Chez11,
Is there some sort of teshuvah to this effect, with sources and argument?
I am extremely skeptical that R’Elyashiv gave any such psak. The age of the universe is not one of the ikkarim in anyone’s listing of ikkarim. At worst it is a mistaken belief of fact. Almost all geirim (and all Jews) will have some mistaken beliefs of facts. This does not make them apikorsim, it just makes them mistaken.
If those who believe that the Universe is billions of years old are correct, are then those who believe that the Universe is less than 6000 years old apikorsim?