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As a light source in the universe (e.g. sun) emits light in different directions, some of the light emitted reaches places like Earth, and some doesn't. So does the light that reaches the Earth disappear or it is reflected in other directions? And for the light that doesn't reach any place, does it keep on going forever? If it does keep on going forever, will the universe become brighter and brighter? Thanks!

Devsman
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Sharona Mily
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    Search for the Olbers paradox ! – Cham Jul 17 '17 at 01:13
  • @Cham It only tells that the universe is expanding and therefore we don't see a sky full of starlight every night. It didn't tell what I am asking about. – Sharona Mily Jul 17 '17 at 01:17
  • Your question isn't clear enough, then. What's the question ? – Cham Jul 17 '17 at 01:18
  • See the answer at https://physics.stackexchange.com/questions/103856/if-the-universe-didnt-expand-faster-than-light-would-our-nights-brighter-like?rq=1 – Bob Bee Jul 17 '17 at 01:30
  • @Cham I have stated clearly in the description. "Do the light that reach the earth disappear or it is reflected to other different directions after it has reach the earth?" "For the light that didn't reach any place, do they keep on going forever?" "If they do keep on going forever, will the universe become brighter and brighter? " – Sharona Mily Jul 17 '17 at 01:30
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    If Olber's paradox does not answer the question for you, then either your question is not clear or you do not understand Olber's paradox. The paradox answers the question as I read it. – Cort Ammon Jul 17 '17 at 03:58
  • @CortAmmon Maybe I should be asking: Is the sun the only star that contributes to Earth's brightness, is there any other star that scatters light on Earth's atmosphere? If there is, then why isn't the universe become brighter and brighter, since there should be more and more photons coming out and the photons are emitted in all directions from the stars, and therefore some of these photons must reach some other planets and more and more photons are also coming out from stars and emitting. Or because they are absorbed by black holes? – Sharona Mily Jul 17 '17 at 04:27
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    @SharonaMily: That latest comment is pretty much the basis for Olber's paradox, as Cham and Cort both commented. – Kyle Kanos Jul 17 '17 at 10:08
  • @KyleKanos The reason I am asking is because I don't see an answer for my question in Olber's paradox. – Sharona Mily Jul 17 '17 at 19:04

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As you probably know, the light that you mention is an electromagnetic radiation, so it is part of a large spectrum in which $\textit{visible light}$ makes only a small part. Even the sun emits tons of radiation, but not all of it is visible light. When you say about the brightness of the Universe (referring only to the visible light which is radiated by the cosmic objects), you need to be careful here: the brightness of the Universe can be calculated actually if you know how much light each cosmic object emits. Of course if you know the brightness of say Andromeda galaxy, then you probably know enough details about its components (i.e. nebulas, stars and so on). Scientists want to measure the brightness of the Universe ( see here). Probably they will obtain a value which of course will be large, but of course, finite. It can also change in value due to normal cosmic events: supernovae, quasars, death of stars (so it can decrease or increase) but I guess the change in value is NOT noticeable in one day :D

Robert Poenaru
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When any source emits light it may hit any objects like Earth and it will be reflected back in universe and yes the light which doesn't hit any thing will travel forever but that doesn't mean that the universe is going to be brighter because as the universe expands the light with in it gets red shifted and eventually get out of visible spectrum

There is a good video which will help you to get it https://m.youtube.com/watch?v=3tCMd1ytvWg

Mr. Robot
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  • Thank you so much for answering the question. So if the light is increased in wavelength while it travels, if there is another planet with living beings that is located in our "red-shifted space" (those places that are out of our visible site), do they have a "red-shifted space" different from us that makes them cannot see the light on us? Thank you. – Sharona Mily Jul 17 '17 at 21:04
  • Oh yes if any alien life is out there but the light reflected from us travels enough in space that it gets red shifted out of the visible spectrum then they can't see us and this limit up to which we can see in space is know as event horizon – Mr. Robot Jul 18 '17 at 02:40