Please excuse the use of incorrect terminology. I've been trying to wrap my head around the fact that the universe has no edge and if you travel in a straight line you would loop back to your original position. When the first light formed according to my Googling 300000 years after the big bang we are told the radius of the universe was 42 million light years.

So my question is have these photons (from the CMB) just been travelling in a straight line for 13 billion years without lap-ing/looping the universe and if so is it because of the expansion of the universe or has there just been that much universe for them to travel through without reaching their point of origin. As you can tell a confused brain finds it very hard to word its question, I hope the question makes sense. But the core of my question is, has the light been moving through the universe without reaching its loop-back point until it hit our detectors on earth?

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    $\begingroup$ "the universe has no edge and if you travel in a straight line you would loop back to your original position" This is just one of the possibilities for how the universe is, and it doesn't seem like the most likely one at the moment. $\endgroup$
    – Javier
    Feb 5, 2019 at 12:50
  • $\begingroup$ Yeah, this question would, to me, more seem like philosophy haha how the endless extent of space really works is a highly debated topic. $\endgroup$
    – DakkVader
    Feb 5, 2019 at 13:27
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    $\begingroup$ no edge doesn't necessarily mean the universe is a sphere or similar in shape. It can be infinite as well. $\endgroup$
    – Kosm
    Feb 5, 2019 at 14:09
  • $\begingroup$ Since Hawking radiation is a necessary response to thermodynamic theory and I don't believe anyone has yet suggested that it may consist of anything but photons, I may have provided an answer to your question in my recent answer to the PSE question, "How can classical black holes even exist?", although I don't feel that it adds enough information to post it as an answer to this one as well. $\endgroup$
    – Edouard
    Feb 5, 2019 at 16:00

1 Answer 1


For a closed universe, if you travel long enough the light should loop itself. But in our universe we don't observe such things. This implies that, If the universe is closed, the radius of curvature must be larger than the hubble radius ($c/H_0$).  Otherwise as I pointed out light will be looping around the universe. 

If the universe is flat then the light will never loop.

  • $\begingroup$ I voted for this, so my vote's locked in. However, as the OP was asking about "early" photons, I have to say that I've since noticed that, in a 2018 paper titled "Big bounce and closed universe from spin and torsion", it's claimed that a closed universe exists only when the product of the scale factor and temperature is higher than a particular threshold, contrary to an open and flat universes (which are not restricted), and hypothesizes that such thresholds increase, thereby implying that a universe starts flatter than it may become. I guess a photon's straight trajectory may, then, curve. $\endgroup$
    – Edouard
    Feb 5, 2019 at 18:10
  • $\begingroup$ I did not understand what you are trying to say $\endgroup$
    – seVenVo1d
    Feb 5, 2019 at 18:15
  • $\begingroup$ Check the abstract of the paper, which is by guys whose credentials are vastly better than mine. It's available free. You can cut and paste the title onto a search engine. (Links don't always work, on PSE.) $\endgroup$
    – Edouard
    Feb 5, 2019 at 18:17
  • $\begingroup$ I read the paper but did not understand your point $\endgroup$
    – seVenVo1d
    Feb 5, 2019 at 18:18
  • $\begingroup$ Well, I think they're saying that a universe that starts flat may curve if it gets hot enough, so, after it starts to curve (into, say, a sphere), the trajectory of a photon which had been moving through flat space may begin curving into a loop that it will eventually complete. If you edit your answer to eliminate its last sentence, it might be consistent with those results. $\endgroup$
    – Edouard
    Feb 5, 2019 at 18:22

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