3

Suppose we're in an unknown place in the universe, only knowing that the Milky Way is somewhere is the sky (but of course, younger that it actually is, due to unknown distance light has to travel to reach our position).

Under detailed observation, could we recognize our galaxy?

Our galaxy alone would probably look like just another younger spiral galaxy. But, its vicinity would also include younger Magellanic Clouds, Andromeda galaxy, etc. And maybe other properties that could hint that galaxy being ours.

Note: I severely edited the initial post to keep the core question, as comments and the proposed "duplicate question" kept focusing on the background, rather than the actual question. I know these kind of edits are discouraged as they can invalidate answers, but the only answer so far is not affected by the edit.

OxTaz
  • 197
  • 3
    Probably worthwhile reading: http://physics.stackexchange.com/q/167999/. It explains the why this isn't true and covers (briefly) what would happen if it were true (so possibly a duplicate). – Kyle Kanos Nov 06 '15 at 15:16
  • first read this paper Planck 2015 results. XVIII. Background geometry & topology. –  Nov 06 '15 at 15:42
  • The universe would have to be smaller than observable and closed. – user235504 Nov 06 '15 at 16:41
  • Curiously, amateur astronomer Jesse Lamplugh makes the assertion that we'd not even be able to find earth within our own galaxy if we've left it far enough behind in his 1962 book Rational Universe (largely unlikely you'll find a copy, I managed one because I'm married to his great-granddaughter; it's full of errors & misunderstandings and is probably wrong on this point but thought it was neat nonetheless). – Kyle Kanos Nov 06 '15 at 20:57
  • @KyleKanos Would it be because there would be too much stuff to look at before you could find Earth, or because from some distance, Earth wouldn't be so different than any other piece of rock? – OxTaz Nov 06 '15 at 21:07
  • IIRC, his argument was based on the sheer number of stars & their positions, that we'd not be able to reconstruct the constellations to get us back. – Kyle Kanos Nov 06 '15 at 21:23
  • @KyleKanos Oh... So it's a problem of finding the needle in the haystack, instead of recognizing a needle from something drawn from the haystack (what I was trying to ask). I could edit the question (again) to clarify, but the question is now marked as duplicated (even after my edit), so I guess I shouldn't expect any new answers. – OxTaz Nov 06 '15 at 22:39
  • That was his argument, but it was made before the prevalence & efficacy of computers & a slew of detection techniques that could narrow the window down (including using the 21 cm line to map the galaxy). – Kyle Kanos Nov 06 '15 at 22:42
  • Also, I've voted to reopen this question because I think it's sufficiently different from the other question. Someone else thought so as well, as there are 2 (of 5) reopen votes. – Kyle Kanos Nov 06 '15 at 22:43

1 Answers1

1

The precise shape and medium details of our galaxy are very unknown: we are at the worst possible place to figure them, even if the first survey try to figure roughly our spirals and (vaguely) sub-spirals.

So we won't recognise our galaxy. But we might recognise it's neighborhood (close as you say, or far), and deduced it's us at the middle. Still if you have no idea where in the sky is supposed to be our "echo image" and under which angle, it will probably require to include a huge set of neighborhood constraints to do the matching.

Now, one more huge difficulty: the images will correspond to very different ages, so the look of every galaxies and their relative positions might be totally different. Here, it's depend the size of "your universe" and thus the age difference of each "echo" (and us). At this stage I would be more pessimistic (at least for huge age differences), excepted if you can either have good prediction of candidates, or have specific enough time-invariant characteristics of galaxies (I'm not sure we know enough yet). Or, preferably, both.

Note that as an equivalent situation occuring in our Universe, you might try to find us near the Einstein disk of gravitational lenses :-) (good luck !)

Fabrice NEYRET
  • 3,874