<p>When astronomers say something is seen "13 billion years in the past", this is (in most cases) a model-dependent statement, but a perfectly valid statement that tells us how long the light from an object has been travelling to us and which <em>does</em> take account of universal expansion.</p>

<p>What has actually been measured is a redshift, and to convert that into a "look-back" time requires an understanding (and a model) of the expansion history of the universe.</p>

<p>In the actual scientific research publication, as opposed to a press release, the details of what assumptions have been made will be stated. The current uncertainties in the cosmological parameters are small - they would not for example lead to uncertainties in the look-back time of even a few percent.</p>

<p>However, there is always a possibility that our understanding of cosmological expansion is wrong. In particular, the assumption that the cosmological constant is constant might be wrong, in which case there will be systematic shifts in estimated look-back times.</p>

CC BY-SA 4.0
2018-12-25T14:47:35.050Z
Plus one. This *finally* answers https://astronomy.stackexchange.com/questions/28791/distance-to-objects as well.
Alchimista
CC BY-SA 4.0
2018-12-24T23:43:12.790Z
2018-12-24T23:43:12.790Z
ProfRob
<p>Welcome new user,</p>

<p>The simple answer is</p>

<p>1) "yes" scientists are absolutely aware of precisely the difficulty you bring up. Google lots about "comving distance", for instance, to see this issue addressed</p>

<p>2) and YES in caps, you are totally 100% correct that the usual language used in such popular science explanations, is, nonsensical / a total simplification  :)</p>

<p>But what can we do?</p>

<p>Note carefully that the phrase</p>

<p>"space is expanding"</p>

<p>(aka "the expanding universe" "cosmological inflation" etc etc) is basically - an utter, total mess.</p>

<p>Many scientists suggest we should never again say "space expanding" and instead talk about the ......... "metric expansion".  (Unfortunately - absolutely nobody, including those who propose it, has a clue what that means.)</p>

<p>Note for example that in the article referenced here:</p>

<p><a href="https://physics.stackexchange.com/a/223697/10319">https://physics.stackexchange.com/a/223697/10319</a></p>

<p>the authors conclude that all the common phrases we hear in discussing this .. are total claptrap.  :O</p>

<p>So just to repeat,</p>

<p>your broad point (1) yes, scientists are 1000% aware of this and deal with it all the time (read about "comoving distances") and your broad point (2) you are spectacularly correct, the language/meaning used is a shambles.</p>

<p>Merry Xmas!</p>

CC BY-SA 4.0
2018-12-24T18:43:12.900Z
Please explain why saying we see something 13 billion years in the past is "claptrap", because that is the impression you are giving.
ProfRob
CC BY-SA 4.0
2018-12-24T20:22:49.540Z
I was trying to say what @KenG said nicely in a comment !
Fattie
CC BY-SA 4.0
2018-12-24T16:39:04.687Z
2018-12-24T16:39:04.687Z
Fattie
<p>Every time astronomers look "13 billion" years into the past, not only is there more space than there normally would be at that instant which increases from observation to observation, but there was also expanding space for eons prior to that which must offset photons over those billions of years. </p>

<p>I haven't seen astronomers ever actually state they've reconciled that difference whenever they talk about their observations, so </p>

<p>1: Do they actually?</p>

<p>2: How do they?</p>

<p>3: How does that change the reality of their public statements?</p>

CC BY-SA 4.0
2018-12-24T00:19:29.957Z
It gets complicated... Take a look at https://en.m.wikipedia.org/wiki/Distance_measures_(cosmology) for an introduction to the various ways that large distances are specified in cosmology. But for popular articles, it's common to specify distances simply in terms of time, eg if we see light that was emitted 1 billion years ago from some galaxy we say the galaxy is 1 billion lightyears away.
PM 2Ring
CC BY-SA 4.0
2018-12-24T06:47:23.180Z
That's true, so of course it means the language used is completely wrong, because it doesn't account for the expansion.  Astronomers do account for that, but it doesn't get translated into the popular language.  It's almost like saying, "we've told you that the universe is expanding, but we don't think you really understand what that means, so we will ignore it when we talk to you."  That's something of a shame.
Ken G
CC BY-SA 4.0
2018-12-24T18:40:15.457Z
I actually can't see what is wrong with saying we see light that was emitted "13 billion years in the past". So commenters, please provide an answer that explains what the problem is. Note there is no mention of distance in the question.
ProfRob
CC BY-SA 4.0
2018-12-25T15:18:41.507Z
Nothing wrong but this is not usually conveyed to the public either by astronomers or the media and this is right the source of the question(s) like this. @Rob Jeffries.
Alchimista
<observational-astronomy>
When astronomers "look into the past" are they accounting for the expansion of space?
user14554
# When astronomers "look into the past" are they accounting for the expansion of space?

(Asked by: username_0 on 2018-12-23)

Every time astronomers look "13 billion" years into the past, not only is there more space than there normally would be at that instant which increases from observation to observation, but there was also expanding space for eons prior to that which must offset photons over those billions of years.

I haven't seen astronomers ever actually state they've reconciled that difference whenever they talk about their observations, so

1: Do they actually?

2: How do they?

3: How does that change the reality of their public statements?

**Tags:** observational-astronomy
> **Comments:**
> **username_1**: It gets complicated... Take a look at https://en.m.wikipedia.org/wiki/Distance_measures_(cosmology) for an introduction to the various ways that large distances are specified in cosmology. But for popular articles, it's common to specify distances simply in terms of time, eg if we see light that was emitted 1 billion years ago from some galaxy we say the galaxy is 1 billion lightyears away.  
> **username_2**: That's true, so of course it means the language used is completely wrong, because it doesn't account for the expansion.  Astronomers do account for that, but it doesn't get translated into the popular language.  It's almost like saying, "we've told you that the universe is expanding, but we don't think you really understand what that means, so we will ignore it when we talk to you."  That's something of a shame.  
> **username_3**: I actually can't see what is wrong with saying we see light that was emitted "13 billion years in the past". So commenters, please provide an answer that explains what the problem is. Note there is no mention of distance in the question.  
> **username_4**: Nothing wrong but this is not usually conveyed to the public either by astronomers or the media and this is right the source of the question(s) like this. @Rob Jeffries.  

---

## 2 answers

### Answer by username_3 on 2018-12-24. Score: 3 | Accepted answer

When astronomers say something is seen "13 billion years in the past", this is (in most cases) a model-dependent statement, but a perfectly valid statement that tells us how long the light from an object has been travelling to us and which *does* take account of universal expansion.

What has actually been measured is a redshift, and to convert that into a "look-back" time requires an understanding (and a model) of the expansion history of the universe.

In the actual scientific research publication, as opposed to a press release, the details of what assumptions have been made will be stated. The current uncertainties in the cosmological parameters are small - they would not for example lead to uncertainties in the look-back time of even a few percent.

However, there is always a possibility that our understanding of cosmological expansion is wrong. In particular, the assumption that the cosmological constant is constant might be wrong, in which case there will be systematic shifts in estimated look-back times.

> **Comments:**
> **username_4**: Plus one. This *finally* answers https://astronomy.stackexchange.com/questions/28791/distance-to-objects as well.  

---

### Answer by username_5 on 2018-12-24. Score: 0

Welcome new user,

The simple answer is

1) "yes" scientists are absolutely aware of precisely the difficulty you bring up. Google lots about "comving distance", for instance, to see this issue addressed

2) and YES in caps, you are totally 100% correct that the usual language used in such popular science explanations, is, nonsensical / a total simplification :)

But what can we do?

Note carefully that the phrase

"space is expanding"

(aka "the expanding universe" "cosmological inflation" etc etc) is basically - an utter, total mess.

Many scientists suggest we should never again say "space expanding" and instead talk about the ......... "metric expansion". (Unfortunately - absolutely nobody, including those who propose it, has a clue what that means.)

Note for example that in the article referenced here:

<https://physics.stackexchange.com/a/223697/10319>

the authors conclude that all the common phrases we hear in discussing this .. are total claptrap. :O

So just to repeat,

your broad point (1) yes, scientists are 1000% aware of this and deal with it all the time (read about "comoving distances") and your broad point (2) you are spectacularly correct, the language/meaning used is a shambles.

Merry Xmas!

> **Comments:**
> **username_3**: Please explain why saying we see something 13 billion years in the past is "claptrap", because that is the impression you are giving.  
> **username_5**: I was trying to say what @KenG said nicely in a comment !  

---

