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This is a question about the properties of the expansion of the universe. I can't say it any better than: If we observe a primordial galaxy that existed soon after the Big Bang, does it follow that the same galaxy, at roughly the same number of years after the big as we are today, can observe our galaxy as it was soon after the Big Bang. I only mean theoretically. Please ignore the galaxy merger issue and issue of new stars since that time, if the question can be constructed logically.

Lucy Meadow
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  • That's correct. So if someone in the Andromeda galaxy with a powerful enough telescope were to look at Earth right now, they would see homo habilis roaming the planet. – lemon Mar 04 '15 at 12:34

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Yes.

The Milky Way is a very old galaxy, having formed roughly half a billion years after the Big Bang. So if we observe a galaxy that has a redshift of ~10, we are looking back in time to approximately this epoch, so an alien astronomer in that galaxy observing the Milky Way today would see it redshifted by the same factor, and would observe it in the process of forming.

If we observe a galaxy at redshift ~0.5, we are looking 5 billion years back in time, so an alien astronomer in that galaxy would see the Milky Way as it looked when it was roughly 8 billion years old, and with a magically powerful telescope. it would be able to see our Sun in the process of forming.

This does not really have anything to do with time symmetry. The light that leaves the distant galaxy and the light that leaves the Milky Way at the same time in each other's directions simply travel through space, meet each other halfway (without interacting) and reaches the other galaxy at the same time.

pela
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  • I would also like to remove my minus, but your statement that this does not really have anything to do with time symmetry, is not sure. I don't know whether it's wrong. We had a talk in the chat room, about the geometry of the time-space. It is not guaranteed whether galaxies that we see, also see us. Well, I got an explanation about the expanding universe and I understood it, but I have difficulty in explaining by myself. That each of the observers sees the past of the other, would be clear, if each of them indeed would see the other. ACuriousMind told me that it isn't trivial. – Sofia Mar 04 '15 at 21:49
  • I never learnt general relativity, but I understood from what was explained in the room, that with an expending universe we may have problems - probably not with galaxies close to us, but with more distant ones. – Sofia Mar 04 '15 at 21:51
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If the Maxwell equations are valid up to the Galaxy of which you talk, and all the time, then the answer is "yes", because these equation are symmetric in time. That means, the trip of the light from the galaxy to us, is reversible in time, i.e. occurs also in reverse.

It's simple to show that the time symmetry is equivalent with rolling the movie in inverse order, i.e. the path of the light is the same if we take the target for source and the source for target. This is one of the laws of the geometrical optics.

EDIT: About the fact that each one of us, the observers in the galaxy, and the observer in our galaxy, would see the past of the other, it is true.

Sofia
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  • Thank you for answering my question. Especially for the Maxwell lead :o) – Lucy Meadow Mar 04 '15 at 13:01
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    In Maxwell's equations weren't valid in THAT galaxy, it would mean that one place in the Universe were profoundly different from another, something that very few people would find plausible. This doesn't really have anything to do with time symmetry. – pela Mar 04 '15 at 13:07
  • @pela I really don't understand what you say. Maxwell's equations guarantee us the time-symmetry of the light-path. If these eqs. aren't valid, who guarantees the time symmetry? So, please before giving minuses, I'd suggest you to ask people about your doubt, and not vice-versa. – Sofia Mar 04 '15 at 14:09
  • @pela I didn't say that the Maxwell eqs. are valid *in* that galaxy, but all up to that Galaxy, and if not clear, then, also, all the time. – Sofia Mar 04 '15 at 14:27
  • I do not think that the time-reversibility of electromagnetic phenomena is exactly what's relevant here. The OP presumes that "we" --- that is, presumably, you and I today --- can see some event in the Andromeda galaxy. You refer to someone in the Andromeda galaxy seeing homo habilis --- but we are not homo habilis, so you certainly have not reversed the path of a single light ray, nor would you want to, since we usually think of light as moving forward in time (as seen from an arbitrary inertial frame). – WillO Mar 04 '15 at 14:50
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    In fact, I think the OP is imagining that somehow, due to the expansion of the Universe, Andromeda is somehow moving along a worldline that represents a velocity $> c$ (relative to us). Given that, you can certainly draw a Minkowski diagram that suggests that light from Andromeda can reach us but light from us can never reach Andromeda (though if you change coordinates, the same diagram will show the opposite, which suggests that the original premise must be flawed). I do not think that the time-reversibility of Maxwell's equations addresses this notion. – WillO Mar 04 '15 at 14:57
  • @WillO if I am not clear, then I am glad to be notified, s.t. many thanks. But, notice that the OP was glad that I mentioned the time reversibility. Leaving for a moment the time interval needed for the light to travel, which is an issue in itself, the fact that I see you, doesn't guarantee that you also see me. It is the property that the path of the light can be rolled in inverse way, that guarantees that, and it emerged from the time-symmetry of the Maxwell's equations. About the fact that each one of us will see the past of the other, there is no doubt. – Sofia Mar 04 '15 at 14:59
  • @WillO If Andromeda moves with respect to us with a velocity > c, we move with respect to it with a velocity > c. But there is no end to the situations that one can imagine. What if we were inside a black hole and we would see things but we couldn't be seen? Andromeda doesn't move > c. – Sofia Mar 04 '15 at 15:08
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    Assuming the OP is concerned about the expansion of the universe, and the resulting horizon, then the argument about time reversibility does not apply because the universe is not time symmetric. – John Rennie Mar 04 '15 at 15:27
  • @JohnRennie then we should have a problem with the Maxwell's eqs. when considering universe-scale distances, shouldn't we? – Sofia Mar 04 '15 at 15:41
  • @sofia: see Maxwell's equations in curved spacetime – John Rennie Mar 04 '15 at 15:50
  • @JohnRennie GR is not my domain. I looked at the article, but what I saw is formulas that tell me nothing. So, if you try to tell me smth., please tell me in words, would you? – Sofia Mar 04 '15 at 16:23
  • Okay, sorry for being fast with the -1. I removed it, because I realize that I don't really understand why time symmetry should be relevant. Btw., let me just clarify that receding faster than c doesn't prevent us from observing a galaxy. In fact, all galaxies that are farther away than roughly 14 billion lightyears do that. – pela Mar 04 '15 at 20:25
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Generally speaking yes.

But looking in detail not necessarily so... Basically, observers today can play peekaboo with the cosmic event horizon by running away from the primordial object fast enough.

Imagine being in a fast spinning binary, as you approach your object of desire everything is fine and blueshifted. As you recede everything is redshifted, very very very redshift.... oops where did that superluminal galaxy just go to ? Oh, there it is again as I reach aphelion.

Coda: this is in a standard cosmology, which I do not believe to be an accurate representation.

cumfy
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