Paper in question: Infinite Ethics. Also LW Wiki Page and a not-particularly-great Reddit thread.

Nobody seems willing to bite the bullet that in fact, if everything possible actually happens, and all parts of the universe are given equal weight, then it is the case that no choice matters. It is the intuition that there is a moral truth which is wrong, not any specific part of it.

To some extent, it boils down to "how do you justify any discount rate if the future is infinite and you weight all parts of it equally"? I think the answer is "you don't. Infinitesimal value approaches 0 for any individual choice".

Note that even in a finite universe, I think this is one the two huge problems with utilitarianism. How do you know what is the proper discount rate and timeframe for your choices? (not today's topic, but to avoid being a tease, the other is: how do you actually assign these values you're aggregating).