If the universe is infinite, does that mean that everything exists somewhere?

Richard87

If the universe is infinite, does that mean that everything exists somewhere, besides obviously impossible things like a star that contains oxygen but doesn't contain oxygen or a 4-sided triangle?

mgb_phys

It would if it was but it isn't

Chalnoth

Well, consider this by way of analogy.

The set of all even numbers is infinite. I can go on counting even numbers for ever and ever and never reach an end.

But clearly the set of all even numbers does not include all possible numbers. It doesn't include, for instance, the number pi.

So even if the universe is infinite (we don't know whether or not it is), then that doesn't necessarily mean that all possibilities are realized.

However, there may be other reasons to believe that all possibilities are realized, mainly stemming from quantum mechanics, where we find, for instance, that if there is the possibility of matter inhabiting a region of space, then particles of that sort of matter will necessarily pop in and out of the vacuum. Another way of saying this is that in quantum mechanics, there mere possibility of existence forces existence. So it is not unreasonable to suspect that perhaps all possibilities must actually be realized.

This doesn't mean that anything and everything we can imagine occurs, of course. We can imagine quite a lot of impossible things, as you mention above. But we can also imagine a great many things that are not obviously impossible, and yet may turn out to be upon deeper inspection.

Entropee

Well so far we think that the Universe is finite, but with no boundaries. I really hate the word infinity >.< It's so unbelievably unfathomable, and people just toss it around as if it's just a large number. Good thoughts on the subject though.

Chalnoth

I don't know who this "we" is, but so far as I am aware there is no consensus on this. Currently there just is insufficient evidence to say anything more than, "the universe is very, very big."

JnWaco

So there might not be another Earth where I'm dating Jennifer Aniston? Dang.

Chalnoth

Well, just because you can imagine it doesn't mean it's possible. Consider, for a moment, how many opportunities you have had to get to know a beautiful actress. Probably not very many.

If, in this parallel world, you were the sort of person that had a life where you were in at least occasional contact with beautiful actresses, would your life be so different that that person even count as being you in the first place?

Entropee

Okay okay.. by "we" I mean Stephen Hawking haha

And if our anti particles formed anti people on an anti earth we could have identical twins down to the last particle...maybe... ;P

Chalnoth

I'm pretty sure if you asked him straight up he'd say basically the same thing I just did. He did, of course, present the no boundary proposal (where the universe has no boundary either in time or in space), but that doesn't mean he would go so far as to claim that he knows it's true. He may think it likely (it's his idea, after all), but I doubt he'd go that far.

Well, we don't have anti-particles. That's one of the requirements of our cosmological observations: that early-on, there was a very small breaking of the symmetry between matter and antimatter.

Entropee

Well yes I agree with you thats why i said its what he "thinks" not knows, and would YOU go so far as to say that you "know" we dont have anti-particles? There may not be symmetry between matter and antimatter but It's still somewhere.

Chalnoth

Yes, because we've actually looked for them. They're not out there. Basically, if the matter and anti-matter were physically separated, then you'd occasionally get clumps of matter running into clumps of anti-matter, causing rather large explosions. We don't see any of that.

Furthermore there's the problem that around the time of the emission of the CMB, our universe was extremely uniform, so that there was no way that normal matter and anti-matter could have been out of contact with one another.

Entropee

Thats true, but why then was the early universe so hot?

Also an unrelated question maybe you can answer for me, if most of the universe is hydrogen, why didnt all the hydrogen undergo nuclear fusion when the universe was as big as a baseball? Was it because the particles were different at the time?

Chalnoth

It appears to be a result of the end of inflation. Basically, whatever it was that drove inflation had to have a whole lot of energy. When it decayed, it reheated our universe to a tremendous temperature.

Right, if the expansion rate would have been slower then, it would have. It would have progressed all the way to producing iron and that'd be the most common element.

But this takes time. First the protons and neutrons condense out of the quark-gluon plasma, so you have hydrogen right away. Then you start to make helium from the hydrogen. Then you start to make heavier and heavier elements.

As it turns out, the expansion rate was such that the universe cooled to where the nuclear fusion basically stopped by the time there was only around 25% helium sitting around, and long before more than trace amounts of anything else formed. Incidentally, this is even faster than it sounds, because much of the helium stemmed from the neutrons that were around early-on (when you have nothing but protons around, fusion takes a heck of a lot of energy, as you have to convert protons to neutrons, and because they repel one another, but when you have lots of neutrons sitting around it's much easier).

Entropee

O.o I thought we didnt have protons and neutrons in the early universe, wasn't there lots of particle decay so they would be different now?

Chalnoth

Oh, well, in the very early universe there weren't any. But when the quark-gluon plasma cooled, well, protons and neutrons were the particles they condensed into: they're the lightest baryons. Heavier baryons are unstable and would have quickly decayed into protons and neutrons.

Entropee

About how long did it take for the quark-gluon plasma to cool?

Chalnoth

Well, either way I don't know off the top of my head and you might be able to find it yourself as quickly as I could off of Google, but what specifically do you mean by this?

That is, are you asking how long the process of nucleosynthesis took, from the time the protons/neutrons condensed out of the plasma to the time that fusion stopped? Or are you asking how long after the end of inflation that this occurred?