# The Naked Scientists Forum

### Author Topic: How long before distant galaxies fade?  (Read 3712 times)

#### Geezer

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##### How long before distant galaxies fade?
« on: 26/09/2009 20:35:53 »
We infer that distant galaxies are accelerating away from us because of observed spectral redshift. Over time, the intensity of the light we receive from these distant objects must also diminish.

How long would would we have to observe a distant galaxy to be able to detect a reduction in light intensity?

#### Geezer

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##### How long before distant galaxies fade?
« Reply #1 on: 26/09/2009 22:45:38 »
Thanks RD. I suppose that means we cannot use reduction in light intensity to confirm recession anytime soon!

Oh well....

Oi! Somebody nicked RD's post.
« Last Edit: 26/09/2009 22:48:36 by Geezer »

#### RD

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##### How long before distant galaxies fade?
« Reply #2 on: 26/09/2009 23:06:03 »
For the intensity of light from a galaxy to drop by 1%, (arbitrary figure), its distance from us would have to increase by 0.5%,
(due to the inverse square law).

The Large Magellanic Cloud  (LMC) is a “nearby” galaxy 160,000 light years from us, moving away at 278 kilometres per second.
So it will travel 8000 light years, (0.5% of present distance),  in  ~8.6 million years.

The LMC is a “close” galaxy, the same 1% drop in intensity would take longer with more distant galaxies.

[someone better check my maths, it's too near my bedtime]
« Last Edit: 26/09/2009 23:13:12 by RD »

#### Geezer

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##### How long before distant galaxies fade?
« Reply #3 on: 27/09/2009 23:35:30 »
Somewhat related:

I understand that distant galaxies appear to be accelerating away form us. Am I correct in assuming we don't actually detect any difference in redshift over time; we infer the acceleration because the more distant galaxies are more redshifted than nearer galaxies?

#### RD

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##### How long before distant galaxies fade?
« Reply #4 on: 28/09/2009 00:15:50 »
I understand that distant galaxies appear to be accelerating away form us.

The further a galaxy is from us the greater it's red-shift, (the faster its recession velocity).
http://en.wikipedia.org/wiki/Hubble%27s_law

Acceleration is a change in velocity.

« Last Edit: 28/09/2009 00:19:18 by RD »

#### Geezer

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##### How long before distant galaxies fade?
« Reply #5 on: 28/09/2009 06:18:30 »
I realize acceleration is the rate of change in velocity (sheesh! )

But I don't think we observe an actual change in the redshift of any galaxy (that was my question), although we can observe that more distant galaxies are receding faster than less distant galaxies. From that, we infer that they must be accelerating, or more accurately, that they did accelerate.

#### Tintin_Triton

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##### How long before distant galaxies fade?
« Reply #6 on: 28/09/2009 07:50:41 »
I realize acceleration is the rate of change in velocity (sheesh! )

But I don't think we observe an actual change in the redshift of any galaxy (that was my question), although we can observe that more distant galaxies are receding faster than less distant galaxies. From that, we infer that they must be accelerating, or more accurately, that they did accelerate.

But, the cosmic Background Radiation is the last bit of light that we can see. How come it is not fading away, or is it?

#### Vern

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##### How long before distant galaxies fade?
« Reply #7 on: 28/09/2009 14:12:18 »
Quote from: Geezer
But I don't think we observe an actual change in the redshift of any galaxy (that was my question), although we can observe that more distant galaxies are receding faster than less distant galaxies. From that, we infer that they must be accelerating, or more accurately, that they did accelerate.
I suspect you are correct. We assume that the red shift we observe in light from distance galaxies is caused by the doppler effect. Based upon that assumption, we notice that some very wierd things seem go be going on. As things get more and more wierd, I would begin to question the original assumpiton.

#### Geezer

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##### How long before distant galaxies fade?
« Reply #8 on: 28/09/2009 18:55:03 »
I suspect you are correct. We assume that the red shift we observe in light from distance galaxies is caused by the doppler effect. Based upon that assumption, we notice that some very wierd things seem go be going on. As things get more and more wierd, I would begin to question the original assumpiton.

Ironically, Hubble had some serious doubts about what he was observing too!

My only point was that because we observe velocities that are constant, we can't directly measure acceleration. We infer that there must have been acceleration and we assume that it continued. (Past tense, because what we observe now existed a very long time ago.)
« Last Edit: 29/09/2009 04:09:22 by Geezer »

#### Geezer

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##### How long before distant galaxies fade?
« Reply #9 on: 29/09/2009 05:06:40 »

But, the cosmic Background Radiation is the last bit of light that we can see. How come it is not fading away, or is it?

Um, er, well, maybe the photons are still whizzing around in all directions? OK - so I'm clueless! Perhaps someone can answer that one.

#### Tintin_Triton

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##### How long before distant galaxies fade?
« Reply #10 on: 29/09/2009 09:55:59 »
I understand that distant galaxies appear to be accelerating away form us. Am I correct in assuming we don't actually detect any difference in redshift over time; we infer the acceleration because the more distant galaxies are more redshifted than nearer galaxies?
Considering the Acceleration being constant, or was constant,  the farther apart the Galaxies the more time the acceleration has been there.
Therefore it is safe to assume that in order to observe a change in Redshift of one galaxy, we must await a huge amount of time.

I guess RD was absolutely correct!

#### Geezer

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##### How long before distant galaxies fade?
« Reply #11 on: 29/09/2009 22:18:22 »

I guess RD was absolutely correct!

Yes, I'm sure he was. The bottom line appears to be that observations of intensity and velocity are, essentially, constant, so we have to make some (not unreasonable) assumptions about the changes that produced the differences that we observe. However reasonable these assumptions are, we should not forget that they are still assumptions (or inferences, conclusions, etc.).

BTW, I'm not challenging these assumptions at all. I'm just trying to get a handle on the evidence we have versus the inferences that we make based on the evidence we have.

#### The Naked Scientists Forum

##### How long before distant galaxies fade?
« Reply #11 on: 29/09/2009 22:18:22 »