Infinite Universe

[Robertlopez]

I live in Australia. As a non-scientist or mathematician cosmology can be hard to understand. This is particularly so when a much loved and respected local Nobel laureate, Brian Schmidt says, “The universe is infinite and it is expanding”.

If the universe is infinite it never ends - I assume it goes on forever!

I can understand infinite in mathematics, the square root of two is a number that, like the size of the universe, goes on forever. But the square root of two is not like the infinity of adding numbers together. The square root of two is infinite but always stays less than one number and bigger than another - it is as if infinity has been constrained.

If you have an infinite universe how can it only point one way? If you look inwards should it not get infinitely smaller? As you look inward, however, the standard model cuts you off – everything seems to just disappear fromview. It is just like the universe - after 13 billion light years or so it also fades from sight! Now, string theory would say it gets a lot smaller still but even string theory has a size that is way bigger than infinitely small.
I find the concept of an infinite universe really confusing.

Having said that I think of the anthropic principle as a basis to explain the existence of life. Our entire universe, the one we can actually see, seems to be perfect for it because even if we are alone somebody is here? It seems you need water, a star for some heat, a steady temperature, a rocky planet and presto it can happen. And so on the basis of all the hundreds of trillions of stars and planets out there you get the numbers you need for life. The problem is I have also heard that the universe does not need to be the way it is. The laws of physics need only be slightly different and all of a sudden you get no stars and I assume life. If you apply the anthropic principle to our whole universe, the one we can see that is, there has got to be more than one because you cannot expect to get all the laws of physics spot on with just the one roll of the dice. All of a sudden infinity is looking good again!

So my question to those who believe in an infinite universe derives from the square root of two. An infinite universe, maybe but it is nevertheless smaller than one thing and bigger than another! Without this constraint sooner or later you end up with an earth like planet that is an exact replica of our own. Moreover, once you get one, by definition there must be an infinite number of them. How do you defend a model that says there must be an infinite number of infinite universes, when just like the string theorists, you know the model can never be seen or empirically tested.

[primummobile]

You really can't defend it. Because of expansion, the area of the universe that we can observe and interact with is necessarily limited. I'm inclined to think that the universe is finite but unbounded rather than infinite.

As for the other part, you are right. There was a finite number of possible quantum states at the beginning of our solar system. That means that the number of different ways our solar system could have evolved, while huge, is still finite. So, in an infinite universe there would be infinite "copies" of everything. I read something where one cosmologist estimated that in the case of an infinite universe that the closest we could expect to find a copy of earth would be 1X10^43,000,000,000,000 light years away. That number is obviously beyond comprehension, and so even if it was true we would never have any interaction with our duplicate.

There are other problems with a truly infinite universe. How does something go from finite to infinite? The universe obviously began from something that would appear to us to be a point. Guth's inflation made it very large very quickly, but infinite?

Defending that there are many other universes isn't as difficult. While there is no experimental evidence, it does answer many questions. If our universe is finite, it only makes sense that there is something outside of it, whatever that means. I think there are experiments going on right now regarding gravity over very small distances that may one day give us experimental evidence for either extra spatial dimensions inside our own universe or other universes. It goes without saying that it will be scant and highly speculative evidence, but if we never chased after scant evidence we would still be gathering wild grain as it grew and wearing animal skins.

The existence of other universes, while not solving the chicken or egg problem, still gives us a possible mechanism for the origin of our universe. As far as evidence goes, that is the most convincing to me that there are other universes outside of our own.

I have a different take on the whole anthropic idea. It is true that our universe seems fine tuned to life as we know it. And having many universes with different physical laws would answer the question of why our universe seems to be the way it is. But I think it is even simpler than that. Life, like we know it, developed here because it could. If the laws were different, a different kind of life would probably develop. I know of no physical law that says sentience requires stars or a specific value for the electroweak force. It just so happens that our physical laws are conducive to our form of life.

The obvious big question is why there is anything, when it seems the natural state would be for there to be nothing. It's tough to wrap your mind around nothingness, and I really think that we'll probably be extinct as a species long before we could ever answer that one. It's still something to think about.

I do have one question, though. You asked how an infinite universe could only point one way. I have no idea what you mean by that. I also don't understand what you mean when you say "look inwards". Could you please explain that?

[Robertlopez]

You asked how an infinite universe could only point one way. I have no idea what you mean by that. I also don't understand what you mean when you say "look inwards". Could you please explain that?

Our earth would appear to be in the middle of the universe because we can only see so far - so as a layperson I think of us as being in the middle of a sphere. If the universe is infinite in size it should also be infinitely small. Accordingly, whether we look outwards or inwards we are in the middle of the spectrum. There are about 200 subatomic particles but there should be an infinite number of them because with infinity there is always room for a new one that is smaller. The standard model stops us seeing in. If the universe is infinitely large why can’t an atom be made up of an infinite number of particles?

[Cougar]

If the universe is infinite it never ends - I assume it goes on forever!

Infinity is not like a really, really big number. It is more like a process. You can start with the number 4 and continually add 1 to it... forever. That's infinity.

If you have an infinite universe how can it only point one way?

That's how the "process" is defined. Keep adding 1. You can't start subtracting 1.

Having said that I think of the anthropic principle as a basis to explain the existence of life. Our entire universe, the one we can actually see, seems to be perfect for it because even if we are alone somebody is here?

I find anthropic arguments very unsatisfying. Yes, the universe exists such that stars can form and life can exist. Otherwise we wouldn't know whether it exists or not. I don't know... there's just not much to the argument...

[mutleyeng]

I find anthropic arguments very unsatisfying. Yes, the universe exists such that stars can form and life can exist. Otherwise we wouldn't know whether it exists or not. I don't know... there's just not much to the argument...

while i agree with you, cosmologists do seem to put a great deal of effort into providing an answer to the "problem".
Seems to me they are uncomfortable with the way the numbers stack up

[kzb]

Personnally I find the anthropic principle very satisfying.

Frequent puzzles are "why is the universe so fine-tuned" or "why are things as they are and not like something else".

It's because the universe HAS to be like this for us to be in it.

At the big bang, there were all kinds of quantum states superimposed. But the only universe that became "real", rather than virtual, is the one that leads to us.

I think this explains a lot, but I know a lot of people don't like it.

[Amber Robot]

How does something go from finite to infinite? The universe obviously began from something that would appear to us to be a point.

Obviously? There is a singularity in the density if you extrapolate the universal expansion to some hypothetical t=0. But we don't really know what happened at t=0 because we don't have any adequate physics to explain anything at that time.

[primummobile]

Obviously? There is a singularity in the density if you extrapolate the universal expansion to some hypothetical t=0. But we don't really know what happened at t=0 because we don't have any adequate physics to explain anything at that time.

That's why I said that it appears to us that it would have been a point. I didn't say that it was a point. It also appears to us that there is a point, a singularity, at the center of a black hole. We don't know that there is because we don't have a quantum theory of gravity. All we currently have is general relativity. If we use general relativity, it appears that there is a point. The fact that we get infinities in these extreme situations is the reason we are looking for a quantum theory of gravity.

[primummobile]

I find anthropic arguments very unsatisfying. Yes, the universe exists such that stars can form and life can exist. Otherwise we wouldn't know whether it exists or not. I don't know... there's just not much to the argument...

I agree with this. We exist because conditions are right. Otherwise we would not, and something else may or may not exist. I don't find it particularly surprising that the laws of nature allow us to exist.

[primummobile]

Our earth would appear to be in the middle of the universe because we can only see so far - so as a layperson I think of us as being in the middle of a sphere. If the universe is infinite in size it should also be infinitely small. Accordingly, whether we look outwards or inwards we are in the middle of the spectrum. There are about 200 subatomic particles but there should be an infinite number of them because with infinity there is always room for a new one that is smaller. The standard model stops us seeing in. If the universe is infinitely large why can’t an atom be made up of an infinite number of particles?

Ok, I'm still not completely clear on what you are asking, but I have a better idea now. The idea that there would be an infinite progression of ever-smaller subatomic particles is not really new. And to be honest, we're still not sure where that progression stops. TOE theories like the different versions of superstring theory or Loop Quantum Gravity say that you can't divide anything smaller than the Planck Length, which is about 1.6 x 10^-35 meters. According to those theories, it is meaningless to talk about anything smaller than that. But the energy required to explore such small distances is many orders of magnitude beyond what our most powerful particle accelerators are able to achieve.

Once something is a point, it has no dimension. It can't get any smaller. In string theory, the use of the word "string" is just colorful language to convey a metaphorical picture of how it would work. It's actually more like a point that has a degree of freedom in either one or two of the three spatial dimensions that we are familiar with. But there couldn't be anything smaller because the string has no size. Regardless of whether string theory is correct, nothing can be smaller than a point. A point, as you would put it, is infinitely small. When we measure dimensions, it really doesn't make any sense to use negative numbers. You can't say that a room is -3 meters wide. That doesn't have any practical meaning. If you were able to "measure" a point, you would get zero as your answer for every measurement you took. You can't get smaller than that.

I hope that helps.

[noncryptic]

If you have an infinite universe how can it only point one way? If you look inwards should it not get infinitely smaller? As you look inward..

An infinitely large anything has no boundary and no center, so there is no direction toward the inside nor a direction toward the outside.

How do you defend a model that says there must be an infinite number of infinite universes, when just like the string theorists, you know the model can never be seen or empirically tested.

The standard model of cosmology does not say whether or not the universe is infinite. The size of the universe is a topic of scientific speculation.

[mutleyeng]

I agree with this. We exist because conditions are right. Otherwise we would not, and something else may or may not exist. I don't find it particularly surprising that the laws of nature allow us to exist.

It is odd though. When i think of it from the perspective of life, i agree - if it wernt the case, we wouldnt be here to see it. When i think of it just as matter, galaxy, the structure of the universe - it starts to seem a little more bizarre for me

[primummobile]

It is odd though. When i think of it from the perspective of life, i agree - if it wernt the case, we wouldnt be here to see it. When i think of it just as matter, galaxy, the structure of the universe - it starts to seem a little more bizarre for me

I agree with that in a sense. I don't find anything in our universe to be that bizarre in terms of existing. But I do think that the fact anything exists at all is strange. I mentioned this in another thread, but it's hard for me to wrap my head around any reason that there would be any kind of order higher than just nothing.

[primummobile]

Think of it this way, mutleyeng. If I take a handful of sand and dump it on to a table, I don't really think anything of how the sand arranges itself. That's just how it happened. I think that's analogous to our physical laws. It's just how they happened to arrange themselves in the moments after the BB. The universe we see is just a reflection of those laws. Were they different, we would simply see a different universe. What I do marvel at is the fact that the table exists for me to dump the handful of sand on to.

I think that explains my position a little better than the mumbling I was doing in the last post.