[DVBVIEW (OP)]

Cosmological distances are expressed in terms of how long the light takes to reach us because the speed of light is constant in a vacuum.  Yet we know that the speed of light varies considerably if it passes through anything but an empty vacuum and has even been stopped and restarted in laboratory experiments.  Is it inconceivable then that, in 13 billion years (less for nearer objects), the light has never had to pass through anything that would affect its speed?  I put this to the team that managed to stop and restart light and they smugly replied that their artificially created laboratory experiment could never occur naturally.  So their laboratory conditions didn’t comply with natural laws and couldn’t occur elsewhere in 13 billion years?  And that’s without the presence of “dark matter” which is assumed to be 85% of all matter and to pervade space.  Hardly an empty vacuum then.  If there is any question over the time light has taken to reach us, maybe the cosmos is not the size we think it is.

[alancalverd]

So far, no space probe has reported anything much between planets other than a hard vacuum, and even less between the solar system and the rest of the universe. AFAIK the observations of distant phenomena that confirm the predictions of relativity all seem to do so with a high degree of precision using the standard value of c, so for all practical purposes it seems that dark matter or whatever else  might pervade the cosmos, has the same electromagnetic properties as nothing at all.

[Halc]

Cosmological distances are expressed in terms of how long the light takes to reach us

Only in the most layman-oriented pop articles are cosmological distances expressed in light-travel-time.  For instance, the most distant galaxy is GN-z11 which is listed in some such articles as being 13.4 BLY distant because that's how long it took the light to get here.  The figure is only useful for comparing ages.  The actual proper distance from here of event of that light being emitted is about 3 billion LY, and the current proper distance to that galaxy is over 30 BLY.

Yet we know that the speed of light varies considerably if it passes through anything but an empty vacuum and has even been stopped and restarted in laboratory experiments.  Is it inconceivable then that, in 13 billion years (less for nearer objects), the light has never had to pass through anything that would affect its speed?

Not significantly, no.  Maybe it has been delayed a few hours at best.  The vast majority of the distance is entirely intergalactic space, devoid of significant mass density.  If there was another galaxy in between, that middle galaxy would obscure the one behind it and we'd not know about it.

I put this to the team that managed to stop and restart light and they smugly replied that their artificially created laboratory experiment could never occur naturally.  So their laboratory conditions didn’t comply with natural laws and couldn’t occur elsewhere in 13 billion years?

That's right. I don't think there's a light-years sized high-speed cube of glass between us and any of the galaxies they mention. Doesn't sound like an naturally occurring thing.

And that’s without the presence of “dark matter” which is assumed to be 85% of all matter and to pervade space.  Hardly an empty vacuum then.

Dark matter doesn't interact with light, so it doesn't have a refractive index.

So I'd say that light years are an excellent measure since they are a known fixed proper distance, and thus they serve the same purpose as using meters, except yielding more reasonable numbers with fewer digits.  Megaparsecs is also a common unit for cosmological distances.

[Janus]

Cosmological distances are expressed in terms of how long the light takes to reach us because the speed of light is constant in a vacuum.  Yet we know that the speed of light varies considerably if it passes through anything but an empty vacuum and has even been stopped and restarted in laboratory experiments.  Is it inconceivable then that, in 13 billion years (less for nearer objects), the light has never had to pass through anything that would affect its speed?  I put this to the team that managed to stop and restart light and they smugly replied that their artificially created laboratory experiment could never occur naturally.  So their laboratory conditions didn’t comply with natural laws and couldn’t occur elsewhere in 13 billion years?  And that’s without the presence of “dark matter” which is assumed to be 85% of all matter and to pervade space.  Hardly an empty vacuum then.  If there is any question over the time light has taken to reach us, maybe the cosmos is not the size we think it is.

While a light year is defined as the distance light travels in a year in a vacuum. This is just a definition. ( in fact, the year used is not even the calendar year of 365.24219 days but the Julian year of 365.25 days)
.
We do not use the time in which it takes light to reach us from some distant star or galaxy to determine its distance, because we have no way of measuring that.  The light year is just a convenient unit of distance like a mile.  Just like a mile (5280 ft) is more  convenient measure when it come to the distance between towns than ft, a light year(9,460,730,472,580.8 km) is a more convenient measure for the distance between stars or galaxies than kilometers.

We use other ways to measure the distances, parallax*, the known relationship between luminosity and period for Cepheid variables, etc. 

We then just use out predefined "yardstick" of a light year to express those distances.

Also, it must be noted that all such measurements will have a certain amount of measurement error that is taken into account.
So even with our closest neighboring star, our best measurement of its distance can be off by a range of 17 light days or about 5% of a light year.
The Andromeda galaxy is listed as being 2.54 million light years away, give or take 110,000 light years, as another example.

The point being that when making these distance measurements, they do take into account those factors which could create errors in the measurement.

* Parallax measurements also inspired a "yardstick" of its own, the Parsec,  which is the distance at which an object needs to be to show 1 arcsecond of parallax.  (you'll also see stars distances given in mas or milli-arcseconds, which is a direct parallax measure, which then can be converted to a distance measurement if needed.

[evan_au]

we know that the speed of light varies considerably if it passes through anything but an empty vacuum

Factors which change the speed of light tend to do so in a way that depends on wavelength (or frequency, if you prefer).
- One example of this is a rainbow, where the water droplets change the speed of light differently for different wavelengths, splitting it out into different colors.
- This characteristic is called "dispersion".

We can actually measure the dispersion of intergalactic space using Fast Radio Bursts (FRB): A very sharp radio outburst lasting just 2-10ms
- The intergalactic medium slows down the longer wavelengths more than the short wavelengths
- The first FRB discovered shows that light is slowed down by about a quarter of a second between the highest and lowest frequencies they were measuring.
- The distance to most FRBs is unknown, but one has been estimated at 3 billion light-years, which is quite a lot of intergalactic space.
See: https://en.wikipedia.org/wiki/Fast_radio_burst

If you were measuring a more steady light source, say the spectrum of a star or a galaxy, the dispersion would have no effect on the measured spectrum.
- The spectrum is used to estimate red-shift and the expansion of the universe
- It is only the extremely short duration of FRBs that allows the dispersion to be measured. And this implies a very low refractive index for intergalactic space.
- And yet the bending of light in intergalactic space is a real thing - see the Einstein ring:
https://en.wikipedia.org/wiki/Einstein_ring

[Bored chemist]

It sounds better than an exainch (well, that's closer to a parsec, but who cares?

[DVBVIEW (OP)]

Thanks for your interesting comments on how we measure stellar distances - most informative for an amateur like myself and the very reason I joined this forum.
 
Only in the most layman-oriented pop articles are cosmological distances expressed in light-travel-time.  For instance, the most distant galaxy is GN-z11 which is listed in some such articles as being 13.4 BLY distant because that's how long it took the light to get here.  The figure is only useful for comparing ages.  The actual proper distance from here of event of that light being emitted is about 3 billion LY, and the current proper distance to that galaxy is over 30 BLY.

OK - treat me as that layman.  If the time it takes for the light to get here is used to represent a distance travelled by light in that time, could you please explain "comparing ages" and the other two apparent conflicting LY or distance figures.
As for dark matter not interacting with light, I understand that dark matter is only postulated because we need its gravity to explain the otherwise anomalous behaviour of large cosmic bodies - a lot of gravity at that!  We also know that gravity interacts with light by warping space-time.  In any case, making any definitive claims for dark matter when almost nothing is known about it other than it should exist seems doubtful. Keep the posts coming and thanks again.

[Halc]

Thanks for your interesting comments

At the upper right of each post is an action button which can be used to reply to any post, or express thanks for any post you find helpful.
If you use the quote(selected) button, the prior post comes up in format as you see here where the quoted text stands out, as opposed to copying and using italics as you have done.

If the time it takes for the light to get here is used to represent a distance travelled by light in that time, could you please explain "comparing ages"

13.4 billion years is a time, not a distance. It means light emitted by the object when the universe was only about 400 million years old.  Most things took longer to get to that luminous state.
So anyway, if there is a row of light bulbs all at different distances, we see the light from the closest one after only a short time for the light to travel the space between. The further away the light bulb is, the longer it takes for the light to get to you, so the light is older, showing us the light from further in the past when the light bulb was younger. So comparing distances is the same as comparing ages. Old light came from an object that is further away than younger light.

and the other two apparent conflicting LY or distance figures.

The other two figures don't conflict. The galaxy mention is hardly standing still in relation to us, so it is natural to have been close by long ago and very far away today.  At the big bang, the material that would eventually become it and us was all in each other's proximity.

As for dark matter not interacting with light, I understand that dark matter is only postulated because we need its gravity to explain the otherwise anomalous behaviour of large cosmic bodies - a lot of gravity at that!

Yes.  Galaxies don't seem to have enough visible mass to account for the rate at which stuff orbits it, so there must be an awful lot of mass that cannot be seen. Not interacting at all with EM radiation is the only way we'd not see it.  We see dust, and clumps of non-emitting things like dead stars and planets. None of it is enough to account for the mass.

We also know that gravity interacts with light by warping space-time.  In any case, making any definitive claims for dark matter when almost nothing is known about it other than it should exist seems doubtful.

The claims are not definitive, but the alternate models (MoND mostly) do not fit the observations as well. Nobody has produced a better idea.

[DVBVIEW (OP)]

I just read an article in Scholar Journal of Applied Sciences and Research that clarified the meaning of the term "proper distance" rather well so I now understand.  Ain't education a great thing - I'm 77 and I haven't stopped learning yet.  Forgive my layman approach.  My speciality is fine art but my interest is cosmology.  On Dark Matter I'll leave you with the thought that science had space full of "ether" as a transmission medium for a long time before before it was determined that waveforms could also be particles.  No-one had a better theory at that stage of knowledge so they had to invent ether.  Sound familiar?

[Janus]

I just read an article in Scholar Journal of Applied Sciences and Research that clarified the meaning of the term "proper distance" rather well so I now understand.  Ain't education a great thing - I'm 77 and I haven't stopped learning yet.  Forgive my layman approach.  My speciality is fine art but my interest is cosmology.

On Dark Matter I'll leave you with the thought that science had space full of "ether" as a transmission medium for a long time before before it was determined that waveforms could also be particles.  No-one had a better theory at that stage of knowledge so they had to invent ether.  Sound familiar?

That's not what happened.  What happened was that when we started to make observations and do experiments to detect the "ether", they all failed to do so, even when they should of.( IOW experiment X should have produced result B if there were an ether, but it didn't)  It had nothing to do with the fact that light could have particle-like properties.
With dark matter we have also continued to make new a better observations.   Again, it was a matter of  "If dark matter exists, then we should be able to find situations where observation A occurs.  We look for such a situation, and lo and behold, we find observation A. 
So what has happened is that as we have increased our observations, other explanations have been falling to the wayside, while dark matter still hangs in there.  If anything, observations have, to date, strengthened the case for dark matter.

The other difference is that the ether model had other problems, the Ether had to have a a set of almost contradictory properties.
This is not the case with DM,  DM just doesn't interact electromagnetically.* 
The point being is that pretty much everything about "normal matter" that gives it the properties we can measure is due to its ability interact electormagnetically.   We see it because of that,  We can touch and hold it or bounce stuff off of it ( all due to electromagnetic interaction) etc.  Remove that ability, and all that goes away.
And it not as if we don't already have an example of a particle that behaves like dark matter in this way.  Neutrinos are particles that have mass and yet do not interact via electromagnetism.  Billions of them pass right through your body every day without ever interacting with it.

* Which in effect, makes it simpler than what we consider "normal matter".  And generally, the simpler things tend to be more abundant than the more complex ( For example Hydrogen, the simplest of atoms, by far the most abundant in the universe).
So, one would think, that being less complex than "normal" matter, one might expect that dark matter would be more abundant, and this seems to be the case.   
So maybe it isn't dark matter that is odd, but it is  the so called "normal" matter that is the outlier, with its "extra" mode of interaction that allows it to form complex structures like atoms, molecules, etc.   Maybe the only reason we think of it being the "norm" is because we are made from it and we are just being baryon-centric in our viewpoint.