Does the Opaqueness of Space Increase?

[mysearch]

By way of clarification, this post is intended as a question, not as some thinly disguised alternative theory. So, as possibly an uninformed generalisation, it seems that the verification of most cosmological models must ultimately depend on observations that are predicated on the detection of photons and, in many cases, the assumption of the constancy of speed of light [c]. In this context, [c] is assumed to be the propagation of light through an 'absolute' vacuum.

However, in optics, there is the general acceptance that [c] reduces to [v] when passing through a given material that has a refractive index greater than 1. As I understand it, this change from [c] to [v] is based on light being delayed as photons collide with matter on-route and, in the process, get absorbed and retransmitted. This is assumption, so any clarification would be welcomed.

As such, I was wondering whether the approximate vacuum of space would represent an increasing opaqueness to the propagation of photons with distance. While the probability of light being ‘delayed’ within the solar system would appear to be small, if we increase the scale towards that of a galaxy, it is assumed that this opaqueness might increase as a function of distance. If we expand beyond a galaxy, some cosmology models now suggest that we might encounter dark matter and dark energy.

So are there any estimates for the refractive index of dark matter or dark energy?

If galaxies are thought to be surrounded by dark matter to account for galactic rotation and the interstellar space is filled with dark energy, do we have any idea of the net effect on the propagation velocity [c] on the very large scale of cosmic space, if the effective opaqueness could increase with distance. Ultimately, there seems to be the suggestion that the probability of photons ever reaching us directly from its source must fall to zero, if the net opaqueness increases due to billions of galaxies and billions of light-years of space. So my general questions are:

Are any of these assumptions valid?
Do current cosmology models account for any delays in the propagation of light?

Thanks

 

[Ich]

As I understand it, this change from [c] to [v] is based on light being delayed as photons collide with matter on-route and, in the process, get absorbed and retransmitted. This is assumption, so any clarification would be welcomed.

I don't know exactly how to describe the process, as I don't fully understand it, but it's not real absorption and retransmission. At least, the process has to be virtual, and from my understanding it's not clear that such a description is valid at all.
However, there is no direct proportionality between opacity and refraction index.

As such, I was wondering whether the approximate vacuum of space would represent an increasing opaqueness to the propagation of photons with distance.

Definitely. But the opacity is small, and the refraction index difference even smaller. The difference is many orders of magnitude smaller than the measurement accuracy.

If we expand beyond a galaxy, some cosmology models now suggest that we might encounter dark matter and dark energy.

Dark energy is everywhere, according to the model, and dark matter has its highest consentration in the middle of galaxies.

So are there any estimates for the refractive index of dark matter or dark energy?

Yes: 1. Both entities, whatever they are, do not interact with light.

Ultimately, there seems to be the suggestion that the probability of photons ever reaching us directly from its source must fall to zero, if the net opaqueness increases due to billions of galaxies and billions of light-years of space.

What really happens is that in the early times, the universe was filled with an opaque bright plasma. This is the barrier for our observations further out. We don't see light from further away (=earlier time), and we see a lot of "light" from the end of that period: the CMB.

 

[mysearch]

Thanks for the feedback.

I don't know exactly how to describe the process, as I don't fully understand it, but it's not real absorption and retransmission. At least, the process has to be virtual, and from my understanding it's not clear that such a description is valid at all. However, there is no direct proportionality between opacity and refraction index.

Not sure what you meant by the process being virtual, but let me try to clarify my own use of ‘refractive index’ and ‘opaqueness’. The former was simply being used in the context [n=c/v], where [v] is the speed of light in some media with a refractive index [n]. As such, I was making the assumption that if the effective speed through a non-perfect vacuum might be reduced to [v], it might be said to have an associated value of [n]. Opacity may also be problematic term in this description, because normally it is used to describe how much the light/photons are blocked. I was assuming that the intensity of light reaching Earth is calculated based on the inverse square law, but presumably opacity would be a factor as would any effective ‘delay’ of a photon through absorption and retransmission in transit.

Definitely. But the opacity is small, and the refraction index difference even smaller. The difference is many orders of magnitude smaller than the measurement accuracy.

On the basis of the clarification above, I agree, that opacity and refractive index are different. However, I was simply speculating that both effects would be a function of distance, but presumably, in this case, distance would also be a function of time in an expanding universe?

Dark energy is everywhere, according to the model, and dark matter has its highest concentration in the middle of galaxies.

Thanks for the clarification.

Yes: 1. Both entities, whatever they are, do not interact with light.

Presumably, there would be some gravitational effects with dark matter and via the indirect effect of dark energy driving the expansion of space while the photons were in transit. However, it seems reasonable, based on the assumed properties of dark matter and dark energy, that a photon could not be directly delayed via absorption and retransmission, only delay by the path becoming longer?

I guess I was simply wondering about the probability of a photon propagating directly from a very distant source to Earth and whether this was something the observations that underpin any cosmology model had to take into account. Thanks

 

[Ich]

However, I was simply speculating that both effects would be a function of distance, but presumably, in this case, distance would also be a function of time in an expanding universe?

Rather, time is a function of distance along the light path. The further you see, the older the light, and the younger the universe where it originated. We don't expect much change wrt distance, but things changed dramatically in the young universe.

Presumably, there would be some gravitational effects with dark matter and via the indirect effect of dark energy driving the expansion of space while the photons were in transit.

Of course. One of these effects is accelerated expansion. You can see its signature in redshift - luminosity distance diagrams.

However, it seems reasonable, based on the assumed properties of dark matter and dark energy, that a photon could not be directly delayed via absorption and retransmission, only delay by the path becoming longer?

Yes, that's what one expects. No interaction.

I guess I was simply wondering about the probability of a photon propagating directly from a very distant source to Earth and whether this was something the observations that underpin any cosmology model had to take into account.

They do. For example, if you see a supernova through a dust cloud, you see it fainter. This could be mistaken as an indication of larger distance. You have to sort these things out, they're part of the systematic errors in the measurements. But that's opacity, not refractivity.