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.

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.

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.

**Explore further:**
Why is space black?

## Denovalin

So is my beard proof there are multiple me's in this same Universe? Whoa. What happens when I shave?

## billpress11

## PointyHairedEE

## antialias_physorg

I find this question slightly nonsensical, as the universe is a four dimensional spacetime (at least) and not just a three dimensional blob. Within a (again: at least) four dimensional spacetime you can create structures that are at one and the same time (!) finite and infinite in three dimensions.

E.g. by the only true and constant ruler we have to measure any kind of distance (light) a black hole (its event horizon) has a finite diameter when measured from the outside. Measured from the inside, however, the diameter of a black hole is infinite (light, traversing the black hole, will take an infinite amout of time to travel from one point on the event horizon) to another. So what's the true size of a black hole? Finite or infinite? The answer is: both (or "it depends" if you prefer)

So whether or not the universe is one-of-a-kind or encased in a 'bulk' (like in brane theory) the question about its size makes not much sense.

## Dethe

## Dethe

## wolf_larson13

## wolf_larson13

www dot astro dot ucla dot edu/~wright/infpoint.html

## Dethe

## greg1357

## Dethe

## ShotmanMaslo

The singularity could really be finite or infinite. We do not know the nature of the singularity. Big Bang does not say whether the universe is finite or infinite, it is compatible with both options. There are Big Bang models of both kinds.

Observable universe. Not the entire universe.

## OdinsAcolyte

## TimLong2001

## Doug_Huffman

## vic1248

http://www.univer...t-147032

I would highlight:

I was just thinking recently that since the universe is expanding, "space" itself is increasing, so doesn't that mean new "space" is being "created" all the time?!

## antigoracle

## Doug_Huffman

## rockyvnvmc

Sorry, but something Has to exist beyond it, if indeed, it is finite... An analogy would be the cells of and bacterium residing in, our bodies, believing our bodies to be a 'universe', in which case, everyone would be a universe, unto themselves, with innumerable 'universes' existing outside of them.

To my way of thinking, we live in a 'multiverse', comprised of an innumerable number of 'universes', existing in the same time/space as ours, but operating at different vibrational frequencies. Regardless of whether our 'universe' is finite, or infinite.

But then I'm not a 'scientist'... merely a 'philosopher'.

## Dethe

There are no "Big Bang model of both kinds" - only the Friedman models, in which the Universe expands into infinity, steady-state or collapsing state. Everything else is just a fantasy.

## Dethe

## adam_russell_9615

## N6FB

The most probable scenario goes back to Occam's Razer proposition; the simplist answer is probably the correct one. In this case, it is that we are in a closed system which oscillates from "big bang" expansion phase to a contraction phase, and then does it again and again and again.

I know this sounds overly simplistic, but makes much sense compared to the speculations being published these days.

## Dethe

## Reg Mundy

Why do you think "the only true and constant ruler we have to measure any kind of distance (light)"? How does this sit with quantum entanglement (spooky action at a distance?) which is instantaneous? Perhaps we should look for a different "ruler"....

## chris35745

## ShotmanMaslo

Again, you are confused about the difference between observable universe and the entire universe. All those statements about the universe being the size of a proton, or a grapefruit etc refer to observable universe, or some similar spatial horizon of some significance. However the size of the entire universe, including parts beyond the horizon, is unknown. It could be both finite or infinite. If it is infinite, then the primordial state was likely also infinite, just extremely compressed. Tough it is hard to reason with any certainty about what really happened in the first instant of existence.

## ShotmanMaslo

There are finite geometries with no boundaries. If you go in one direction far enough, you would ultimately return back to where you started. Like walking on the surface of a sphere, but in 3 dimensions instead of 2.

## Dethe

## vidyunmaya

http://www.scribd...Dec-1999

http://www.scribd...del-2003

http://www.scribd...rse-2003

## cdt

## astroboy2002

## Dethe

## Benni

Albert Einstein 97

"If we are to have in the universe an average density of matter which differs from zero, however small may be that difference, then the universe cannot be quasi-Euclidean. On the contrary, the results of calculation indicate that if matter be distributed uniformly, the universe would necessarily be spherical (or elliptical). Since in reality the detailed distribution of matter is not uniform, the real universe will deviate in individual parts from the spherical, i.e. the universe will be quasi-spherical. But it will be necessarily finite. In fact, the theory supplies us with a simple connection 1) between the space-expanse of the universe and the average density of matter in it."

Above quote is taken directly from a little paper called General Relativity, a much hated document by those who have never seen a Differential Equation they could solve.

## antialias_physorg

Speed of light is a limit for massive particles (i.e. anything that travels at the speed of light must have a mass of zero). It may not be the limit for other propagation forms (none of the proposed ones have yet been detected, of course)

The current big bang model does posit a period of inflation between 10E-35 and 10E-33 to 10E-30 seconds in which space expanded much more rapidly than light could have travelled. Though this is a volumetric expansion and not an expansion of some outside border. The idea of a 3D border to the universe is not a sensible model.

## Accata

## Multivac jr_

But that doesn't mean it's not fun (and a way to pass the time) to chase our proverbial tails contemplating the fundamental nature of existence, and who knows? One day we might just nail it.

Anyway, there's no math I know of that can prove the existence of three dimensions, and in any case dimensions are abstract constructs/models and so are not real in and of themselves. Maybe we've become too stuck on our extant models to consider other possibilities (e.g. a 2-D, but highly-folded Universe? Or a 1-D, balled-up Universe?). That seems like it would have some bearing on the answer to the question posed by the article, or maybe not.

## Dethe

## Multivac jr_

That is, assuming a Big Bang resulted in the creation of the space-time continuum we call "the Universe," it may be that our Bang wasn't the only Bang. So while our expanding, deformed bubble of a Universe marks the theoretical limits of our perceptions, I see no reason why there can't be many other Big Bangs (past, present, and future), with each a discreet "Universe" all it's own. [cont.]

## Multivac jr_

Time seems interchangeable with motion because most if not all phenomena can be explained via the physics of motion, so it may not exist as a discreet force at all (no force-carrier... …particle for it has been discovered yet but then we haven't found gravitons yet, either).

So maybe there are countless (but finite… at least for now) numbers of Big Bangs banging' out spacetimes (or spacemotions, as it were) in an expanding, "foamy" Metaverse made of all the Big Bangin' bubbles.

## Multivac jr_

## Benni

But the big difference between what you comprehend about the Universe & what Einstein in his General Relativity understood about the mechanics of the universe is "energy distribution". Simply put, you understand just about zero when it comes to the Laws of Conservation of Energy.

Your posts are absolutely boring mundane nonsense, paraphrasing basic grade school level material. Go take a course in Differential Equations so you can better comprehend why Einstein was very clear about the structure of the universe, follow that up with some courses in Thermodynamics, in short.......get out of grade school.

## stardust magician

The BIG BANG stopped, so much came out and no more!

## russell_russell

And you are allow to ask yourself the same questions you asked yourself as if you were still human.

As light what is 'measure' for you?

Do you have duration (time)? Do you have distance? You are moving. Your path is a geodesic.

When you were human you said light 'travels'. Now you are light. As light what do you say about the word 'travel' now?

When you were human you assigned light properties. Paths, motions, oscillation , correspondence, entanglement and more. The mechanics packaged in math.

There was no shortage of imagination to assign properties.

When you were human you assigned the universe properties.

There was a shortage of imagination to assign properties.

As light what properties can you assign the universe?

As light is the universe infinite or finite?

You did so well to help flatlanders understand their universe.

Giving predictions about what happens in their finite or infinite universe coming in contact with our universe.

## solen