What's our universal address?

If we were trying to reach out to somewhere outside of our universe, what would our overall universal address be? The closest I’ve come is this: (personal address), state. USA North America, Earth, Solar System, Global Cluster, Milky Way, Local Group, Virgo Cluster.

This is a surprisingly tricky thing to work out, but the difficulty lies in being able to locate ourselves in three dimensions on large scales more than anything else.

Your addressing scheme (as far as you’ve written it) is pretty accurate up to the Virgo supercluster, which itself is really a conglomeration of other clusters, both small and large, shown above. (This is distinct from the Virgo Cluster, which is about 65 million light years away from our local group.) But that address would really only help people if they know where to look for the Virgo supercluster. The next step up from the supercluster is a loose affiliation of other superclusters, so if you could get everyone in that supercluster affiliation to look at the right supercluster, they might be able to zoom in from there and find us using your address.

We have dealt with this problem on a much smaller scale before. Both the Pioneer and Voyager spacecrafts tried to relay our position in the unlikely event that they were ever encountered and picked up by another intelligent race of beings. The Pioneer plaque showed the location of our planet within our solar system, but it (and the Voyager Golden record) also gave a map that would be useful to someone else within our galaxy. The set of lines radiating out from a single point on the left hand side of the Pioneer plaque and the Voyager record is this map. This gives the positions and frequencies of a set of pulsars - incredibly rapidly spinning objects that beam light out into the cosmos like a lighthouse. Each pulsar has its own unique frequency - the number of times per second it flashes in our direction. Pointing out the distances and frequencies of 14 pulsars, as we’ve done here, should allow someone else, observing the same set of pulsars, to triangulate our position.

However, this can only be scaled up so far. If you’re trying to tell someone outside our universe where we are - or really, anyone sufficiently far away in our own universe - you swiftly become tangled in a different problem, which is that the more distant from our planet you would like to map out for our faraway friends, the more you travel back in time. At some “distance” from our planet, our exploration becomes one much more of time than of space. Even just the Virgo supercluster (a tiny corner of the universe) is some 110 million light years across. Undoubtably, between light leaving that side of the cluster and our receiving it, things have changed over there in the intervening 100 million years. If our visitor happened to be at a 90 degree angle to the line that connects us to the other side of the cluster we observe, they would receive light from both of us at the same time. This means that particular observer wouldn’t have the same time delay that we observe between our current time and the time that light left the other side of the cluster. They would have a different, but equal, delay on their observation of both of us. They might see a structure that looks a little different from the way we understand it, as a result.

This becomes an increasingly disruptive problem the further back in time we go. At some point, we are looking at structures and objects that no longer exist. Many things can happen in a couple billion years, and they usually do, so using those objects on our map isn’t very useful for an inter-universe traveller.

So what can we use as a universal reference frame? Unfortunately, one of the fundamentals tenets of cosmology tells us that there are no “preferred” or “special” perspectives on the universe. This means that there’s no overall zero point everyone can agree on, which makes it difficult to give directions. I’ve talked about this total lack of a universe-wise reference frame once before, and there’s no easy solution at hand.

The best thing we may be able to do right now is to create a really good map of our little local set of superclusters, and tell our visitors to stop by if they happen to see something that matches it in their travels.

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