<p>I'm not 100% sure if I'm understanding what your asking, but if your question could be rephrased as "how do we measure distances in an expanding universe?", then I can try to answer that.</p>

<p>Depending on what astronomers measure they use different <em>distance measurements</em>. For example the <strong>comoving distance</strong> between two objects takes into account the expansion, and so does not change with time. If you know the redshift of the galaxy, for example by measuring the spectrum, and have a cosmological model, then you can calculate the comoving distance. Here cosmological model means constraints on the amount of dark energy, matter (both dark and regular) and radiation. The current accepted model is that the universe is around 70% dark energy and 30% matter, most of which is dark (with negligible radiation). The percent here refers to the fraction of energy density. Note that these values change over time, mostly since dark energy is like a property of space and so increases as the universe expands.</p>

<p>For some more info see:
<a href="https://en.m.wikipedia.org/wiki/Distance_measures_(cosmology)" rel="nofollow">https://en.m.wikipedia.org/wiki/Distance_measures_(cosmology)</a></p>

<p>Note that light years is a definite measurement, and so we can use it a valid unit for all distance measures.</p>

CC BY-SA 3.0
2016-02-08T18:35:38.023Z
Thank you for your answer. Comoving distance - is that like the distance between 2 points on an expanding balloon? Is the distance changing between 2 objects changing proportionally? Or are the objects in question expanding themselves so the distance between them is constant? Also my point about the light years was that it was unlike a map on earth where you can travel between the distance between 2 cities (if you control traffic and other factors) at a constant speed in the same time back and forth ex. about 2 hours travel time @ 60 m/hr either way for cities that are 120 miles apart.
signsgeek
CC BY-SA 3.0
2016-02-09T03:06:25.010Z
Yes you have the right idea about comoving distance - it factors in the expansion of the universe (or Hubble Flow) - but this comoving distance does not change over time as the universe expands. As the universe expands the time it would take to travel between these two points would change - there is no way to avoid this. Light years are just a useful unit, because we can say something like 10 thousand lightyears rather than 100 billion billion meters (which is the equivalent SI measurement).
Robbie
CC BY-SA 3.0
2016-02-06T10:55:08.467Z
2016-02-06T10:55:08.467Z
Robbie
<p>Isn't it better to say that the light from the galaxy took 5 billion light years to reach us and it is a very old picture of it? However, if light travelled back to the galaxy at the constant speed and an observer was stationed to record it wouldn't it take longer to reach because of the expansion of the Universe? </p>

<p>Also if it still exists after 10 billion years or not we could not predict. So is the picture of the Universe more of a timeline of events rather than like a conventional map that shows distance between points.</p>

<p>How do current day theories tackle these issues?</p>

CC BY-SA 3.0
2016-02-06T03:14:52.927Z
I don't agree with "unfamiliar" cause we can extrapolate and predict their current location with pretty good accuracy and can make a "current map" fairly well if desired.   Also, galaxies don't cease to exist in 10 billion years because there's regular new star formation, though they probably grow less bright as they grow older, fewer large stars, but more smaller ones over 10 billion years.  They can also merge with near by galaxies.   But I agree with everything else you wrote.
userLTK
CC BY-SA 3.0
2016-02-08T18:53:18.950Z
@userLTK Thank you. I removed the "unfamiliar" as part of an edit and also after reading your answer.
signsgeek
<observable-universe>
Since the Universe is expanding, is it accurate to say that a galaxy is 5 billion light years away?
signsgeek
# Since the Universe is expanding, is it accurate to say that a galaxy is 5 billion light years away?

(Asked by: username_0 on 2016-02-06)

Isn't it better to say that the light from the galaxy took 5 billion light years to reach us and it is a very old picture of it? However, if light travelled back to the galaxy at the constant speed and an observer was stationed to record it wouldn't it take longer to reach because of the expansion of the Universe?

Also if it still exists after 10 billion years or not we could not predict. So is the picture of the Universe more of a timeline of events rather than like a conventional map that shows distance between points.

How do current day theories tackle these issues?

**Tags:** observable-universe

> **Comments:**
> **username_1**: I don't agree with "unfamiliar" cause we can extrapolate and predict their current location with pretty good accuracy and can make a "current map" fairly well if desired.   Also, galaxies don't cease to exist in 10 billion years because there's regular new star formation, though they probably grow less bright as they grow older, fewer large stars, but more smaller ones over 10 billion years.  They can also merge with near by galaxies.   But I agree with everything else you wrote.  
> **username_0**: @username_1 Thank you. I removed the "unfamiliar" as part of an edit and also after reading your answer.  

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## 1 answers

### Answer by username_2 on 2016-02-06. Score: 3

I'm not 100% sure if I'm understanding what your asking, but if your question could be rephrased as "how do we measure distances in an expanding universe?", then I can try to answer that.

Depending on what astronomers measure they use different *distance measurements*. For example the **comoving distance** between two objects takes into account the expansion, and so does not change with time. If you know the redshift of the galaxy, for example by measuring the spectrum, and have a cosmological model, then you can calculate the comoving distance. Here cosmological model means constraints on the amount of dark energy, matter (both dark and regular) and radiation. The current accepted model is that the universe is around 70% dark energy and 30% matter, most of which is dark (with negligible radiation). The percent here refers to the fraction of energy density. Note that these values change over time, mostly since dark energy is like a property of space and so increases as the universe expands.

For some more info see:
<https://en.m.wikipedia.org/wiki/Distance_measures_(cosmology)>

Note that light years is a definite measurement, and so we can use it a valid unit for all distance measures.

> **Comments:**
> **username_0**: Thank you for your answer. Comoving distance - is that like the distance between 2 points on an expanding balloon? Is the distance changing between 2 objects changing proportionally? Or are the objects in question expanding themselves so the distance between them is constant? Also my point about the light years was that it was unlike a map on earth where you can travel between the distance between 2 cities (if you control traffic and other factors) at a constant speed in the same time back and forth ex. about 2 hours travel time @ 60 m/hr either way for cities that are 120 miles apart.  
> **username_2**: Yes you have the right idea about comoving distance - it factors in the expansion of the universe (or Hubble Flow) - but this comoving distance does not change over time as the universe expands. As the universe expands the time it would take to travel between these two points would change - there is no way to avoid this. Light years are just a useful unit, because we can say something like 10 thousand lightyears rather than 100 billion billion meters (which is the equivalent SI measurement).  

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