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I was wondering about this. I've always heard that (for example) Andromeda being 2.5 million light years away means that what we see is from 2.5 million years ago. However, taking into account the expansion of the universe, shouldn't the distance be greater to correspond to 2.5 million years? For example, if we could see something at the "edge" of the observable universe it would be 46 billion light years away, but of course it couldn't be that old for the universe itself isn't.

What am I missing here? Maybe the difference isn't noticeable for Andromeda or anything in the order of millions instead of billions?

As a bonus question, is there a way to calculate how old what we see is, taking into account the expansion of the universe and knowing how far, in light years, it is from us?

EDIT: I used Andromeda as an example, but it could any x light-year-away object.

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    $\begingroup$ That's maybe not a good example, since Andromeda is heading towards the Milky Way. $\endgroup$
    – PM 2Ring
    Nov 8, 2021 at 10:22
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    $\begingroup$ To add to @PM2Ring's comment, the expansion of the universe doesn't pertain to galaxies that are close to one another. The Milky Way and Andromeda are close enough to one another that gravitational attraction overwhelms the Hubble expansion. $\endgroup$ Nov 8, 2021 at 11:17
  • $\begingroup$ Related: How will Andromeda collide with Milky Way in spite of Hubble's law? $\endgroup$ Nov 8, 2021 at 13:21
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    $\begingroup$ I think I figured out the answer to my question. For Andromeda the answer is no because it is too close for expansion to be significant enough (and also, as someone else pointed out, Andromeda is coming towards us and, in fact, it blueshifts). For objects in the order of billions of light years away, this difference becomes more significant. The problem boils down to the difference between light travel distance and comoving distance (or present proper distance). For example, here you can see the difference: en.wikipedia.org/wiki/GN-z11 $\endgroup$ Nov 9, 2021 at 9:31
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    $\begingroup$ Yes, exactly! Andromeda is too close, but for instance GN-z11 is some 32 billion lightyears away, despite the light having traveled only 13.4 billion years. $\endgroup$
    – pela
    Nov 9, 2021 at 14:01

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i will try to answer it as good as i can. First of all your bonus question. The Problem is that we dont really know the exact rate at which the Universe is expanding. Right now i think to remember its something about 82+- 8 km/s/MPC. So whenever we calculate something with the Hubble-Constant (thats what the expansion rate is called) we have to take in account that it is wrong, but as precise as we can calculate it so we have to be satisfied with that.

Now to your main question: We know that as further away something is from us the faster it is moving away from us. To be precise, galaxies move at speeds proportional to their distance from Earth, away from Earth. The problem is, that Hubble-constant also applies on light rays and as u maybe know, galaxys arent acually moving away in the normal way, rather the space between it expands, if we apply that on light rays, the rays get stretched which leads to a redshift. So in the far future galaxys will appear red.

Hope i could help u understand it, if im wrong at some point, correct me, its already been some time that i habe activly dealt with it.

Greetings

Alex

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  • $\begingroup$ Thanks for your answer. I understood the answer to my bonus question, but I'm not sure I got the rest. What I mean is: if Andromeda is 2.5 Mly away, shouldn't the light we see be a little younger than 2.5M years? As I see it, while that light traveled to us, space between us and Andromeda has expanded, so the distance should have increased. $\endgroup$ Nov 8, 2021 at 10:55
  • $\begingroup$ I think i understood your question wrong, if i now got what u meant right, i can recommend an articel: physics.stackexchange.com/questions/331741/… $\endgroup$
    – Alex
    Nov 8, 2021 at 11:09
  • $\begingroup$ PS. let me now if thats what u meant, if not, tell me and il try to explain it to you if i can. $\endgroup$
    – Alex
    Nov 8, 2021 at 11:11
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In space there are some inconceivably large voids. Here is the biggest:

https://en.wikipedia.org/wiki/Bo%C3%B6tes_void

This void is 30 million light years away. Now imagine you have a future spaceship capable of warp, that is, just 'teleport' somewhere - anywhere.

And you warp to the centre of the above void. Fortunately, your spaceship is a scout so you go to the telescope to see around.

Surprisingly, you will see stuff around you, because light travels anyway. In other words, light did not wait for you to go somewhere to start watching. It is already there as well as it was where you left from; so you observe the void from its centre.

It almost certain that what you observe, the void directly from it's own centre, will not be exactly the same as we observe it from here, earth. How much different it will be, i cannot tell.

I suppose that if that was possible and we compare both observations regarding their differences we would discover many more amazing things, but that is another story.

Now while looking around, your spaceship alarm rings. You check and see another spaceship just 1 light year away from you. Obviously somebody else wanted to go observing too. This however happened 1 year ago. That ship may not even be there now.

Your warp drive is in cool down so you travel there with regular means, thrust. For convenience let's say the spaceship travels deadly close to the speed of light.

So you travel directly to that spaceship with the speed of light. You need 1 year to get there. To be honest here, you need 1 year for an observer of that other ship, for you inside the ship you need less time.

If that other ship is still there and observing exactly at you, it will never see you coming until you get there, because your light image, your light, does not travel faster than you. In other words, you will be there as soon as your first light image reaches that other spaceship. Or, to be more practical, if you stay 1 day observing after 1st warp, need another 1 day to accelerate to speed of light and 1 day to decelerate, that other ship will see you 3 days before you reach it, approximately.

If that other ship left at any time lesser than a year from your arrival at your original warp destination, they will never see you coming even if they observe exactly at your warp destination spot. That is because what you see was 1 year older as that ship was there, but your ship warp 1 light year close so light it emits (its light image) need 1 year to go around 1 year radius!

If that other ship left (with warp, like you) 6 months from that spot you observe, you will stop see it after 6 months travelling with the speed of light towards it, always approximately.

Now assume that ship did not left so it is there when you arrive, travelling at the speed of light. You now have 1 light second distance, and perfectly see each other at a state most humans understand, in 'real time'.

Now assume that they are alien ship, they are frighten and thrust away. For any reason, they do not use or they do not have warp. So they use regular thrust, like you do.

If they travel at the speed of light or greater, one would expect you will never see them, they will just vanish because their light image would never reach you because they travel at the speed of light or faster. The reverse of what happened when you approach them. That is not the case here, because light always travels anchored from any spacetime.

That is, even if that alien spaceship travels 10 times the speed of light with instant acceleration, you will be still able to observe it. You wont be able to know where exactly it is (how much far away from you) at any given time, but with comparing all observed images in time you will be able to understand the ship velocity and therefore, distance, approximately. If the ship keeps going with 10 times the speed for light constantly for 1 year of flight your observations will conclude that it is now 10 light years away from you, while your 'life' at the duration of this phenomenon is only 1 year.

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