Extragalactic radiation and the ultra-high-energy cosmic-ray spectrum

The high-frequency (submillimetre) side of the 'black-body' radiation spectrum has now become available for study from balloons and the soon-to-be-launched Cosmic Background Explorer (COBE) satellite. There is recent evidence of an unexpectedly large excess of radiation beyond the peak of the black-body curve, which seems to be generally isotropic and therefore of extragalactic extent. Such radiation will have a significant effect on the propagation of ultra-high-energy cosmic rays. Because of the importance of this new discovery and the timeliness of the subject,the effect of extragalactic microwave and submillimetre-radiation fields on the ultra-high-energy cosmic-ray spectrum are examined. General characteristics of the spectrum can be derived from fairly simple analytical arguments. The various spectral features obtained by numerical calculations ('pile-up' bumps and 'dips') are explored. and the technique is illustrated with a newly derived lifetime-energy relation, based on the new submillimetre observations. (author)