Is the Universe Finite or Infinite?

Two possiblities exist: either the Universe is finite and has a size, or it’s infinite and goes on forever. Both possibilities have mind-bending implications.

In another episode of Guide to Space, we talked: “how big is our Universe”. Then I said it all depends on whether the Universe is finite or infinite. I mumbled, did some hand waving, glossed over the mind-bending implications of both possibilities and moved on to whatever snarky sci-cult reference was next because I’m a bad host. I acted like nothing happened and immediately got off the elevator.

So, in the spirit of he who smelled it, dealt it. I’m back to shed my cone of shame and talk big universe. And if the Universe is finite, well, it’s finite. You could measure its size with a really long ruler. You could also follow up statements like that with all kinds of crass shenanigans. Sure, it might wrap back on itself in a mindbending shape, like a of monster donut or nerdecahedron, but if our Universe is infinite, all bets are off. It just goes on forever and ever and ever in all directions. And my brain has already begun to melt in anticipation of discussing the implications of an infinite Universe.

Haven’t astronomers tried to figure this out? Of course they have, you fragile mortal meat man/woman! They’ve obsessed over it, and ordered up some of the most powerful sensitive space satellites ever built to answer this question.Astronomers have looked deep at the Cosmic Microwave Background Radiation, the afterglow of the Big Bang. So, how would you test this idea just by watching the sky?

Here’s how smart they are. They’ve searched for evidence that features on one side of the sky are connected to features on the other side of the sky, sort of like how the sides of a Risk map connect to each other, or there’s wraparound on the PacMan board. And so far, there’s no evidence they’re connected.

In our hu-man words, this means 13.8 billion light-years in all directions, the Universe doesn’t repeat. Light has been travelling towards us for 13.8 billion years this way, and 13.8 billion years that way, and 13.8 billion years that way; and that’s just when the light left those regions. The expansion of the Universe has carried them from 47.5 billion light years away. Based on this, our Universe is 93 billion light-years across. That’s an “at least” figure. It could be 100 billion light-years, or it could be a trillion light-years. We don’t know. Possibly, we can’t know. And it just might be infinite.

If the Universe is truly infinite, well then we get a very interesting outcome; something that I guarantee will break your brain for the entire day. After moments like this, I prefer to douse it in some XKCD, Oatmeal and maybe some candy crush.

Artist's conception of Planck, a space observatory operated by the European Space Agency, and the cosmic microwave background. Credit: ESA and the Planck Collaboration - D. Ducros
Artist’s conception of Planck, a space observatory operated by the European Space Agency, and the cosmic microwave background. Credit: ESA and the Planck Collaboration – D. Ducros

Consider this. In a cubic meter (or yard) of space. Alright, in a box of space about yay big (show with hands), there’s a finite number of particles that can possibly exist in that region, and those particles can have a finite number of configurations considering their spin, charge, position, velocity and so on.

Tony Padilla from Numberphile has estimated that number to be 10 to the power of 10 to the power of 70. That’s a number so big that you can’t actually write it out with all the pencils in the Universe. Assuming of course, that other lifeforms haven’t discovered infinite pencil technology, or there’s a pocket dimension containing only pencils. Actually, it’s probably still not enough pencils.

There are only 10 ^ 80 particles in the observable Universe, so that’s much less than the possible configurations of matter in a cubic meter. If the Universe is truly infinite, if you travel outwards from Earth, eventually you will reach a place where there’s a duplicate cubic meter of space. The further you go, the more duplicates you’ll find.

Ooh, big deal, you think. One hydrogen pile looks the same as the next to me. Except, you hydromattecist, you’ll pass through places where the configuration of particles will begin to appear familiar, and if you proceed long enough you’ll find larger and larger identical regions of space, and eventually you’ll find an identical you. And finding a copy of yourself is just the start of the bananas crazy things you can do in an infinite Universe.

The Hubble Ultra Deep Field seen in ultraviolet, visible, and infrared light. Image Credit: NASA, ESA, H. Teplitz and M. Rafelski (IPAC/Caltech), A. Koekemoer (STScI), R. Windhorst (Arizona State University), and Z. Levay (STScI)
The Hubble Ultra Deep Field seen in ultraviolet, visible, and infrared light. Image Credit: NASA, ESA, H. Teplitz and M. Rafelski (IPAC/Caltech), A. Koekemoer (STScI), R. Windhorst (Arizona State University), and Z. Levay (STScI)

In fact, hopefully you’ll absorb the powers of an immortal version of you, because if you keep going you’ll find an infinite number of yous. You’ll eventually find entire duplicate observable universes with more yous also collecting other yous. And at least one of them is going to have a beard.

So, what’s out there? Possibly an infinite number of duplicate observable universes. We don’t even need multiverses to find them. These are duplicate universes inside of our own infinite universe. That’s what you can get when you can travel in one direction and never, ever stop.

Whether the Universe is finite or infinite is an important question, and either outcome is mindblenderingly fun. So far, astronomers have no idea what the answer is, but they’re working towards it and maybe someday they’ll be able to tell us.

So what do you think? Do we live in a finite or infinite universe? Tell us in the comments below.

How Big is the Universe?

The Universe is big, but how big is it? That all depends on whether the Universe is finite or infinite. Even the word “big” is tough to get clear. Are we talking about the size of the Universe we can see, or the Universe’s actual size right now?

The Universe is big, but how big is it? And what the heck kind of question is that? Are elephants big? Trucks? Dinosaurs? Cheese? Is cheese big? How big is cheese? How big is big?

The word “big” is tough to get clear. Are we talking about the size of the Universe we can see, or the Universe’s actual size right now? This becomes even more complicated when we are trying to work under assumptions of either the Universe is finite or the Universe is infinite.

One difficulty with talking about the size, is that the Universe is expanding. Light takes time to travel from distant galaxies, and while that light travels, the Universe continues to expand. So our problem with talking about how big it is, is that there is no single meaning to distance when it comes to the universe. For this reason, astronomers usually don’t worry about the distance to galaxies at all, and instead focus on redshift, which is measured by z. The bigger the z, the more redshift, and the more distant the galaxy.

As an example, consider one of the most distant galaxies we’ve observed, which has a redshift of 7.5. Using this, we can determine distance by calculating how long the light has traveled to reach us. With a redshift of 7.5, that comes out to be about 13 billion years. You might think that means it’s 13 billion light years away, but 13 billion years ago the universe was smaller, so it was actually closer at the time the light left that galaxy. Using this, if you calculate that distance, it was only a short 3.4 billion light years away.

Now the galaxy is much farther than that. After the light left the galaxy, the galaxy continued to move away from us. It is now about 29 billion light years away. Which is definitely more than 13, and quite a bit more than its original 3.4.

Usually it is this big distance that people mean when they ask for the size of the universe. This is known as the co-moving distance. Of course, we can only see so far. So, how far can we see? The most distant light we are able to observe is from the cosmic microwave background, which has a redshift of about z = 1,000.

This means the co-moving distance of the cosmic background is about 46 billion light years. Sticking us at the center of a massive sphere, the currently observable universe has a diameter of about 92 billion light years. Even with this observed distance, we know that it extends much further than that. If what we could see was all there is, we would see galaxies tend to gravitate towards us, which we don’t observe.

Multiverse Theory
Artist concept of the multiverse. Credit: Florida State University

In fact we don’t see any kind of galaxy clumping to a particular point at all. So as far as we know the universe could extend forever. It could be even stranger than that. Despite some media controversy, if the BICEP2 detection of early inflation is correct, it is likely the Universe undergoes a type of inflation with the intimidating moniker of “eternal inflation”. If it is the case, our observable universe is merely one bubble within an endless sea of other bubble universes. This is otherwise referred to as… the multiverse.

So, in the immortal words of Douglas Adams, “Space,” it says, “is big. Really big. You just won’t believe how vastly, hugely, mindbogglingly big it is. I mean, you may think it’s a long way down the road to the chemist’s, but that’s just peanuts to space”

What do you think? Does the Universe go on for ever? Tell us in the comments below. And if you like what you see, come check out our Patreon page and find out how you can get these videos early while helping us bring you more great content!

Astronomy Cast Ep. 267: Infinities

Forever is a funny thing. Today we’re going to talk about infinities. That’s right, all the different kinds of possible infinities. How you add them, subtract them, and use them to think about the scale of the Universe.

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“Infinities” on the Astronomy Cast website.

And the podcast is also available as a video, as Fraser and Pamela now record Astronomy Cast as part of a Google+ Hangout:
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