As far as the redshift is concerned, it has been suggested
that it might be caused by light losing energy during its
long voyage through space, the tired-light theory.
Halton Arp argues that redshift is primarily a function of
age, and that tired light plays no more than a secondary
The gravitational mass of light is determined by the acceleration produced by the red shift.
see page 12 of
http://www.angelfire.com/scifi2/zpt/chapter7.html
Frank Z
