Why might light from distant galaxies not reach us?

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  • #26
DaveC426913
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Dave, is it not difficult to maintain scientific veracity without mathematical rigour?
It's not difficult at all. This is the real world; physics is not paralyzed just because mathematical definitions don't fit well.

Remember the context here:

For a photon to have zero frequency, it would need to have a wavelength infinitely long. But a photon whose wavelength is longer than the diameter of the universe is effectively infinite. To-wit: being 100 billion ly long and being infinitely long makes no change in it the property of its frequency; it's still undefined.

So: a wavelength greater than the diameter of the universe is "as close to being infinity as is meaningful".
 
  • #27
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DaveC426913 said:
This is the real world; physics is not paralyzed just because mathematical definitions don't fit well.
I absolutely agree! I still thing there is considerable scope for confusion arising from the variety of ways in which "infinite" is used. Thanks to Cantor, we have an infinity of mathematical infinities, then, as you point out, there are effective infinities, and that's before you get to the infinity (oops!) of cases in which "boundless" would be a better term. I am not suggesting that we should hog-tie ourselves with sterile terminology or mathematical pedantry, only that we should make an effort to ensure that there is a reasonable degree of unity in our usage.
 
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Davec..... Now the layman is totaly out of track. Can be the frequency les than 1 (One) ?!.
May be i need to shut up! Sorry for intrusion.
 
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mquirce, Surely there is no such thing as "intrusion" with a legitimate question. Don't apologise when you have as much right in a thread as anyone else.
 
  • #30
marcus
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The expansion of the universe IS causing them to move away from us faster than the speed of light. It is not merely an optical illusion, but a consequence of an expanding universe.
That's essentially right. I wouldn't go so far as to say that the distant galaxies MOVE away from us faster than c. There are problems with defining distance and motion at cosmic scale so to avoid confusion you have to say what DEFINITION of distance. In this connection (Hubble Law expansion) the commonly used definition is what cosmologists call "proper distance" which basically means imagine you freeze the expansion process at a given moment (to give yourself time to measure) and then use any ordinary means like radar or a giant tape measure. Proper distance is the distance at a specified moment.

The distances to most of the galaxies which we observe today are increasing faster than c, and were already doing that back when they emitted the light we are now getting from them. The "charley" link in my signature explains how the light gets here despite this.

It is a bit confusing to think of that as MOTION because they aren't going anywhere. The distances to them are simply increasing. Ordinary idea of motion is you are traveling towards some destination. But they aren't getting closer to anything. Aside from some petty random local motions what we witness is just pure distance-expansion without any motion towards.

Superluminal expansion of distance is certainly real. In fact it is typical---most of the galaxies we can see today are participating. But this is not the kind of motion relative to local surroundings that would conflict with the Special Rel "speed limit".
Special Rel is a geometry that only applies approximately in local patches over small distances like within one cluster of galaxies. It is the nonexpanding local approximation to General Rel. So its "speed limit" is meaningless at cosmic scales and can be ignored at those scales.

DaveC made a good point, that is based on the discovery that the expansion process is gradually intensifying---the slope of the "size-curve" is getting steeper with time. That leads into a whole other discussion.
Not all light will reach us. Most of the universe is forever beyond our ability to see.

As the universe ages, more and more of the universe will slip outside our observable bubble. It is speculated that billions of years from now, observers looking up (from the tropical paradise of their homeworld Pluto) will see only our Milky Way Galaxy and its close neighbours floating in an otherwise empty universe, and will have no way of understanding how the universe came to be. Cosmology, like archeaology, is a science doomed to attrition.
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Much of what 94J says here is misleading. "Superluminal motion" refers to an illusion we get with jets of luminous matter aimed in our direction. An interesting but comparatively rare optical illusion. This is a different topic from the superluminal expansion of distance we typically encounter over cosmic scales (more exactly beyond redshift 1.7)
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1) firstly, superluminal motion is a phenomenon caused by the observation of matter traveling almost directly TOWARD the observer at relativistic speeds, not AWAY from the observer. due to the nature of the propagation of light, matter moving more or less directly toward us at relativistic speeds will appear to be moving toward us faster than it truly is (sometimes to the point where it appears to exceed the speed of light - this is the phenomenon of superluminal motion). likewise, matter moving more or less away from us at relativistic speeds will appear to be moving away from us slower than it truly is, again, due to the nature of the propagation of light.

2) nothing in our universe actually travels at superluminal speeds with respect to its observer. if the speed of light, c, is observed to be the same by any and every observer regardless of the position and motion of the source, and if nothing can travel faster than c, then it stands to reason that superluminal motion is an optical illusion, which it is.

3) if there are objects so far away from us in our universe that they would seem to be moving away from us at superluminal speeds (these objects would have lie outside our visible universe of ~13.7 billion light years in all directions...in other words, they would have to be more than 13.7 billion light years distant), it is not b/c they are actually moving away from us faster than the speed of light - it is b/c the expansion of the universe is contributing greatly to the illusion of superluminal motion as these objects recede from us at relativistic (but not superluminal) speeds.
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There is a lot of confusion in this post, no time to touch on all the points. Properly speaking, our currently visible universe extends way far beyond 13.7 billion lightyears.

Most of the galaxies which we observe with telescope, if you could stop expansion to give yourself time to measure, are at this moment much farther than 13.7.
And, as said earlier, the distances to them are typically increasing faster than c.
 
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  • #31
DaveC426913
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I absolutely agree! I still thing there is considerable scope for confusion arising from the variety of ways in which "infinite" is used.
Except that the word infinity is not being misused.

Think of this analogy:
That purse's colour is Tuesdayish red.

The word red is not being misused - it is most definitely an appopriate word, considering the context here is colour. However, the modifier creates a new concept, that of 'Tuesdayish red' which, I'll grant, might cause the raising of some eyebrows.


Regardless, if you are still uncomfortable with the concept of 'almost infinity' then mentally substitute the word 'effectively' to make the perfectly reasonable phrase 'effectively infinity'.

Davec..... Now the layman is totaly out of track. Can be the frequency les than 1 (One) ?!.
Precisely, which is why I said undefined.
 
  • #32
cepheid
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It occurred to me that this thread was in need of some expert attention, and so I am glad to see that marcus stepped in. I don't claim to be an expert, but I am able to comment on a couple of the ideas that were raised in this thread:

1. The "Tired Light" Hypothesis

This was proposed as an alternative explanation for the observed redshifts of distant objects (i.e. instead of cosmological expansion). It was not really meant to be something that went along with expansion as sophiecentaur suggested in post #2. In any case, to answer mquirce's question in post #10, the tired light hypothesis has been thoroughly ruled out. There are a couple of fairly obvious reasons why it can't work, including:

i) Scattering at this level would require that distant objects be blurred, with the blurring getting worse the farther out you looked. We don't see this.

ii) The scattering and/or absorption would have to be frequency dependent. We don't observe this. The amount of redshift is independent of the original wavelength (in the rest frame of the emitter). The entire spectrum of the emitter gets shifted by the same amount

iii) More generally, the tired light hypothesis just doesn't explain a broad range of observations as well as big bang cosmology does (e.g. the CMB, primoridal abundances of light elements, large scale structure, Type Ia supernova data etc. More details here: http://en.wikipedia.org/wiki/Tired_light#Criticisms).

That's not to say that there isn't any absorption or scattering of photons along the line of sight at all. It just isn't the dominant effect and can't adequately explain the observed cosmological redshifts.

2. Event Horizons

It may be true that light from some objects is redshifted to the point that we can no longer see it, but that is not the sole answer to the question of why we may never see the light from some distant galaxies. Depending on what cosmological model you use, there may very well be a distance beyond which light will never reach you, at any time in the future. We call this distance the radius of the event horizon, since you as an observer can never have information about events that take place beyond it. This is not the same thing as the horizon that cosmologists speak about more frequently, which is the particle horizon: the maximum distance that light can have travelled so far (since the beginning of the universe). The particle horizon sets the size of the region of everything that you can see now, but it expands to encompass more and more volume as time goes on. The particle horizon scale is therefore the radius of our currently observable universe (which is how you may also have heard of it referred to).

Getting back to event horizons -- again, whether or not one exists depends on your cosmological model (i.e. the parameters of the universe you think you live in). For some models, there is no finite event horizon radius. It turns out to be infinite, meaning that if you could just wait long enough (i.e. forever) then you would eventually see everything. However, I seem to recall (and I will double check this) that our standard lambda-CDM cosmological model does have a finite event horizon. Due to the accelerating expansion, there is a distance beyond which you will never be able to see. As for how this can be reconciled with the argument the OP made about how we should always see photons travelling towards us at c, I suspect it has something to do with general relativity that he/she wasn't taking into account (sorry I don't know enough to explain further). I should point out that the OP failed to take into account that the universe is expanding, so that the distance you would infer from the light travel time does not correspond to the proper distance to the object (neither at the time when the light was emitted, nor at the time when it was received).
 

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