Sentence:  So the issue of unification only comes up when the geometry can no longer be treated as a mere background, or <WSD>conversely</WSD>, when the classical theory is no longer accurate. But these circumstances exist (as far as we know) in only two places: the earliest moments of the Big Bang, and the immediate vicinity of singularities hidden behind black hole event horizons. 
Meaning of the word marked with <WSD> tags:  From another point of view; on the other hand.