Are We Living In An Island Universe? | Unveiled

Are We Living in an Island Universe?

It’s human nature to look up at the night sky and wonder what could be happening in space. We’ve been doing it, in some capacity, for thousands of years. But it’s only really in the last one hundred years or so that we’ve started to truly accelerate our knowledge. And our understanding of the cosmos has changed immeasurably in just a few decades. So much so that now we can revisit some terms of the past… and perhaps find new uses for them.

So, this is Unveiled, and today we’re answering the extraordinary question: are we living in an island universe?

The words island universe may well be applied to various contemporary, cosmological theories, of which we’ll get to in the second half of this video. But, first, it’s a turn of phrase that does have prior history in astronomical circles. As far back as in the 1920s, leading minds were discussing island universes as they tried to make sense of space - its size and scope. It’s just that, back then, the islands in question were what we now understand to be galaxies.

The Great Debate, as it’s popularly known, took place in Washington DC on April 26th, 1920, and it’s remembered as arguably one of the most significant moments in the history of astronomy. The debate itself primarily featured two astronomers promoting opposing points of view. First there was Harlow Shapley, then of the Mount Wilson Observatory in Los Angeles, who sought to argue that the Milky Way and the universe were one and the same thing. That is, that everything in the night sky was also contained within the Milky Way. At the time, Shapley and the rest of the scientific world were well aware of other major structures, such as Andromeda… but were they to be separate from the Milky Way (as we now know they are), then Shapley knew that there’d need to be some truly mind-boggling distances between us and them - which he just couldn’t entertain.

His opponent was another American astronomer, Heber Curtis, who was made head of the Allegheny Observatory at the University of Pittsburgh, also in 1920, the same year as the debate. Curtis was more open to there being huge distances at play in space, and so he put forward an alternative model, and one that’s now almost unanimously accepted. He argued that Andromeda (and others like it) weren’t in fact part of the Milky Way, but that they were galaxies in their own right. Only, Curtis didn’t yet use the term galaxies, and instead he referred to island universes. So, already we have one answer to today’s question… because if by “island universes” we mean what contemporary science refers to as “galaxies”, then yes we live in an island universe. We live in the Milky Way, which we know is a galaxy, which was at one time thought of as an island universe… and so, the answer is quite straightforward. And, when we consider that the latest estimates on the number of galaxies in space reach as high as there being two trillion of them… that’s a lot of islands that we’re just one of!

But, of course, science has moved on since the days of the Great Debate. And, as much as that one discussion helped to shape all that came afterwards, it might be argued that the very notion of island universes can today take on new meaning. Naturally, whenever we refer to the universe in modern conversation, we mean everything in existence. And, traditionally, as is indicated by the “uni” prefix to the word itself, we’ve thought of the universe as being the one and only. The whole of reality inside one massive, potentially infinite, structure. But modern talk of a multiverse may have put paid to all that, and here’s where the universe as an island begins to make sense once again.

Today, the multiverse is so much more than just one idea, and not every iteration of it does produce island-like, universal structures. Of the physicist Brian Greene’s much-cited nine types of multiverse, for example, the Simulated Multiverse is built into an incredibly complex computer system, no islands required! Meanwhile, the Quantum Multiverse is more concerned with decisions and splits made at the quantum level, arguing that it’s these which set off parallel worlds that are beyond our reach. But others of Greene’s nine types do lend themselves to visualising this universe (and others) as islands. Including the Inflationary and Landscape multiverses.

While both these models differ significantly in many ways, they’re linked by a broad proposal that whole universes could live and die across a wider structure of false vacuums. In multiverse models like these, space is expanding and (during its expansion) it isn’t always consistent. It becomes scattered with regions or bubbles of spacetime, and these bubbles may differ from one another, with suggestions that they could retain alternate fundamental properties to our own. Gravity might work differently in one, there may be changes to the elementary particles in another, and so on. No multiverse model has yet been proven beyond doubt but increasing numbers of scientists suggest that whenever we look deep enough at our physical reality… a multiverse is what we invariably arrive at. It’s as though, in modern science, the existence of the multiverse has become the ultimate problem-solver, revealing to us why things are as they are in this universe… because there’s all kinds of other possibilities for reality in all those other universes.

At present, the term “island universes” hasn’t been widely applied to a multiverse model. But it might be argued that there are similarities between Heber Curtis’ early understanding of galaxies during the Great Debate in 1920, and our recent attempts to picture the multiverse. The bubbles that feature across so many multiverse models may reasonably double up as islands again, as we begin to consider whether even something as seemingly massive and profound as the universe could in fact be one of many.

It’s no secret that when it comes to space travel, human beings are still at an extremely early stage. Our knowledge of space has certainly grown, and quickly, in the last century or so, but only a tiny percentage of us has ever left planet Earth, and the furthest anyone has reached is the moon. Often, then, the scale of just this universe can feel daunting enough… what with its 13.8-billion-year age, and the 93-billion lightyear diameter of just the observable part of it. But now we’re proposing that when we think of the universe, it could be as though we’re on the islands Bali, or Barbados, Crete, or Madagascar, and looking out across the ocean. The next landmass, or in this case universe, could be close to or far away from us, relatively speaking… but, in the meantime, could we ever hope to pay it a visit? To set sail across space and disembark somewhere new? To island hop across the cosmos? The answer right now is a resounding no.

Clearly, we’ve yet to master (or even begin) long distance space travel in just this universe. If (and when) we ever get to Mars, it’ll surely stand as one of humanity’s greatest ever achievements… but that in itself will also go to show just how small an impact we’ve so far made on deep space. To embark on an interstellar voyage, we’d need to be aiming for another star system… and a journey that’s hundreds of times longer than even the currently impossible trip to Mars will be. And then, to travel to the edge of the observable universe, well, there’s no real-world technology that comes even close to getting that done. Sci-fi style wormholes probably stand as our best bet, and those remain totally hypothetical.

If this universe were an island, then, we’re still so far away from getting off it. To think of the task another way, consider how the first humans to travel across the seas of Earth might’ve felt. Even if they were confident of the land, new countries, and islands that they would eventually find, their pioneering journeys will’ve seen them headed into an incredible unknown. That’s now how we view something like a trip to Mars. We’re sure it’s there, we’re not sure how to get there, but we’re hopeful that one day we will. But, what if those early seafarers of Earth had rather skipped travelling to other parts of this planet, and instead had headed straight for the moon? Upwards and away from Earth, to explore space. Such a monumental jump forwards might’ve 1) been impossible to conceive and 2) would’ve surely been impossible to do. And that’s how thoughts of the multiverse rank with us. We can theorise that other universes are out there… other islands in space waiting to be explored… but we’ve little hope of getting to any of them until we can master the universe we’re currently in. And, unfortunately, we’re still so, so far away from doing that!

Still, perhaps it doesn’t hurt to set our sights high? What’s your opinion? First Mars, then Andromeda, then another universe entirely? How long could it be before we’re ready to take that final step? Or will it never happen at all? For now, that’s how we are living in an island universe, in Heber Curtis’ traditional sense… but also how we can’t be sure when it comes to the many bubbles of a potential multiverse.