Recent measurements of the cosmic microwave background confirm that it is d
escribed by a Planckian distribution with high precision. It is non-extensi
vity bounded to be less than some parts in 10(5), or to some parts in 10(4)
at most. This deviation may appear minuscule, but may have a non-negligibl
e effect on a particle propagating through this background over the course
of millions of years. In this Letter we analyze the possible influence of s
uch a slight deviation upon the propagation of nuclei and protons of ultra-
high energy. These particles interact via photopion and photodisintegration
processes which we examine taking into account a slight non-extensive back
ground. We show that such a deviation does not exhibit a significant differ
ence in the energy attenuation length of extremely high energy cosmic rays.
(C) 2001 Elsevier Science B.V. All rights reserved.