# Light travel just after the big bang

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1. Apr 16, 2015

### Pjpic

If the universe had stayed the size of a baseball, would it have taken light 13.5 billion years to travel across it?

2. Apr 16, 2015

### student07

What is your thought about it?

3. Apr 16, 2015

### phinds

The universe has never been "the size of a baseball" so I'm not clear why you are asking what would have happened if it has "stayed" that size.

4. Apr 16, 2015

### Pjpic

I don't know, maybe something about how the density(?) of spacetime effects light.

5. Apr 16, 2015

### phinds

This posits an incorrect assumption that spacetime is an "ether". It is not.

6. Apr 16, 2015

### Staff: Mentor

The universe, as best we can tell, is spatially infinite, so, as phinds said, it has never been "the size of a baseball".

Spacetime doesn't have a density. The matter and energy present in spacetime does, but spacetime itself doesn't.

7. Apr 20, 2015

### Pjpic

Maybe the question s/h/b stated as: If the currently visible universe had stopped expanding at an earlier time, would the greater energy density cause light to take the same 13.5 b.l.y. to traverse the radius?

8. Apr 20, 2015

### Staff: Mentor

The only way this could have happened would be if the universe were closed and that earlier time was the moment of maximum expansion. So the universe would currently be contracting.

First of all, it's years, not light-years, since you're talking about time.

The time it takes light to cover a given distance in the universe is not affected by the energy density, except in so far as the energy density determines how the expansion proceeds. All you need to know to figure out light paths is the expansion profile.

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