  A Fourier transform analysis of 2.5 million spectra in the SDSS survey was
carried out to detect periodic modulations contained in their intensity versus
frequency spectrum. A statistically significant signal was found for 223
galaxies while the spectra of 0.9 million galaxies were observed. A plot of the
periods as a function of redshift clearly shows that the effect is real without
any doubt, because they are quantized at two base periods that increase with
redshift in two very tight parallel linear relations. I suggest that it could
be caused by light bursts separated by times of the order of 10-13 seconds
because it was the original reason for searching for the spectral periodicity
but other causes may be possible. As another possible cause, I investigate the
hypothesis that the modulation is generated by the Fourier transform of
spectral lines, concluding that it is not valid. Although the light bursts
suggestion implies absurdly high temperatures, it is supported by the fact that
the Crab pulsar also has extremely short unresolved pulses (<0.5 nanosecond)
that also imply absurdly high temperatures. Furthermore, the radio spectrum of
the Crab pulsar also has spectral bands similar to those that have been
detected. Finally, decreasing the signal to noise threshold of detection gave
results consistent with beamed signals having a small beam divergence, as
expected from non-thermal sources that send a jet, like those seen in pulsars.
Considering that galaxy centers contain massive black holes, exotic black hole
physics may be responsible for the spectral modulation. However, at this stage,
this is only a hypothesis to be confirmed with further work.
