hep-ph/0112212
Effects of the Intergalactic Plasma on Supernova Dimming via Photon-Axion Oscillations
Authors: Csaba Csaki, Nemanja Kaloper, John Terning
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.