So in the early universe where the universe was really small what would happen to a photon is it didn't collide with any material and made it to the edge of the universe? Because yes the big bang expanded faster than light at one point, but there must have been a point were the universes expansion rate was slower than the speed of light. So if a photon got to the edge of the universe what would happen?

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    $\begingroup$ The universe doesn’t have an edge. $\endgroup$
    – G. Smith
    Oct 9, 2019 at 22:54
  • $\begingroup$ No, but in the early universe like a few picoseconds after the big bang wouldn't it have been smaller than our universe now? $\endgroup$
    – T. Fisher
    Oct 9, 2019 at 22:57
  • $\begingroup$ Possibly. That would be the case if it has positive spatial curvature, but there is no evidence that it does. (It appears to have zero spatial curvature.) Even in this case, there is no edge for the photon to reach. If the universe has zero or negative spatial curvature, then after the Big Bang (even picoseconds after) it is infinite. $\endgroup$
    – G. Smith
    Oct 9, 2019 at 23:01
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    $\begingroup$ "So if a photon got to the edge of the universe what would happen?" - Related if not a duplicate: What happens when a photon reaches the “edge” of the universe? $\endgroup$ Oct 10, 2019 at 0:02
  • $\begingroup$ I'm sorry I didn't know that had been asked. $\endgroup$
    – T. Fisher
    Oct 10, 2019 at 0:06

1 Answer 1


The universe was not transparent to light until about 380,000 years after the Big Bang, because it was a hot ionized plasma. However, hypothetically, if a photon existed shortly after the Big Bang and happened to hit nothing ever, no matter how unlikely, then it would still be traveling in space. Of course, this photon would be dramatically reshifted and become a radio wave. Photons of Cosmic Microwave Background emitted 380,000 years after the Big Bang have redshifted by now approximately by 1,000 times. A photon existing since earlier conceptually would be redshifted much stronger and be difficult if even possible to detect.


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