You say multiverse, I say galaxies

(Photo credit: Wikipedia)

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.