To clarify what I meant by "past uncertainty" and tie it into this thread title, "Which universe are we in", it is necessary to adopt a certain view towards instantiations of consciousness.
If my mind, as a physical or computational system, is instantiated in multiple places, whether multiple universes, multiple branches of a many-worlds interpretation (MWI), or even multiple places and times in one universe, the question is how that appears to me from the first-person perspective. I am adopting a position that from my point of view, it is indeterminate which of those instantiations I am now experiencing. There is no "fact of the matter" as to which one is me, now. The alternative is to say that although all of these instantiations are in some sense indistinguishable, nevertheless the instances of consciousness produced by these systems are all distinct. That is, for each instance of consciousness, there is a single physical system which creates that consciousness. All of the physical systems are similar, or even locally identical, so that the consciousness instances produced are all structurally the same. But nevertheless we would not say that there is one consciousness which spans all the implementations; rather, there are multiple consciousnesses which merely "look alike from the inside". Anyway, that is the opposite of the view I am taking for the purposes of this discussion. I am assuming the former position, that my present consciousness is being instantiated widely throughout the multiverse and I can with equal justification think that I am experiencing any of those instantiations. My consciousness, in that sense, spans many parts of the multiverse, and the question of "which universe am I in" has no unique answer. That is how we can reconcile the notion of a cleanly determined "third person" past with an indeterminate "first person" past. Each universe has a uniquely defined past history (we will stipulate), but since it is ambiguous which universe my mentality is currently occupying, there is also ambiguity about which past is mine. We might note that in the MWI we also have a somewhat similar effect with regard to the future. In the MWI, the universe is completely deterministic. There is no randomness. Yet we are able to reconcile a deterministic "third person" view with an indeterminate, random, "first person" perspective. It is quite possible to have objective determinism while having subjective indeterminism, as long as "identity" is sufficiently slippery. And note too that as we move into the future, although we face subjective indeterminism, we will still have a consistent consensus reality. The events of the quantum future are undefined for us, but we know that we will all experience consistent versions of those events. In the same way, subjective indeterminism in the past still allows for consistent consensus reality. As we gain information about the past, we will all learn consistent information. The way this works, in both the future and the past, is that as we gain this information, there is a subjective "split" and different versions of ourselves will gain different sets of information. But each group of observers which is communicating together will see a consistent view. The future splitting is that which is described by the QM wave function, the formation of a relative state. The past splitting is different and relies on the speculative philosophical interpretation I described above; as we learn about the past, mind instances which were formerly indistinguishable become distinct. Hal Finney