<p>Contracting objects can and do exhibit a measurable red shift, and this has actually been measured.</p>

<p>For example, <a href="http://en.wikipedia.org/wiki/Cepheid_variable" rel="nofollow">Cepheid variable</a> are stars that expand on contract in a cycle lasting days to months.</p>

<p><a href="http://adsabs.harvard.edu/full/1998ASPC..135..201B" rel="nofollow">This paper</a> by R. Paul Butler discusses the measurement of Doppler shifts in Cepheid variables. According to the paper, radial velocities as small as 3 meters/second have been measured. Cepheids have been measured to have Doppler velocities of 30 to 1000 meters/second.</p>

<p>The paper was published in 1997. It says that the precision of such measurements had improved by more than two orders of magnitude (a factor of 100) in the preceding decade. I presume the technology has continued to improve in the nearly two decades since then.</p>

CC BY-SA 3.0
2015-01-06T21:17:29.183Z
2015-01-06T21:17:29.183Z
Keith Thompson
<blockquote>
  <p>shouldn't contracting objects show a redshift?</p>
</blockquote>

<p>Yes, they should. But do they, actually? That's the better question.</p>

<p>To have any appreciable redshift, the speed of the object needs to be huge. That will be a very short implosion. Also, there aren't many mechanisms that can accelerate implosions that much.</p>

<p>What I'm saying is - it's theoretically possible, but in practice you won't see it that often, if ever.</p>

<blockquote>
  <p>And if so it should be less towards the edges than towards the centre,
  right?</p>
</blockquote>

<p>Sure. If the object is transparent to its own radiation, then you'd see blueshift from the other side, too. Again, this is very theoretical.</p>

CC BY-SA 3.0
2015-01-06T19:48:01.797Z
"*To have any appreciable redshift, the speed of the object needs to be huge.*" The speed doesn't need to be huge for the redshift to be *detectable*.
Keith Thompson
CC BY-SA 3.0
2014-05-02T02:41:59.540Z
"in practice you won't see that often" . Has anyone ever observed such a thing?
Yashbhatt
CC BY-SA 3.0
2014-05-01T18:06:11.757Z
2014-05-01T18:06:11.757Z
Florin Andrei
<p>If we consider a point on the central part of a contracting object as observed from earth, the point is continuously moving away from us as the object contracts. Moreover, light emerges from a deeper gravitational well as the object contracts. So, shouldn't contracting objects show a redshift? And if so it should be less towards the edges than towards the centre, right?</p>

CC BY-SA 3.0
2015-01-06T19:49:53.740Z
You can ask a followup question by posting a new question. And I think you've misunderstood the OP's question. It doesn't imply anything in particular about distant galaxies; it's about contracting objects, and galaxies typically don't contract significantly.
Keith Thompson
CC BY-SA 3.0
2015-01-07T01:58:41.637Z
I'm not sure how I can be clearer. If you have a question, you can post it *as a question*. If you're asking for clarification, it should be a comment (you'll need a few more reputation points to be able to post comments). And the question was about red shifts of contracting objects, not distant galaxies. Are you under the impression that galaxies contract?
Keith Thompson
CC BY-SA 3.0
2014-05-01T18:00:51.847Z
What do you mean by central part of the contracting object? 'Earth expands'? Where is the light emerging from?
christopherlovell
CC BY-SA 3.0
2014-05-02T02:39:45.147Z
@ChrisLovell I mean as viewed from the earth, a spherical object would appear circular. So, the part we see as the center of that apparent circle would be moving away faster in the direction opposite to us.
Yashbhatt
CC BY-SA 3.0
2014-05-02T03:16:04.193Z
The earth expands??
Py-ser
CC BY-SA 3.0
2014-05-02T03:23:28.287Z
Sorry, typed the question late at night. Made the edits.
Yashbhatt
<light><redshift>
Do contracting objects show redshift?
Yashbhatt
# Do contracting objects show redshift?

(Asked by: username_0 on 2014-05-01)

If we consider a point on the central part of a contracting object as observed from earth, the point is continuously moving away from us as the object contracts. Moreover, light emerges from a deeper gravitational well as the object contracts. So, shouldn't contracting objects show a redshift? And if so it should be less towards the edges than towards the centre, right?

**Tags:** light, redshift
> **Comments:**
> **username_2**: What do you mean by central part of the contracting object? 'Earth expands'? Where is the light emerging from?  
> **username_0**: @ChrisLovell I mean as viewed from the earth, a spherical object would appear circular. So, the part we see as the center of that apparent circle would be moving away faster in the direction opposite to us.  
> **username_3**: The earth expands??  
> **username_0**: Sorry, typed the question late at night. Made the edits.  
> **username_1**: You can ask a followup question by posting a new question. And I think you've misunderstood the OP's question. It doesn't imply anything in particular about distant galaxies; it's about contracting objects, and galaxies typically don't contract significantly.  
> **username_1**: I'm not sure how I can be clearer. If you have a question, you can post it *as a question*. If you're asking for clarification, it should be a comment (you'll need a few more reputation points to be able to post comments). And the question was about red shifts of contracting objects, not distant galaxies. Are you under the impression that galaxies contract?  

---

## 2 answers

### Answer by username_1 on 2015-01-06. Score: 2 | Accepted answer

Contracting objects can and do exhibit a measurable red shift, and this has actually been measured.

For example, [Cepheid variable](http://en.wikipedia.org/wiki/Cepheid_variable) are stars that expand on contract in a cycle lasting days to months.

[This paper](http://adsabs.harvard.edu/full/1998ASPC..135..201B) by R. Paul Butler discusses the measurement of Doppler shifts in Cepheid variables. According to the paper, radial velocities as small as 3 meters/second have been measured. Cepheids have been measured to have Doppler velocities of 30 to 1000 meters/second.

The paper was published in 1997. It says that the precision of such measurements had improved by more than two orders of magnitude (a factor of 100) in the preceding decade. I presume the technology has continued to improve in the nearly two decades since then.


---

### Answer by username_4 on 2014-05-01. Score: 3

> shouldn't contracting objects show a redshift?

Yes, they should. But do they, actually? That's the better question.

To have any appreciable redshift, the speed of the object needs to be huge. That will be a very short implosion. Also, there aren't many mechanisms that can accelerate implosions that much.

What I'm saying is - it's theoretically possible, but in practice you won't see it that often, if ever.

> And if so it should be less towards the edges than towards the centre,
> right?

Sure. If the object is transparent to its own radiation, then you'd see blueshift from the other side, too. Again, this is very theoretical.

> **Comments:**
> **username_0**: "in practice you won't see that often" . Has anyone ever observed such a thing?  
> **username_1**: "*To have any appreciable redshift, the speed of the object needs to be huge.*" The speed doesn't need to be huge for the redshift to be *detectable*.  

---

