Passionately Curious – Living on the Edge

The universe is weird. Like, really friggin’ weird. There are things like a cosmic speed limit and magical buzz words like ‘Dark Matter’ and ‘Dark Energy’, and a plethora of other strange and mysterious subjects that even those who work on them their entire lives might never truly understand. And today, we’ll be talking about one of these strange and admittedly rather complicated things: the edge of the universe.

Now, a good first question would be: “why should there be an edge to our universe? Could it not simply have existed forever, and stretch into infinity?” Well, it might. It might also not be infinite in time, yet infinite in space, and vice versa. Or it might be finite in both. Amusingly enough, we don’t know for certain, and there is reason to suggest that we might, in fact, never know.

It begins with one funny little aspect of our universe, which was almost accidentally discovered in the 19^th century: the fact that light has a finite, constant speed. As Albert Einstein would later prove, this is actually one of the most fundamental aspects of our universe. It imposes upon us a kind of cosmic speed limit¹. For now, merely the finite speed suffices, as it allows us to grasp that when we look outward into our universe, we in fact look back in time. A photon from a star one light-year away takes a year to get to us, and so it shows us what the star looked like one year ago! Once we apply this to the light we receive from the entire universe, we encounter what astrophysicists have dubbed the Observable Universe.

Yup, that’s pretty frigging big alright.
(Image credit: Wikipedia)

The Observable Universe is, fittingly enough, the edge of the universe that we can see. As light has a finite speed, this is not so much the physical (or to be more precise: spatial) edge of our universe, but rather the temporal. We can’t look beyond it for the simple reason that light from beyond this limit hasn’t yet had the time to get to us! In a sense, this means that this truly is the edge of the universe; the universe simply did not exist yet if you look any further².

Of course, you might consider this a matter of semantics. While the Observable Universe only extends as far as we can see, that does not mean that that is all there is. Someone might be standing on a planet beyond what we call the Observable Universe, whom we won’t be able to see for billions of years, wondering the very same thing right now. For every point in space, there is such a thing as the Observable Universe, as it is simply defined by how far back we can see in time. For all we know, the actual universe stretches a hundred times further than our temporal edge. Perhaps a thousand – perhaps even 10²³ times. And perhaps, the universe truly is infinite in space.

As you can see, we’ve got fairly strong reasons to believe that the universe is probably not infinite in time³. After all, with infinite time we’d expect to be able to see things from infinitely far away, but we can’t. Alright, so we don’t have infinite time – now we might like to see if we can figure whether the universe is finite or infinite in space. And I’m going to make you very sad here with one very fundamental truth: this is an unknowable fact. However, we’ve got a very good idea about the existence the edge of the universe (which is really what we’re here for). “But wait, don’t those two questions have basically the same answer?” Interestingly enough, they don’t! The shape of the universe gets in the way.

You see, the shape of space is a tricky thing; to simplify, let me compare the 3D space of our universe with the 2D surface of a sphere⁴. We are simply creatures locked in 2D, living on that two-dimensional surface, never realizing that it actually is spherical. Compare this to how, in our everyday experience, the earth appears to be a flat surface– while in fact, it is a gigantic sphere curving into a 3D universe⁵. Now imagine the very same thing for our perception of the universe: what we observe is merely the surface of the structure, which is so enormous that we can’t see that the whole thing curves out into the third dimension. The only problem being, of course, that the sphere we live on is actually four-dimensional – making our universe the three-dimensional surface of a gigantic four-dimensional sphere⁶!

The surface of our confusing little sphere.
(Image taken from Here)

If this is the case it means that our universe, despite being finite⁷, has no edge because it closes in upon itself. This seems to be supported by our observation that the universe is uniform; it is what we would expect from a universe without an actual edge, because that would mean every point in this universe is fundamentally the same as every other point. Every point on a finite sheet of paper, however, is not made equal, and this is what we would expect to see reflected in the structure of our universe – but we don’t. We observe a universe that appears to be fundamentally homogeneous. This means that either the universe is infinite, or it is finite and it closes in upon itself. In the end, even if we don’t know whether the universe is finite or infinite in space, we can at long last conclude: there is actually no edge to the universe regardless!

And still, that’s not the end of the tale. Although we now have good reason to believe that the universe doesn’t have an edge no matter whether it is finite or infinite, we simply can’t know for sure. After all, we could be in a very strange part of the universe that just happens to be homogeneous by chance. And that is perhaps the strangest thing: we’ll probably never get much further than increasingly accurate guesses, because we’ll never get to actually go there and see what’s out there. In a way that’s a saddening prospect, but I also think there’s a kind of beauty in the fact that there are simply some things we’ll never get to know for certain.

 

Footnotes

1. This is hardly a trivial notion, and in fact may well be considered the most crucial insight of Einstein’s entire career. I hope to be writing an article on special relativity in the future, but for those who are want to go investigate themselves: Wikipedia is usually a very good start for reading into scientific subjects: Link.
2. This is actually not quite true, and I hope to be going a little deeper into this in a future article. For now, if you want to read up on the reason why we can’t see any further than the limits of the observable universe, you might consider starting Here.
3. For those interested, the Wikipedia articles on the Big Bang and Steady State theories may be enlightening reads.
4. Please do mind that this is actually a really bad example for an actual description the true shape of the universe, but this is the typical example used to explain the ‘shape’ of the universe as it tends to make more sense intuitively.
5. It is important to distinguish here that the surface of a sphere is actually two-dimensional, even though it is curved into a third dimension!
6. Don’t even bother trying to wrap your head around that: it won’t work.
7. It is of course possible to walk forever in a straight line in such a surface, and as such an ant (or a person who is very tiny compared to the size of the sphere) trapped on it might think it is infinite. To us, however, it is plain to see that the surface itself is actually of finite size.

 

References:

Ryden, B. (2014). An Introduction to Cosmology. Pearson Education Limited, Edinburgh.

In addition, the following articles have been massively helpful, and should serve anyone who wants to dive into this subject a bit further quite well.
http://en.wikipedia.org/wiki/Michelson-Morley-experiment
http://en.wikipedia.org/wiki/Olbers’_paradox
http://en.wikipedia.org/wiki/Observable_universe
http://en.wikipedia.org/wiki/Cosmological_principle
http://scienceblogs.com/startswithabang/2012/07/18/how-big-is-the-entire-universe/
http://en.wikipedia.org/wiki/Physical_cosmology
http://io9.com/5743624/how-can-we-measure-the-size-of-the-universe