Cosmological redshift
The first issue to be addressed is Hubble's law of redshifts. We assume a model in which Hubble's law is due to absorption of galactic light by the interstellar and intergalactic matter and not due to a Doppler effect. To investigate the underlying mechanisms, we can write Bouguer's law (Bouguer 1729; Mach 1926; Curtis 1978) as applied to the energy of a photon as:
(1) E(r) = [E.sub.0][e.sup.-[[alpha].sub.L]r]
where [E.sub.0] is the initial photon energy, and [[alpha].sub.L] is the mean absorption coefficient of light in the line of sight connecting the source and the earth. Using the Einstein relation (Einstein 1905) namely, E = hv = hc/[lambda], we can obtain a law of redshifts from Eq. (1):
(2) z(r) [equivalent to] [[lambda](r) - [[lambda].sub.0]/[[lambda].sub.0] = [e.sup.-[[alpha].sub.L]r] - 1 [congruent to] [[alpha].sub.L]r]
In this equation z is the fractional spectral shift, [[lambda].sub.0] is the wavelength emitted by the source, and [lambda](r) is the wavelength detected at the Earth. Since we know that z [congruent to] [H.sub.0]r/c where [H.sub.0] is Hubble's constant, Eq. (2) yields [[alpha].sub.L] = [H.sub.0]/c. This is essentially the tired-light model. We will not present any new theory to explain this tired-light behaviour; we will merely review the main models which result in a derivation of Eq. (1).
The first to propose such an idea was Zwicky (1929), and later Hubble himself discussed this hypothesis with an open mind (Hubble and Tolman 1935; Hubble 1936a, b and c, 1937 and 1953). The main obstacle to acceptance of the tired-light model has always been the mechanism behind the loss of energy. On the other hand, as has been pointed out clearly by G. Reber (1986), the main reason for adopting the hypothesis of a Doppler effect as the cause of redshifts has been the assumption that intergalactic space is a void and that nothing happens to light in its journey from a galaxy to the Earth. Nowadays we know this is not the case: interstellar and intergalactic space is full of cosmic rays, clouds of dust, etc. .
