total energy emitted. We verify that in the sudden approximation there is an efficient production of photons, and further that the main contribution to this dynamic Casimir effect comes from a volume term, as per Schwinger's original calculation. However, we also demonstrate that the timescales required to implement Schwinger's original suggestion are not physically relevant to sonoluminescence. Although Schwinger was correct in his assertion that changes in the zero-point energy lead to photon production, nevertheless his original model is not appropriate for sonoluminescence. In other works (see quant-ph/9805023, quant-ph/9904013, quant-ph/9904018, quant-ph/9905034) we have developed a variant of Schwinger's model that is compatible with the physically required timescales.

quant-ph↗

arXiv1999

On the Interaction Between Cosmic Rays and Dark Matter Molecular Clouds in the Milky Way. l Basic Considerations

We explore some basic observational consequences of assuming that the dark matter in the Milky Way consists mainly of molecular clouds, and that cosmic rays can penetrate these clouds. In a favoured model of the clouds, this penetration would have the following consequences, all of which agree with observation: (i) cosmic ray nuclei would be fragmented when they enter a cloud, giving them a lifetime in the Galaxy of $10^{15}$ sec (for relativistic nuclei). (ii) Pionic $γ$ rays emitted by the clouds, after pp collisions, would have a flux in the galactic plane comparable to the flux from known sources for photon energies $> \sim 1$ Gev. (iii) The heat input into the clouds from cosmic rays would be re-radiated mainly in the far infra-red. The resulting radiation background agrees, in both intensity and spectrum in different directions, with a known excess in the far $i - r$ background of the galaxy over emission by warm dust.

astro-ph↗

arXiv1999

Sonoluminescence: Bogolubov coefficients for the QED vacuum of a time-dependent dielectric bubble

We extend Schwinger's ideas regarding sonoluminescence by explicitly calculating the Bogolubov coefficients relating the QED vacuum states associated with changes in a dielectric bubble. Sudden (non-adiabatic) changes in the refractive index lead to an efficient production of real photons with a broadband spectrum, and a high-frequency cutoff that arises from the asymptotic behaviour of the dielectric constant.

quant-ph↗

arXiv1999

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.

quant-ph↗

arXiv1998

Diffuse Ionization in the Milky Way and Sterile Neutrinos

We propose the radiative decay of sterile neutrinos which fill a fraction of the halo dark matter with a mass of 27.4 eV and lifetime of $\sim 10^{22}$ sec. as a way to explain the observed diffuse ionization in the Milky Way galaxy. Since the sterile neutrino number density in the present universe can be adjusted by arranging its dynamics appropriately, the resulting hot dark matter contribution to $Ω_m$ can be small as required by many scenarios of structure formation. On the other hand a 27.4 eV neutrino could easily be the partial halo dark matter. One realization of this idea could be in the context of a mirror universe theory where the gauge and matter content of the standard model are completely duplicated in the mirror sector (except for an asymmetry in the weak scale); the three mirror neutrinos can mix with the known neutrinos via some strongly suppressed mechanism such as the gravitational or heavy right handed neutrino mediated forces. Two of the mirror neutrinos (say $ν'_μ$ and $ν'_τ$) could play the role of the above sterile neutrino.

hep-ph↗

arXiv1998

Are Neutrino Oscillations Compatible with the Decaying Neutrino Theory for the Ionisation of Hydrogen in the Interstellar Medium?

Recent Super-Kamiokande data have greatly strengthened the hypothesis that the atmospheric neutrino anomaly is mainly due to nearly maximal oscillations between $ν_μ$ and either $ν_τ$ or a sterile neutrino $ν^s_μ$.In this letter we point out that the decaying neutrino theory for the ionisation of hydrogen in the interstellar medium, in conjunction with the assumption that the cosmological constant $λ$ is zero, requires these oscillations to be between $ν_μ$ and $ν^s_μ$. This requirement can be tested by forthcoming Super-Kamiokande measurements. A further requirement can be derived if the solar neutrino deficit is energy dependent and if both it and the LSND data are also due to neutrino oscillations. In that case the decaying neutrino theory alone (without any assumption about the value of $λ$) requires both that the atmospheric anomaly is due to $ν_μ-ν^s_μ$ oscillations and that the solar neutrino oscillations are between $ν_e$ and a second sterile neutrino $ν^s_e$. This latter requirement can be tested by forthcoming SNO and BOREXINO measurements. If these are also $ν_τ-ν^s_τ$ oscillations the decay line would be split into two lines of nearly equal intensity and spacing less than $\sim$ 0.2 eV.

astro-ph↗

arXiv1998

CMB Anisotropy in the Decaying Neutrino Cosmology

It is attractive to suppose for several astrophysical reasons that the universe has close to the critical density in light (~30 eV) neutrinos which decay radiatively with a lifetime of ~10^{23} sec. In such a cosmology the universe is reionized early and the last scattering surface of the cosmic microwave background significantly broadened. We calculate the resulting angular power spectrum of temperature fluctuations in the cosmic microwave background. As expected the acoustic peaks are significantly damped relative to the standard case. This would allow a definitive test of the decaying neutrino cosmology with the forthcoming MAP and PLANCK surveyor missions.

astro-ph↗

arXiv1998

Optical Emission Lines from Warm Interstellar Clouds - a Decisive Test of the Decaying Neutrino Theory

Recently developed instruments such as the Taurus Tunable Filter and WHAM should be able to detect some or all of the optical emission lines H$α$, [OI] $λ$6300, [SII] $λ$6717, [NI] $λ$ 5200 and [NII] $λ$6584 from warm interstellar clouds such as those observed by Spitzer $&$ Fitzpatrick (1993) (SF) along the line of sight to the halo star HD93521. The strengths of these lines should resolve the debate as to whether the free electrons, which SF held responsible for the observed excitation of CII in the clouds, are located mainly in the skins of the clouds or in their interiors. If the free electrons are indeed mainly located in the cloud interiors, then the substantial electron density derived by SF, and its constancy from cloud to cloud for the slow-moving clouds, when combined with their opacity to Lyman continuum radiation, lend strong support to the decaying neutrino theory for the ionisation of the interstellar medium (Sciama 1990, 1993 a, b, 1997). If the [OI] and [NI] lines are relatively strong but the [NII] line is weak, then this would lend further, decisive, support to this theory, since decay photons are unable to ionise N, although its ionisation potential is only 0.9 eV greater than that of H.

astro-ph↗

arXiv1998

Updated Parameters for the Decaying Neutrino Theory and EURD Observations of the Diffuse UV Background

Various recent observational developments are here used to make a more critical analysis of the parameter space of the decaying neutrino theory for the ionisation of the interstellar medium. These developments involve phenomena inside our Galaxy, outside the Galaxy but at essentially zero red shift, and at large red shifts. This new analysis leads to a viable theory with a decay lifetime of $2{\pm}1\times 10^{23}$ sec, a decay photon energy of $13.7{\pm}0.1$ eV, and a mass for the decaying neutrino of $27.4 {\pm}0.2$ eV. These parameters, when combined with some known astronomical quantities, lead to predictions for the intensity, the wavelength and the width of the decay line produced by neutrinos lying within one optical depth of the sun $({\sim} 1/2 pc)$. One finds an intensity in the line of $350^{+350}_{-117}$ photons $cm^{-2} sec^{-1}$, a wavelength of $905\pm7Å$, and a width $\sim1Å$. These predictions are relevant for the observations about to be made by the EURD ultra-violet detector which is currently in Earth orbit on board the Spanish MINISAT 01 satellite.

astro-ph↗

arXiv1998

Resonant spin-flavour precession of neutrinos and pulsar velocities

Young pulsars are known to exhibit large space velocities, up to $10^3$ km/s. We propose a new mechanism for the generation of these large velocities based on an asymmetric emission of neutrinos during the supernova explosion. The mechanism involves the resonant spin-flavour precession of neutrinos with a transition magnetic moment in the magnetic field of the supernova. The asymmetric emission of neutrinos is due to the distortion of the resonance surface by matter polarisation effects in the supernova magnetic field. The requisite values of the field strengths and neutrino parameters are estimated for various neutrino conversions caused by their Dirac or Majorana-type transition magnetic moments.

hep-ph↗

arXiv1997

Decaying Neutrinos and the Flattening of the Galactic Halo

The recently constructed Dehnen-Binney set of mass models for the Galaxy is used to show that the decaying neutrino theory for the ionisation of the interstellar medium (Sciama 1990a, 1993) requires the neutrino halo of the Galaxy to be as flattened as is observationally permitted (axial ratio q=0.2 or shape E8). The argument involves an evaluation of the contribution of red-shifted decay photons from the cosmological distribution of neutrinos to the extragalactic diffuse background at 1500 Angstroms. This contribution must be as large as is observationally permitted. These two requirements depend on the decay lifetime in potentially conflicting ways. For consistency to be achieved the lifetime must lie within 30 per cent of 10^23 seconds.

astro-ph↗

arXiv1997

An updated precision estimate of the Hubble constant and the age and density of the Universe in the decaying neutrino theory

We here update the derivation of precise values for the Hubble constant H_0, the age t_0 and the density parameter Omega*h^2 of the universe in the decaying neutrino theory for the ionisation of the interstellar medium (Sciama 1990 a, 1993). Using recent measurements of the temperature of the cosmic microwave background, of the abundances of D, He^4 and Li^7, and of the intergalactic hydrogen-ionising photon flux at zero red shift, we obtain for the density parameter of the universe Omega*h^2=0.300\pm 0.003. Observed limits on H_0 and t_0 then imply that, for a zero cosmological constant, H_0=52.5\pm 2.5 km. sec^{-1} Mpc^{-1}, t_0=12.7\pm 0.7 Gyr and Omega=1.1\pm 0.1. If Omega=1 exactly, then H_0=54.8\pm 0.3 km. sec^{-1} Mpc^{-1}, and t_0=11.96\pm 0.06 Gyr. These precise predictions of the decaying neutrino theory are compatible with current observational estimates of these quantities.

astro-ph↗

arXiv1997

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 $\sim$ 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 $\sim$ 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 $\sim$4, are proportional to the column density of HI to within $\sim$20 per cent. This proportionality is used to show that the free electrons exciting the CII to CII$^*$ 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$α$ 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.

astro-ph↗

arXiv1996

Resonant neutrino spin-flavor precession and supernova shock revival

A new mechanism of supernova shock revival is proposed, which involves resonant spin--flavor precession of neutrinos with a transition magnetic moment in the magnetic field of the supernova. The mechanism can be operative in supernovae for transition magnetic moments as small as $10^{-14}μ_B$ provided the neutrino mass squared difference is in the range $Δm^2 \sim (3 \;{\rm eV})^2-(600 \;{\rm eV})^2$. It is shown that this mechanism can increase the neutrino--induced shock reheating energy by about 60\%.

hep-ph↗