Recent developments in cosmology indicate that every history having a nonzero probability is realized in infinitely many distinct regions of spacetime. Thus, it appears that the universe contains infinitely many civilizations exactly like our own, as well as infinitely many civilizations that differ from our own in any way permitted by physical laws. We explore the implications of this conclusion for ethical theory and for the doomsday argument. In the infinite universe, we find that the doomsday argument applies only to effects which change the average lifetime of all civilizations, and not those which affect our civilization alone.
It seems if you count all possible universes (or call them parts of our multiverse, whatever) as normatively relevant, none of your actions matter in consequentialist terms.
As to how our world, and our decisions, matter at the margin, we delve into the murky waters of infinite expected values. With an infinity of alternatives out there, our little add-on doesn’t seem to make any difference for the grand total. Why should even you raise the average outcome across universes? (TC yesterday: "No, Bryan, we are not leaping up Cantorian levels of infinity, it is just one version of you getting another Klondike bar.")
One option is that only our universe, or some other "in-group," matters. The other universes cannot count for less, rather they must count for nothing. I recoil at such a thought, but it does avoid the mess of infinities. Alternatively, we might embrace some version of Buddhism.
On the bright side, philosophic talk about modality is no longer so problematic but rather refers to facts about other existing universes. Since that problem threatened to bring morality to its knees anyway ("what do you mean, you "could" have done something different? You did what you had to do."), maybe I don’t feel so bad after all. And who should care if I do feel bad? The other me feels fine. Infinity has its benefits, and there are many worse problems.
You should lower your probability that God exists, since the Anthropic Argument will dispense with the Argument from Design. Only the ordered pockets of the multiverse can wonder about why we are here and why things seem to run so smoothly.
That’s a lot to swallow in one day, but it seems the probability of all those propositions just went up.
Addendum: Have I mentioned that inflationary cosmology and its implications fit my crude, pathetic intuitions? Since we have a universe, I feel it must somehow be a kind of cosmic "free lunch." And once you open the door for free lunches, why stop at just one? There is no good reason to rely on our locally-evolved common sense intuitions when doing philosophic cosmology.