So it seems the overall thought is that the universe is infinite. I don't get how.

Most depictions I read mention something to the effect of "at 1^-20 seconds the universe was the size of _". (Essentially mentioning a finite size at a very small length of time). So it came from a singularity, and had a finite size at some point. No matter how fast inflation went, any multiple of that finite size is NOT infinity though. If it was finite once, it is still finite.

If it's infinite then it was always infinite which means it was infinite density, infinitely large, etc. That's too crazy to swallow.

Isn't it more likely that the universe is really really big but not infinite? And we are at the center of the observable universe (to us), however we are off center in some grander universe that's maybe 1 trillion light years across? And nothing is past that because space and time are the same thing. Therefore if there is no light/matter/events, there is no time and hence no space.

  • $\begingroup$ I have to say, I don't think the community is leaning one way or the other. I can say that you're obviously right about us being in the center of the observable universe, but that because there is no center to the universe (see other questions on this stack exchange), we're definitely not in the center. $\endgroup$
    – HDE 226868
    Aug 13, 2014 at 22:40
  • $\begingroup$ If it were finite in size then there is certainly a center. We just can't see it. $\endgroup$
    – Paul
    Aug 14, 2014 at 13:27
  • $\begingroup$ In the universe (on the largest scales), everything is moving away from everything else. I can assure you, there is no center. See this: physics.stackexchange.com/questions/25591/… $\endgroup$
    – HDE 226868
    Aug 14, 2014 at 13:31
  • $\begingroup$ Whoops, sorry, no single point that is the center. $\endgroup$
    – HDE 226868
    Aug 14, 2014 at 13:34
  • $\begingroup$ @HDE226868 If it's finite in size, then there certainly is a center. We just don't know where it is because we are the center of our observable one. If it's infinite in size then there is of course no center. But if the universe is 1 trillion light years across, then there's a very obvious center which may or may not be in our own observable universe. $\endgroup$
    – Paul
    Aug 14, 2014 at 19:05

2 Answers 2


Depending on the mass of the universe, the universe has three different possible shapes. http://en.wikipedia.org/wiki/Shape_of_the_universe

It can be finite or infinite depending on the shape it has, and there are some ways we can discard possible shapes (if it started to shrink instead of expand).

Hope this helps

  • 1
    $\begingroup$ Well, it technically has three different curvatures, no three different shapes. $\endgroup$
    – HDE 226868
    Aug 14, 2014 at 20:34
  • $\begingroup$ Universe topologies, if you like :) $\endgroup$
    – saken
    Aug 15, 2014 at 20:13

Space and time are total different things.

These citations are about the observable universe, and not from the whole.

We can see only a 13.7billion light year sphere from our spacetime. It is not very big.

The current measurement of the WMAP satellties says, the universe is probably infinite big.

Your questions can be explained with the Friedman equations. These equations are the solutions of the Einstein Field Equations for the infinite vacuum of our spacetime.

And its spacelike sections of our Friedman-spacetime is planar, at least in the limits of the precision of the measurement. What currently can be said, is that "if the whole universe is finite, and has a (hyper-)spherical geometry, then its radius must be at least 300billion light years".

  • $\begingroup$ I will look into the Friedman equations. $\endgroup$
    – Paul
    Aug 14, 2014 at 19:06
  • 1
    $\begingroup$ The Friedmann equations are much more general than either the vacuum condition or being spatially infinite. The whole section is more than a bit confused. @HDE226868 The implication, which this answer alludes to but fails to communicate clearly, is that the WMAP observations indicate spatially flatness, which under the modeling assumptions of the FRW family of solutions (global isotropy and homogeneity) would imply an infinite universe. $\endgroup$
    – Stan Liou
    Aug 15, 2014 at 7:10
  • $\begingroup$ We can see only a 13.7billion light year(radius or diameter?) sphere, nah the universe, as many observations seem to show is about 13.5 billion years old, but the sphere of our visible universe is much bigger, almost 50 billion lightyears in radius $\endgroup$ Aug 15, 2014 at 8:40
  • $\begingroup$ @StanLiou Thanks for the information about WMAP. $\endgroup$
    – HDE 226868
    Sep 6, 2014 at 21:58

Not the answer you're looking for? Browse other questions tagged .