The reversibility of classical mechanics is broken by the Carnot princ
iple; in quantum mechanics, the time inversion invariance is broken by
the principle of state decoherence. In both cases, a greater complexi
ty produces a faster irreversibility. These principles are applied to
problems related to absorption and diffusion of light. It appears that
Raman scattering in extremely low-pressure gases shifts the frequency
of the incident light, is space coherent and thus may be confused wit
h the Doppler effect: the Raman scattering produces at least a part of
the galactic redshift; the expansion of the universe is reduced or po
ssibly cancelled.