Are there any galaxies that are partially with the visible universe? It seems unlikely as most of the universe is empty space however our circumference makes it seem likely as well. It would be a rather interesting thing to know about.
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3$\begingroup$ While I don't know enough to confidently put down an answer, I am willing to wager that the answer would be "no". Remember, light has a finite speed, so the further we look, the more we look back in time. At the edge of the observable universe we're seeing the cosmic microwave background. The most distant galaxies we'd see would be ones that are early in their formation. We won't ever just see half a galaxy. $\endgroup$– PhiterosFeb 26, 2017 at 1:56
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$\begingroup$ Ah, of course, It completely slipped my mind that the light we would be receiving would be from the beginning of the galaxy. Forgive me for my question. $\endgroup$– TheGamerPlayzFeb 26, 2017 at 2:20
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$\begingroup$ @Phiteros The observable universe goes further back than the CMB. Its a boundary defined by arbitrary light-speed signals, not just electromagnetic ones. Gravity, for example. Possibly neutrinos can go back before the CMB, too, though since they are massive and move at sub-light speeds, such neutrinos may lag behind the CMB photons. We could choose to talk about the "practically observable universe", which is restricted to those (electromagnetic) signals our (current) technology can actually observe. I'm not sure if this changes the technical possibilities, though. Seems plausible to me. $\endgroup$– zibadawa timmyFeb 26, 2017 at 16:35
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2$\begingroup$ @zibadawatimmy Phiteros' point still remains. The edge of the observable universe (by any means of observation) will be too early in the Universe's formation for any galaxies to exist. There won't be a galaxy at the edge that we can observe. $\endgroup$– zephyrFeb 27, 2017 at 13:48
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$\begingroup$ @zephyr Have our telescopes hit the point where they could resolve a galaxy at a certain distance, but in reality can't because there necessarily couldn't be one? Last I checked we were still getting "astronomers discover oldest galaxy at the edge of the universe" headlines. While headlines aren't exactly science, it seems to suggest that we are well short of being able to observe the strictly furthest galaxies. So that there is a transition of "region we can detect galaxies in" to "region we can't, but they should be there". So why couldn't a galaxy be centered at the border between them? $\endgroup$– zibadawa timmyFeb 27, 2017 at 14:08
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2 Answers
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Perhaps, on rare occasions this will occur. Quoting from Phil Plait in an astronomycast ,
Dr. Plait: Yeah, because the universe is expanding ever faster every day, basically the distance we will be able to see, away from the earth, is going to get closer and closer to us. Anything past the distance will be moving away from us too rapidly to see and so eventually, all that is going to be left is some sort of super galaxy that is us and the Andromeda galaxy and all of the little dwarf galaxy’s, will have all combined into, sort of, one thing. Of course, all of the stars will be dead and then that’s it.
So, you know, if you are a civilization, somehow, that evolves a hundred trillion years from now, the universe is going to look way different than it does now. You are not going to see any other galaxies; it’s just going to be you.
Frazier: You are not going to see any stars, because it will all be dark.
Dr. Plait: That’s right and for a long period of time that is all there is going to be, it’s just going to be that. Even on the scale of what we are talking about, we are talking about a long period of time, now instead of an octillion years, which is what, ten to the twenty seven, now we are starting to talk about ten to the fifty and sixty years, so far into the future, that the present age of the universe, 13 billion years, is so small, that I don’t even have, ten to the sixty divided by ten to the thirteen is ten to the forty seven, that is such a huge number, that there is no ratio that exists today.
On those expansion scales, a galaxy is small (!!) enough that it'll pretty much disappear all at once.
answered Feb 27, 2017 at 15:23
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The observable universe, by definition, is the area of the universe from which light has reached us.
While this boundary could lie in the middle of a galaxy, some of the oldest light (at the edge of the observable universe) could be old enough that it originates from a time before galactic formation. In this case, the cutoff could exist on a semi-congealed mass of gas or whatever structure was present at the time, but there would be no galaxies to speak of.
