  We have recently proposed a mechanism of photon-axion oscillations as a way
of rendering supernovae dimmer without cosmic acceleration. Subsequently, it
has been argued that the intergalactic plasma may interfere adversely with this
mechanism by rendering the oscillations energy dependent. Here we show that
this energy dependence is extremely sensitive to the precise value of the free
electron density in the Universe. Decreasing the electron density by only a
factor of 4 is already sufficient to bring the energy dependence within the
experimental bounds. Models of the intergalactic medium show that for redshifts
z<1 about 97% of the total volume of space is filled with regions of density
significantly lower than the average density. From these models we estimate
that the average electron density in most of space is lower by at least a
factor of 15 compared to the estimate based on one half of all baryons being
uniformly distributed and ionized. Therefore the energy dependence of the
photon-axion oscillations is consistent with experiment, and the oscillation
model remains a viable alternative to the accelerating Universe for explaining
the supernova observations. Furthermore, the electron density does give rise to
a sufficiently large plasma frequency which cuts off the photon-axion mixing
above microwave frequencies, shielding the cosmic microwave photons from axion
conversions and significantly relaxing the lower bounds on the axion mass
implied by the oscillation model.
