What is at the edge of the universe?

And why you shouldn't care.

Nov 11, 2021

While that cosmic donut looks appetizing, it’s probably a bit too much to chew. All those planets, stars, and galaxies make eating the entire universe almost impossible. Notice how I said almost. I like to stay optimistic. Actually, according to one theory, the total energy in the universe is equal to zero. Because it’s a closed system, and energy cannot leave or enter, it has a constant energy, and all the positive and negative energy cancels out. This means that, technically, the universe has zero calories. Take that celery, who’s the best low-calorie snack now?

That got off track remarkably quickly, I apologize for the tangent but I had to get you hooked somehow. Now that you’re here, we might as well learn something. The universe is large (I think we can all agree). It’s so large, in fact, that we can’t cross it. We really can’t even leave our own galaxy, let alone explore the next ones. So, the question that we’ll ask today is not one that will be relevant to your or my life, or any human’s for that matter, but it’s still important. Our species has spent our whole existence looking up wondering our place among the stars; and what’s on the outside of it all?

The universe is expanding. We know this because we can measure the distance between us and other galaxies—and this distance is changing. These galaxies have velocities left over from the Big Bang, and because there’s no air in space, there’s not a whole lot to slow them down. So, unless they run into another galaxy, they’ll just keep on keepin’ on. So if the universe is expanding (think of blowing up a balloon), what is it expanding into? If space itself is growing, what could be at the edge of it? This is where our common misconception of space starts to set in. The Big Bang wasn’t like blowing up a balloon. Space didn’t expand into nothingness, rather, space itself expanded everywhere, at the same time. Because of this, space doesn’t technically have an edge. Confused yet? Me too. The only edge that the universe has is time. As far as we know, the universe appeared 13.8 billion years ago, so in that sense, the edge of the spacetime continuum was 13.8 billion years ago. But in terms of space, from what we can gleen, if you just keep flying in one direction, there will always be stars, galaxies, and space.

This is also a catch-22, because if you’re traveling less than the speed of light, there are some parts of the universe that are moving away from us faster than we can travel to get to it. Well, wait a minute, how can galaxies move away from us faster than the speed of light? Physically, it’s because of dark energy but conceptually, it’s because of relative speeds. We don’t know much about dark energy apart from that it exists, and that it’s causing the universe’s expansion to not only be faster than the speed of light, but it’s actually causing it to accelerate. To achieve this faster than light travel, we have to look at relative velocities. If you’re in a car, traveling 50 mph on a highway where the speed limit is 50 mph, not only are you on a lame highway, but you’re abiding by the law—even lamer. (For legal reasons that was a joke) But if another car passes you going the other direction, also at 50 mph, that car will look like it is coming at you at 100 mph. The difference in velocities is what you, the observer, will see. The same applies to galaxies. If the Milky Way is moving away from a neighbor at 75% the speed of light, and our neighbor is also moving away from us at 75% the speed of light. To us, it will look like that galaxy is traveling away at 150% the speed of light—impossible with absolute speed, but very possible with relative speed.

Now we can get to the core of the discussion. Even though the universe doesn’t have an edge, per se, ours does. By ours I mean the observable universe. This is the universe that we can see with telescopes. This isn’t defined by how powerful our telescopes are, rather, it’s defined by the amount of light that has reached us. The universe is 13.8 billion years old, which means light from all around has had 13.8 billion years to get to us. This, combined with the expansion of space that we’ve previously discussed, has unlocked access to 46 billion years worth of light. That is the radius of the sphere of our universe: 46 billion light-years. And, as time goes on and more light arrives at Earth, the larger that sphere gets. It’s exciting that there is so much out there to explore but the expansion of space “faster than the speed of light” sadly means we can physically only access 3% of our observable universe, even with light-speed travel.

We now have a lot of information and not a whole lot of answers. The universe is so large that we can barely see it, and the vast majority of things that we can see we’ll never reach. And even if we tried to reach them, they wouldn’t be there anymore. If you went towards a star 10 light-years away, you’d miss it completely, because you were traveling towards where the star was 10 years ago. While we’ve arrived at a sad result about our access to the universe, the good news is there’s still relatively a lot we can access. Scientists estimate that it will take humans millions of years to fully explore the Milky Way galaxy. That ought to keep us busy for a while.

In the meantime, I hope you enjoyed this week’s edition of It’s Not Rocket Science. Don’t forget to leave a like and subscribe if you haven’t already!

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