# An infinite universe can expand?

1. Jun 21, 2013

### MathJakob

Is it true that the universe can be infinitely large but still expand, so that at every particular moment in time the universe is infinitely large, but then becomes 100 light years larger for every second that time goes on for example?

For example $$\infty+100=\infty$$

I know infinity isn't a number but something that is infinite can still increase?

Last edited: Jun 21, 2013
2. Jun 21, 2013

### phinds

Google "Hilbert's Hotel"

3. Jun 21, 2013

### marcus

Sure! and what I mean by expansion of a place is what the people living there experience from WITHIN, namely distances between things increase at some percentage rate.

By expansion I do not mean anything that anybody could see from OUTSIDE because that idea has no valid basis. There is no outside, and no outside observer, that we know of. So that is not what I mean by expansion.

You should think of expansion as what people living deep inside some dough experience when they see the raisins getting farther apart. It's raisin bread dough and has yeast and was put in a warm place to rise, before being baked. And they say "You know what? I think our dough is expanding! The raisins are getting farther apart!"

As far as they know there is no outside. No limit, no other special kind of dough outside their dough, that has the raisins. There is no known SIZE of the whole, indeed logically it could be infinite. All they know is a regular pattern of expansion of distances by a certain percentage each half-hour measured by their clocks.

It is useless to talk about the overall size of the universe if we have no way to measure it. So when we say it is expanding we are talking about a pattern of growth of distances between socalled isotropic observers (observers who are not moving relative to the ancient light, but who are getting farther apart). this is a minor technical point which is rarely explained.

So you have to know what expansion means and get used to that, for the idea to make sense.
BTW try the "charley" link in my signature. It is a good SciAm article that a lot of people have used as intro to cosmology. It gets recommended a lot here.

4. Jun 21, 2013

### MathJakob

Thanks!

5. Jun 21, 2013

### Chronos

Local distances can obviously increase, but, distance become increasingly meaningless as it approaches cosmological proportions in a spatially infinite universe.

6. Jun 24, 2013

### Chalnoth

Perhaps more to the point, even if there is an outside, they can't observe the goings-on of our universe.

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