Corey Powell has an interesting post up on what he calls the Four Great Eras of Exploration. The first era was Galileo’s discovery of the vastness of the universe, the second that stars were composed of chemical elements, and the third was Hubble’s discovery of other galaxies. The fourth, and main topic of his post, is the current age of discovery of exoplanets.
But for this post, I’m focusing on the third:
Overnight, the Andromeda Nebula became the Andromeda Galaxy, and our home galaxy became just one of a multitude. A scant 6 years later, Hubble measured the motions of those other galaxies and discovered that they were systematically moving away from us, with their speed directly proportional to their distance. This was the discovery of the expanding universe, which led to the idea of the Big Bang, galaxy evolution, dark energy, and all the other wild concepts of modern cosmology.
I still find it mind-boggling that less than a century ago nobody even knew whether other galaxies existed. The pace of astronomical discovery is truly shocking when you step back and look at it.
I very much agree with that last paragraph. Less than a century ago, we thought our galaxy was all that was, the entire universe. Reading this post reminded me of an old archaic term for galaxies: ‘island universes’, which I recall seeing in very old science fiction stories. The term didn’t stick because ‘universe’ was then understood to mean all of reality.
Thinking about our multiverse discussions, and Tegmark’s levels of multiverse, it occurs to me that galaxies could have been considered other universes at one time, and the space they’re all in, the multiverse. This is a counter-factual of course, since that’s not where the terminology went.
But it makes me wonder what might happen if we ever actually did discover other universes as described in one or more of the multiverse theories. Would we end up calling them universes, or something else like bubble, region, brane, or whatever ended up being descriptive? Maybe the term ‘universe’ might continue to encompass all of reality.
What if our currently observable universe was part of a large structure that was just one of several such structures separated from each other by vast voids. Would those other remote structures count as other universes, or just new regions of the current one? At what point is another aspect of reality another universe instead of just a previously unknown aspect of the universe?
Depending how we spliced it, the word ‘universe’ might become like ‘world’, where it would figuratively be used to refer to all of reality, but technically mean a distinct subset of it, similar to how ‘world’ now effectively means ‘planet’.
This line of thought (admittedly largely semantic) reminds me of something cultural anthropologists are always warning us about, that much of how we perceive reality is essentially convention. Conventions which are often ultimately the result of historical accidents.
SelfAwarePatterns 
Good point, SAP, and one that occurred to me as I read Tegmark’s book too. Tegmark defines the universe to be the region of space which is in principle observable to us, so that’s just the region where light has had time to travel to us since the big bang. There are certainly other regions beyond this which Tegmark defines as outside our universe, which is where he gets the idea of a Level I multiverse.
What he describes is reasonably consistent so I have no major problem with it, but it’s not the terminology I would use. One problem I have with Level I in particular is that it’s not discrete – the different universes overlap, and in fact my universe is ever so slightly different to yours because we occupy slightly different locations. This despite the fact that you are in my universe and I am in yours. So I prefer to think of the Level I multiverse as the universe.
I think in general I like to consider “The Universe” as the region of space we could visit if we could travel back in time or faster than light. The Level II multiverse might also be included in this, although that’s debatable. Level III is not included in this, but I can see a case for calling the Level III multiverse the true universe if we adopt the perspective that reality is the evolving wavefunction of the universe and not the set of “collapsed” views of it.
I can’t see much reason to call Level IV the universe, because each mathematical object is a completely distinct reality from all others. They are not separated by mere space, they are completely and entirely separate logical entities. From our perspective universes elsewere in Level IV can reasonably be said not to exist, just as our universe can be said not to exist by observers in those. If a universe is all that exists from the perspective of its inhabitants, the multiverse is the set of possible universes.
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Interesting insights. I think I’d agree with all of them, although your point about Level I multiverses reminds me that astronomers generally consider ‘universe’ to be a synonym for the observable universe. This is probably where Guth and Tegmark are coming from.
I’ve read some physicists who’ve said that anything outside of our cosmological horizon is forever causally separated from us and therefore, as far as we’re concerned, doesn’t exist. But the idea that we can’t conclude anything beyond our observations based on the patterns that are within our observations, seems a bit too logically positivist to me.
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I think we can legitimately say that the universe beyond the observable does exist, otherwise we would see “edge effects”. We may not be causally connected to it, but we can deduce that it’s very much like the part of the universe we can see.
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It is fascinating that before Hubble, we imagined that our galaxy was all there was, and that the stars were fixed in space, uniformly distributed throughout the galaxy, unchanging and long-lived.
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