first of all, is the universse infinite or finite?

If finite, then the universe is itself expanding, so if you assume the farthest star in the farthest galaxy emits light. The photon would be able to reach the edge of universe and then what would happen to it would it? Would it travel out and be the cause of expansion or would something else would happen to it.

If Universe is infinite - then I am sorry to ask the question, because light would never be able to reach the edge.

  • 2
    $\begingroup$ What does "outside the universe" mean? $\endgroup$
    – Kyle Kanos
    Commented Mar 19, 2018 at 10:05
  • $\begingroup$ I have clarified myself in the body now for your review ..... $\endgroup$
    – C.Jain
    Commented Mar 19, 2018 at 12:03
  • $\begingroup$ Edited the question too... $\endgroup$
    – C.Jain
    Commented Mar 19, 2018 at 12:05
  • $\begingroup$ Your question is unclear. Why and how would light be the cause of expansion? $\endgroup$ Commented Mar 19, 2018 at 12:16
  • $\begingroup$ Do you mean the apparent energy loss when light travels through space? $\endgroup$ Commented Mar 19, 2018 at 12:23

1 Answer 1


You need to first define what’s ‘outside’ the universe. Since outside the Big Bang is meaningless hence outside the universe is also meaningless. That’s because space and time started at the big bang.

Here’s another way to think of it:

When the universe expanded, the very ‘fabric’ of space time expanded. During the inflationary period the universe accelerated exponentially. Due to this light hasn’t had the time to travel the entire universe. (It is possible for space time to expand faster than light but matter cannot move faster than light. This is in accordance with general relativity). This problem is also known as the Horizon problem in cosmology. If you look at the universe as a whole it appears to be smooth. But light didn’t have the time to reach every end of the universe then how could other ends of the universe know how smooth to be. This problem is active in the physics community as it hasn’t been solved. Thus if light could not reach the far ends of the universe how could light travel outside the universe.

Hope this helps.

  • $\begingroup$ please explain what is inflationary period of the universe....... $\endgroup$
    – C.Jain
    Commented Mar 21, 2018 at 16:45
  • $\begingroup$ @C.Jain inflationary period of the universe is a period of time in the universe when it expanded at an exponential rate. The exact cause is unknown at the atomic level is unknown but the theory predicts in a good way the rate. The universe expanded in volume by as much as 10^78 Times more. Without it you cannot account for the fine tuned structure. $\endgroup$ Commented Mar 21, 2018 at 16:51
  • $\begingroup$ "If you look at the universe as a whole it appears to be smooth". @physics2000 - what does smooth means? $\endgroup$
    – C.Jain
    Commented Mar 22, 2018 at 2:37
  • $\begingroup$ @C.Jain m.esa.int/spaceinimages/Images/2013/03/Planck_CMB see the first photo on the site. What do you see as the most striking thing? If you look overall not on the minute scale but the whole image, it looks so smooth. It looks almost ordered. This is the cosmic microwave background radiation of the universe. Check more on Wikipedia $\endgroup$ Commented Mar 22, 2018 at 2:59

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