B What is the recession speed today of the matter which created the CMBR?

[PeterDonis]

It explicitly excludes galaxies in the Local Group from the entry.

But the Local Group is not the largest gravitationally bound system of which our galaxy is a part. The article gives no explanation of why it only considers the Local Group as a gravitationally bound system that doesn't participate in the Hubble flow.

 

[Tanelorn]

Peter, in this case wiki is saying:
"The Universe's expansion causes all galaxies beyond the former Milky Way's Local Group to disappear beyond the cosmic light horizon, removing them from the observable universe".
However according to George, this is incorrect because all the galaxies which can be seen today will always be seen, they just get more and more redshifted.

https://en.wikipedia.org/wiki/Timeline_of_the_far_future
https://arxiv.org/abs/1102.0007

Wiki also claims that "All the c. 47 galaxies of the Local Group will coalesce into a single large galaxy".
They reference the following papers:
https://arxiv.org/abs/astro-ph/9701131
http://www.messier.seds.org/more/local.html

 

[PeterDonis]

"The Universe's expansion causes all galaxies beyond the former Milky Way's Local Group to disappear beyond the cosmic light horizon, removing them from the observable universe".

Yes, but I don't think limiting this claim to the Local Group is correct, since, as I have pointed out several times now, the Local Group is not the largest gravitationally bound system of which our galaxy is a part. The correct statement would be that any galaxies in the same gravitationally bound system as ours will remain visible.

However according to George, this is incorrect because all the galaxies which can be seen today will always be seen, they just get more and more redshifted.

No, these statements are not contradictory, they are just using vague ordinary language to refer to different things in the math. See my post #37.

Wiki also claims that "All the c. 47 galaxies of the Local Group will coalesce into a single large galaxy".

That doesn't change the fact that the Local Group is not the largest gravitationally bound system of which our galaxy is a part. The large galaxy that our Local Group eventually coalesces into, if it does, will still be part of a larger gravitationally bound system.

 

[Tanelorn]

mfb, thanks for the link that the ELT is using direct frequency measuring variation of the expansion rate of the Universe over time. Very interesting.
I wish the same could also be done with the CMBR, but it appears to diffuse and without any sharp lines. Up around 500GHz and around 3GHz the CMBR signals are dropping more rapidly. Perhaps if we subtracted the extremely narrow band average power levels at these two frequencies from each other and then averaged this result over decades?
Or perhaps if the CMBR ever gets sufficiently delayed when gravitationally lensing around a supercluster then we might get chance to see some changes in center frequency over a longer time interval? Again, it is probably still too diffuse and chaotic.Another somewhat related and interesting thing is that the graphics I have seen showing galaxies, clusters and super clusters appear to be as they are when viewed from Earth now, which I think is somewhat deceptive. I have not seen similar graphics for what these same structures and voids would actually look like now, today. We would of course have to simulate the effects of galaxy mergers over time, as well as show how far the same structures would have receded from us until now.

 

[PAllen]

I thought I read various articles which state that only our local group of galaxies remain in our observable universe many 100s Billions of years in the future?

The number of galaxies whose 'present state*' we will ever see (up to the 'infinite future') can shrink. The number of galaxies we see at some point of their history can never shrink.

*present state being defined as same time coordinate in standard cosmological coordinates.

 

[PeterDonis]

The number of galaxies whose 'present state*' we will ever see (up to the 'infinite future') can shrink.

But note that this only applies to galaxies that are comoving. It does not apply to galaxies that are part of the same gravitationally bound system as ours. We will be able to see the "present state" of those indefinitely.

 

[Tanelorn]

This is an example of one such graphic, showing local superclusters, which I believe shows the relative positions of superclusters as viewed from Earth today. I do not know how accurate this is supposed to be. If it was updated to show proper distances, then I assume that the effects of redshift (largest z < 0.07) would not be great enough to significantly change how this graphic would look.

https://en.wikipedia.org/wiki/Supercluster

I believe this recent graphic here shows what I was asking for, which is lower supercluster density over proper distance from us:
https://en.wikipedia.org/wiki/Observable_universe

 

[Tanelorn]

On second thoughts, I find this graphic also misleading because it suggests that the proper observable universe (diameter 93BLys) is not actually homogenous and isotropic in all locations, which is important.

If instead it represents the supercluster light which we currently receive here on Earth and not the proper universe as it is today, then it should probably show the diameter of the observable universe as it was when the light was first emitted, when it had a diameter of 3.924BLyrs (z=16.7 current highest redshift).

However, if it is also intended to show the position of the matter which emitted the CMBR, which we are currently receiving here on earth, then somehow, we should also show the matter at the distance when the CMBR was first emitted (41MLys)?

These graphics can be as misleading as the videos showing the BB as a point like explosion.
The solution is probably to have then and now graphics for both the CMBR and the most distant superclusters. It could then also show both time, distance and recession velocity information for then and now on the same graphics.