Astron. Astrophys. 349, 317-322 (1999)

1. Introduction

Cosmic ray showers as observed with the Super-Kamiokande observatory (Fukuda et al. 1998) have lent strong new support to the hypothesis of nonzero neutrino masses. While there is at present no consensus on the actual values of these masses (Valle 1998), various extensions of the standard model of particle physics imply neutrino rest masses (for instance) of order  0.07 eV (Glashow et al. 1998), 4 eV (Foot 1998), and even 15-100 eV (Shi & Fuller 1999). This is of great interest for cosmology, since neutrinos near the upper end of this mass range could collectively provide enough hot dark matter to close the Universe (Gawiser & Silk 1998). The most direct connection between cosmology and particle physics is via background radiation, since massive dark matter particles will in general decay, adding photons to the intergalactic radiation field, about which we have good data in various wavebands. Specifically, in Sciama's theory neutrinos with rest masses of about 30 eV decay and contribute to the extragalactic background light (EBL) at ultraviolet wavelengths (see for review Sciama 1997). In a previous study (Overduin & Wesson 1997; hereafter "OW") we showed that the strength of this neutrino decay signal was at or near upper limits on EBL intensity, especially at far ultraviolet (FUV) wavelengths. We did not include the effects of absorption by dust in that study, since preliminary work indicated that this would be of little importance compared to absorption by neutral hydrogen. Recent developments, however, have prompted us to revisit this issue. In particular, it appears that intergalactic dust may consist of smaller particles than has usually been assumed in the past (Duley & Seahra 1998 , Zubko et al. 1999 , Weingartner & Draine 1999), and these would be more efficient at absorbing short-wavelength ultraviolet radiation. There are also tighter new observational limits on the intensity of the FUV EBL, based on a re-analysis of data from the Voyager spacecraft (Murthy et al. 1999 , Henry 1999). Taken together, we believe that the Super-Kamiokande results, the change in thinking on dust, and the new observational data warrant a new look at the question of decaying neutrinos and the EBL. We will find that Sciama's theory appears inconsistent with the new Voyager numbers, but that other data do not rule it out.

© European Southern Observatory (ESO) 1999

Online publication: August 25, 1999
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