5.5.3. Is there spatial variation in D/H towards quasars?
It seems highly likely that the D is low in the three quasars which show low D, and we discussed above why it is hard to imagine how this D could have been depleted or created since BBN. Hence we conclude that the low D/H is primordial.
Are there other places where D is high? All quasar spectra are consistent with a single low D/H value. The cases which are also consistent with high D are readily explained by the expected H contamination. We now explain why we have enough data to show that high D must be rare, if it occurs at all.
High D should be much easier to find than low D. Since we have not found any examples which are as convincing as those of low D, high D must be very rare. If D were ten times the low value, the D line would be ten times stronger for a given NHI , and could be seen in spectra with ten times lower signal to noise, or 100 times fewer photons recorded per Å. If such high D/H were common, it would have been seen many times in the high resolution, but low signal to noise, spectra taken in the 1980's, when the community was well aware of the importance of D/H. High D would also have been seen frequently in the spectra of about 100 quasars taken with the HIRES spectrograph on the Keck telescope. In these spectra, which have relatively high signal to noise, high D could be detected in absorption systems which have 0.1 of the NHI needed to detect low D. Such absorbers are about 40 - 60 times more common than those needed to show low D/H, and hence we should have found tens of excellent examples.