Is the Universe Infinite?

Question posed by @LaVonneEllis.

We don't know.

The portion of the universe that we can see is finite. This limit is imposed on us by the speed of light: we can't see anything beyond a certain distance because any light from objects further away simply hasn't had time to reach us yet.

The edge of the observable universe, then, is thought to be around 46 billion light years away from us in every direction, meaning that it is a sphere with a diameter of a bit more than 92 billion light years.

The WMAP mission to measure the Cosmic Microwave Background (CMB) in great detail has implications for the size of the universe as a whole. If the universe is finite and smaller than the observable distance (yes, that sounds weird, but bear with me), then we would see strange patterns in the CMB that were an effect of space folding back on itself. From the WMAP results, no such patterns have been discerned. If this is accurate, it means that the universe must be bigger than the 92 billion light year sphere that we can observe around us.

How much bigger is a more difficult question. It is possible that the universe is infinite in extent, and this idea is somewhat supported by the measurements from WMAP which suggest that the universe is flat. However, it could be that space is flat only locally, and curves beyond our visible horizon.

Beyond this conjecture, it may well be impossible to know whether the universe is infinite in extent as we are causally disconnected from the rest of the universe outside our observable bubble.


  1. 46 billion light years? in 13.7 billion years? I don't think so....

  2. If you've got an issue with something that has been posted, it would be helpful to future readers if you'd explain exactly what that issue is.

    But you haven't, so I'll have a guess...

    My guess is that you're struggling with the idea that, knowing that the speed limit for every particle in the universe is the speed of light, and that the universe is around 13.7 billion years old, then the furthest anything could have travelled is 13.7 billion light years. I.e., the most distant photon that we can conceive of having hit our telescopes can only have started travelling 13.7 billion years ago, 13.7 billion light years away. So nothing, not even a photon, could have travelled 46 billion light years in that amount of time.

    Am I right?

    If I have managed to guess your issue correctly, then allow me to attempt to respond to it:

    The problem with the model in your issue is that it doesn't take into account the expansion of space, which has been happening since the very beginning. That photon left, 13.7 billion years ago, a galaxy which was, at the time, 13.7 billion light years away from us. Over the intervening time, the photon has travelled inexorably towards our telescope lens. At the same time, space itself has expanded, taking the original galaxy further away from us than its original 13.7 billion years.

    It is using the fact of the universe's continuing expansion along with what we observe in our most powerful telescopes that allows us to calculate the edge of the 'known' universe as being about 46 billion years away from us.

  3. Oh, by the way, this post might be interesting / relevant!


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