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

[Tanelorn]

These are the numbers I passed on:

At t = 379,500 years after the BB, the CMB photons we receive today were located at a distance of 0.04144 Gly from our location with redshift 1092. The matter which emitted those photons had a recession speed of 66.416c at that time. Now, today, that matter is located at a distance of 45.277 Gly from our location with a recession speed of 3.133c.

Also, the light from CEERS-93316 (redshift 16.7) was emitted 235.8 million years after the BB, when it was 2Gly from us and its recession velocity at that time was 6c. Its current distance is 34.7Gly and its current recession velocity Vp is 2.53c.

This all happened in what was our observable universe. Today's observable universe for photons emitted today is called the event horizon and it currently has a radius of 16GLyrs. The observable universe gets larger over Billions of years, but there will be fewer and fewer galaxies within it as the time goes by.

 

[JimJCW]

Also, the light from CEERS-93316 (redshift 16.7) was emitted 235.8 million years after the BB, when it was 2Gly from us and its recession velocity at that time was 6c. Its current distance is 34.7Gly and its current recession velocity Vp is 2.53c.

This all happened in what was our observable universe. Today's observable universe for photons emitted today is called the event horizon and it currently has a radius of 16GLyrs. The observable universe gets larger over Billions of years, but there will be fewer and fewer galaxies within it as the time goes by.

Similar to the result for z = 1091.6 (see Post #10), for z = 16.7, we can get the following information:

At t = 230.329 Myr, the CEERS-93316 photons we receive today were located at a distance of 1.9621 Gly from our location. At that time, CEERS-93316 had a recession speed of 2.40314c. Now it is located at a distance of 34.7291 Gly from our location with a recession speed of 2.40314c. The Hubble parameter at z = 16.7 was 2817.7 (km/s)/Mpc.​

About the event horizon, I copied from a website the following:

The cosmic event horizon is also a sphere centered on us, which is the boundary inside which light, if it is emitted today, may still reach us sometime in the future. If it is emitted outside this horizon, the expansion of the Universe ensures that the light will never reach us.​

​

The current cosmic event horizon has a radius of 16.5803 Gly.

 

[George Jones]

The observable universe gets larger over Billions of years, but there will be fewer and fewer galaxies within it as the time goes by.

No. Anything that is now in our observable universe remains in our observable universe. Some (but not all) things that now are not in observable universe will move into our observable universe in the future; once in, always in.

 

[PeterDonis]

Anything that is now in our observable universe remains in our observable universe.

This is probably a good time to again reference Davis & Lineweaver 2003, which I referenced in post #2:

https://arxiv.org/abs/astro-ph/0310808

Fig. 1, particularly the bottom diagram in it, is helpful in understanding exactly what the various statements being made in this thread mean. The "light cone" in that diagram is the current boundary (in spacetime) of our observable universe, i.e., of the set of events we have observed up to now. The "event horizon" is the boundary in spacetime that includes all of the events we will ever observe. As we move into the future, the "light cone" obviously approaches the "event horizon".

The quoted statement above from @George Jones means that, as we move into the future and the "light cone" moves up the diagram and approaches the "event horizon", more and more comoving worldlines (vertical lines up the diagram) become included in the "light cone", but no comoving worldline ever leaves the "light cone"--once some part of a comoving worldline is in the "light cone", a part of it will always be in the "light cone". However, we see the objects following those comoving worldlines, as they enter the "light cone" region and become part of our observable universe, as they were near the beginning of the universe, and the later and later we see them enter our observable universe, the less and less of their total lifetime we are able to observe (because their comoving worldline intersects the "light cone"--which ultimately reaches the "event horizon"--closer and closer to the bottom of the diagram, i.e., at earlier and earlier times).

Now, consider this statement:

Today's observable universe for photons emitted today is called the event horizon and it currently has a radius of 16GLyrs.

If we look at the horizontal "now" line in the bottom diagram of Fig. 1, and see where it intersects the "event horizon" line, we find that this is at a comoving distance of about 16 GLyrs; and since at the "now" instant, comoving distance and proper distance are equal, this also corresponds to a proper distance of 16 GLyrs. So a photon emitted "now" from a point that far away will be on the event horizon and will never quite reach us; a photon emitted just inside that point will reach us very, very far in the future.

As was noted in earlier posts, the comoving distance to the event horizon decreases with time, but the proper distance to the event horizon increases, though more and more slowly, approaching a fixed value (which depends on the dark energy density aka cosmological constant) asymptotically. So more and more comoving objects go behind the event horizon as time goes on.

So, in appropriate senses, comoving objects are both entering our observable universe (in the very far past, near the Big Bang) and leaving it (passing behind the event horizon so we will never see their more recent history) as we move into the future.

 

[Tanelorn]

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?

 

[Bandersnatch]

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?

There's some nuance to such statements. In terms of signals being able to reach us, it's as George said: once observable, always observable. But those signals, coming from receding galaxies, end up ever more redshifted and ever dimmer. At some point they fade away beyond the capabilities of any assumed detector. The signals are technically still arriving, though.
The key takeaway here is that, from an observer's standpoint, distant galaxies don't disappear from sight because they cross the event horizon - they just fade away beyond detectability over time. This means you can have forever more galaxies in your observable universe, but still have all but the closest bound ones be unobservable. The reason for their disappearance is not them leaving the observable universe, is the point.

 

[PeterDonis]

from an observer's standpoint, distant galaxies don't disappear from sight because they cross the event horizon - they just fade away beyond detectability over time

More precisely, once a galaxy has entered our observable universe (meaning some portion of its worldline is within the "light cone" portion of the spacetime diagram at the bottom of Fig. 1 in Davis & Lineweaver), we will continue to receive light signals from that galaxy forever. However, those signals will be coming from that galaxy's location in the very distant past. If we look at the universe "now", i.e., at a spacelike surface of constant time (a horizontal line in the diagram) instead of the region of spacetime in our past light cone, the fraction of galaxies that are within our event horizon gets smaller, because the "now" surfaces cross the galaxy worldlines at later and later times, when more and more of the worldlines have exited the region of spacetime within our event horizon.

In other words, as I said before, it is possible to describe what is going on, using vague ordinary language, both as "once observable, always observable" and as "more and more galaxies pass beyond our event horizon", depending on how you match up the vague ordinary language to the actual mathematical description.

 

[PeterDonis]

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?

Our local group of galaxies is a gravitationally bound system. With a gravitationally bound system, only the center of mass of the system is comoving; the individual galaxies within the system are not. So the individual galaxies within the system do not recede from each other over time, on average (provided we wait long enough for the bound system to reach equilibrium).

Our local group is part of larger bound systems as well, so unless you give us a specific reference, we have no way of telling whether the articles you speak of were truly claiming that only the local group will remain visible, and if so, why the same logic was not extended to the larger gravitationally bound systems of which our local group is a part.

 

[Tanelorn]

Wikipedia also has a page regarding the far distant future of the Universe.
They mention what they think will be observable at 100 Billion years in the future:
https://en.wikipedia.org/wiki/Timeline_of_the_far_future

Ethan here says that 100B years from now even the closest galaxy will be unobservable:
https://www.forbes.com/sites/starts...ve-background-ever-disappear/?sh=30c192cb40e0

Here is another link which is related:


Perhaps the light from all these distant galaxies can continue to reach us for 100s Billions of years even though the galaxies as they are at that time are not gravitationally bound and have also left the observable universe at that time.

Are there terms for these two different Observable Universes?
The observable universe which we can still see today.
The observable universe where light can still reach us if it was emitted today. Perhaps proper observable universe?

 

[Tanelorn]

Reading through the replies I see there is a disagreement on the recession velocity Vp of CEERS-93316 (redshift 16.7) when the light we see on Earth was first emitted. We have 6c and 2.53c.
2.53c is the same recession velocity as today.
The recession velocity for the matter which created the BB has changed over time though correct?

 

[Bandersnatch]

Ethan here says that 100B years from now even the closest galaxy will be unobservable:

As he says in that article, the closest outside the Local Group. He seemingly contradicts himself a bit later, when he says that nothing but our galaxy will be observable - but in 100 Gy the entire LG might end up coalescing into a single galaxy, so it could very well be technically correct.
But, as was mentioned earlier in this thread, this might actually be an understatement, as the larger Virgo cluster is also thought to be gravitationally bound. It's fuzzy.

Are there terms for these two different Observable Universes?
The observable universe which we can still see today.
The observable universe where light can still reach us if it was emitted today. Perhaps proper observable universe?

That's the aforementioned distinction between the OU and the event horizon.

Reading through the replies I see there is a disagreement on the recession velocity Vp of CEERS-93316 (redshift 16.7) when the light we see on Earth was first emitted. We have 6c and 2.53c.
2.53c is the same recession velocity as today.

If you're talking about post #32, then that's a mistake. JimCW seems to have copied twice the same number from the V_now column in the calculator.

The recession velocity for the matter which created the BB has changed over time though correct?

For any material point comoving with the expansion the following is true: its recession velocity was much higher in the distant past, and has been (much more slowly) getting higher again. The switch moment is somewhere around half the time since the BB.
That's why you get ~6c at emission and ~2.5c at reception for that old galaxy - in such early history of the universe the deceleration was still relatively rapid.

 

[PeterDonis]

Wikipedia

Ethan here says

Neither Wikipedia nor Forbes are good sources for something like this. We have already discussed repeatedly in this thread the fact that gravitationally bound systems are not comoving and the logic that says that comoving objects will eventually become unobservable does not apply to them.

 

[PeterDonis]

Are there terms for these two different Observable Universes?
The observable universe which we can still see today.
The observable universe where light can still reach us if it was emitted today. Perhaps proper observable universe?

I have discussed these issues in several posts now, and referred you twice now to a good paper, Davis & Lineweaver 2003, that discusses them. You don't appear to have read any of those things. Please do so.

 

[Tanelorn]

Thanks all for clarifying the 2.53c misprint. 6c was correct.

Peter, thanks for replies, so this is another wiki inaccuracy it seems.
I have been looking at Lineweaver 2003.
https://arxiv.org/pdf/astro-ph/0310808.pdf
I have also used this one at various times:
https://www.mso.anu.edu.au/~charley/papers/LineweaverDavisSciAm.pdf

Bandersnatch said, "As he says in that article, the closest outside the Local Group. He seemingly contradicts himself a bit later, when he says that nothing, but our galaxy will be observable - but in 100 Gy the entire LG might end up coalescing into a single galaxy, so it could very well be technically correct. But, as was mentioned earlier in this thread, this might actually be an understatement, as the larger Virgo cluster is also thought to be gravitationally bound. It's fuzzy."

Thanks Bander, that is what I was wanting to confirm. In the far distant future, there will be fewer galaxies present in our event horizon observable universe for light leaving those galaxies at that future time. However, even the light from distant CEERS-93316 will always be observable, it will just get more and more redshifted.

I think I need better words or definitions.
e.g. I believe that Vp is proper recession velocity for a galaxy receding from us today. Proper distance D is a correct term also.
The observable universe is everything from all ages which we can still see including CMBR.
The observable universe for light being emitted today is referred to as the event horizon.

I guess I am going to have to work through the meaning of these graphs in my own time, there is a lot going on there.

 

[PeterDonis]

I think I need better words or definitions.

I think it can be very helpful to view things in terms of regions of spacetime instead of locations in space. The "light cone" region of spacetime in the diagrams in Davis & Lineweaver is the region of spacetime from which we, here and now, have received light signals. The "event horizon" region of spacetime is the region of spacetime from which we, here, at any time into the infinite future, can receive light signals. This gets rid of the confusing term "observable universe" altogether.

I guess I am going to have to work through the meaning of these graphs in my own time.

I would recommend looking at the bottom graph in Fig. 1 (the one you didn't show), because it is conformal, i.e., the paths of light rays are always 45 degree lines, so it is easy to tell what events can or cannot send light signals to what other events. That is the key thing to look at to determine what is or is not "observable" to a given observer.

 

[Bandersnatch]

I guess I am going to have to work through the meaning of these graphs in my own time, there is a lot going on there.

You might find this post of mine of some use: https://www.physicsforums.com/threa...increase-bc-of-expansion.912881/#post-5754083
The question it's directly addressing has not been brought up here, but the first 2/3rds or so talk about how to read those graphs. I'm not terribly proud of how it's put together (or the English it's written in), but there's probably some insight to be found in there.

 

[Tanelorn]

Peter this one correct?

 

[PeterDonis]

this one correct?

Yes.

 

[mfb]

Peter, thanks for replies, so this is another wiki inaccuracy it seems.

What would be inaccurate? It explicitly excludes galaxies in the Local Group from the entry.

4. Also, roughly how long would it be before we can start to detect a shift in the CMBR microwave frequency due to increasing redshift? I was just looking for an estimate, it will depend on instrumentation accuracies, but we do have a lot of time for averaging!

For the CMB with its smooth spectrum this will take too long, but for light emitted later ELT (under construction) should be able to see the time-dependence of the redshift within its lifetime. Even the effect of dark energy should be measurable directly by studying the time-dependence at different redshift.

https://academic.oup.com/mnras/article/382/4/1623/1145693

Figure 2 assumes 30 years difference between first and last measurements.

 

[JimJCW]

If you're talking about post #32, then that's a mistake. JimCW seems to have copied twice the same number from the V_now column in the calculator.

You are right. Thanks for pointing out my mistake. Post #10 and #32 use the following LightCoe8 output:

 

[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.