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Astron. Astrophys. 335, 12-18 (1998)

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1. Introduction

A variety of recent observational developments, including Hipparcos data and the probable discovery of the HeII Gunn-Peterson effect (Zheng et al 1998), make it desirable to update the parameters of the decaying neutrino theory for the ionisation of hydrogen in warm opaque regions of the interstellar medium (Sciama 1990a, 1995, 1997a). In particular we need to prepare for the forthcoming observations of the extraterrestrial diffuse background at wavelengths below [FORMULA] to be made by the EURD detector (Bowyer et al 1995, Morales et al 1997, Bowyer, Edelstein & Lampton 1997) on board the Spanish MINISAT 01 satellite. This satellite was successfully launched on April 21 1997, and at the time of writing the detector is working well. One of its tasks is to search for a decay line emitted by neutrinos within half a parsec of the sun.

The decaying neutrino theory makes a number of specific predictions for a variety of phenomena within individual galaxies,in intergalactic space at red shifts in the range 0 to 5, and in the early universe at red shifts between 5 and 1000. Two recent successes may be noted here. The first is the observational verification of its prediction that the density of free electrons in the interiors of warm opaque interstellar clouds near the sun should be (a) substantial ([FORMULA] cm-3) and (b) the same in each cloud (Spitzer & Fitzpatrick 1993, Sciama 1997a).

The second success concerns its rather precise predictions (Sciama 1997b) that the Hubble constant should be [FORMULA] km sec-1 Mpc-1 and that the age of the universe should be [FORMULA] Gyr. While the actual values of these quantities remain controversial, it is noticeable that there has been a general tendency recently for the higher estimates of both these quantities to be significantly reduced. Moreover a number of these recent estimates are in good agreement with our predictions, although the observational uncertainties are still in the range 10 to 20 [FORMULA], rather than our uncertainty of [FORMULA].

The decaying neutrino theory is based on three parameters, namely, the rest mass [FORMULA] of the decaying neutrino, its radiative lifetime [FORMULA], and the monochromatic energy [FORMULA] of the decay photon in the rest frame of its parent neutrino. For simplicity we follow popular (but unproved) particle physics models (such as the see-saw model (Yanagida 1978, Gell-Mann, Ramond & Slansky 1979)) in which the secondary neutrino in the decay has a much smaller mass than [FORMULA]. In that case [FORMULA], and there are only two parameters to be determined, namely, [FORMULA] and [FORMULA].

In Sect. 2 [FORMULA] is determined from the observed ionisation in the interstellar medium and from observational limits on the diffuse extragalactic background at [FORMULA]. Its value is found to be [FORMULA] x [FORMULA] sec. In Sect. 3 [FORMULA] is determined from upper limits on the extragalactic hydrogen-ionising background at various redshifts to be [FORMULA] eV. Finally Sect. 4 discusses the implications of these results for the forthcoming observations by EURD. In particular, with the help of additional astronomical parameters, estimates are given for the intensity, the wavelength, and the width of the decay line postulated to be emitted by neutrinos in the vicinity [FORMULA] of the sun.

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© European Southern Observatory (ESO) 1998

Online publication: June 12, 1998