I Any direct measure of Universe expansion via galaxy subtended angles?

[hkyriazi]

TL;DR Summary

Is there anyone examining a large sample of galaxies' subtended angles over time, as an independent test of the universe's supposed expansion?

As indicated in the title and summary, I'm wondering if there is any large scale astronomical effort to assess directly the universal spatial expansion assumed by the Doppler interpretation of the redshift/distance relationship, by measuring individual galaxy subtended angles over time. The predicted decreases in size over time should be small, to be sure, but with a large enough sample size and good enough telescopes, may be measurable over a 5-, 10-, or 20-year period.

 

[phinds]

... the universe's supposed expansion?

Are you seriously questioning the expansion of the universe?

 

[hkyriazi]

Are you seriously questioning the expansion of the universe?

I suppose I am. A Doppler shift is one interpretation of the redshift/distance relationship. Others are possible. I'd be happy to see any direct confirmation of the expansion, not just one based on individual galaxy subtended angle measurements over time.

 

[phinds]

I suppose I am.

Amazing. You are seriously suggesting that all of the thousands of scientists who have confirmed it in various way have no idea what they are doing but YOU know the truth. You REALLY might want to rethink that.

 

[Vanadium 50]

Are you seriously questioning the expansion of the universe?

That's the thing you are questioning? My question for for @hkyriazi would be "Are you seriously asking if we can look at the sky today and look at it again tomorrow and tell that the galaxies are slightly smaller?"

 

[phinds]

That's the thing you are questioning? My question for for @hkyriazi would be "Are you seriously asking if we can look at the sky today and look at it again tomorrow and tell that the galaxies are slightly smaller?"

Well he did say up to 20 years

 

[PeterDonis]

I'm wondering if there is any large scale astronomical effort to assess directly the universal spatial expansion assumed by the Doppler interpretation of the redshift/distance relationship, by measuring individual galaxy subtended angles over time.

Of course there is. What you describe is called "angular size distance" or "angular diameter distance" in cosmology and is one of the primary observational parameters used to constrain possible models of the universe.

https://en.wikipedia.org/wiki/Angular_diameter_distance#Angular_size_redshift_relation

 

[PeterDonis]

The predicted decreases in size over time should be small, to be sure, but with a large enough sample size and good enough telescopes, may be measurable over a 5-, 10-, or 20-year period.

Actually, such time periods are far too short compared to the age of the universe to show measurable change in angular sizes of individual galaxies. But we can simply look at different galaxies with widely varying redshifts and widely varying distances from us, and therefore look back through a time span of billions of years.

 

[PeterDonis]

A Doppler shift is one interpretation of the redshift/distance relationship. Others are possible.

Others are logically possible, but they have been considered over decades and ruled out based on observational evidence.

 

[Bandersnatch]

The predicted decreases in size over time should be small, to be sure, but with a large enough sample size and good enough telescopes, may be measurable over a 5-, 10-, or 20-year period.

IIRC, expansion dominates observed motion of galaxies at scales >500 MPc. Let's say we want to use one such galaxy, picking one of the closest viable ones (let's say it's exactly at 500 MPc distance). The recessional velocity predicted by Hubble law should be ~35 000 km/s. Over 20-year time span, that's total receded distance of approx. $22*10^{12} km$, which is a grand total of 2 light years and change. This in turn means 1/750 000 000 change in distance and a corresponding change in the angular diameter of a diffuse object whose size is measured in arcseconds or fractions thereof.
This is not the kind of precision that's currently achievable.

I'd be happy to see any direct confirmation of the expansion

 

[Buzz Bloom]

Summary: Is there anyone examining a large sample of galaxies' subtended angles over time, as an independent test of the universe's supposed expansion?
As indicated in the title and summary, I'm wondering if there is any large scale astronomical effort to assess directly the universal spatial expansion assumed by the Doppler interpretation of the redshift/distance relationship, by measuring individual galaxy subtended angles over time.

Hi hkyriazi:

I am very surprised that no one has brought up the point I make below.

Galaxies do not expand in the manner that astronmers know that the universe expands. That is, stars in a galaxy to not move apart (due according to the phenomenon of the Hubble constant) with a speed proportional to distance. Objects in the universe that are gravitationally bound to each other do not speed away from each other proportionately to distance.

Regards,
Buzz

 

[PeterDonis]

Galaxies do not expand in the manner that astronmers know that the universe expands.

That's right, and that's why the angular size of galaxies is a useful datum--because their actual physical size can be expected to be stable.

 

[hkyriazi]

IIRC, expansion dominates observed motion of galaxies at scales >500 MPc. Let's say we want to use one such galaxy, picking one of the closest viable ones (let's say it's exactly at 500 MPc distance). The recessional velocity predicted by Hubble law should be ~35 000 km/s. Over 20-year time span, that's total receded distance of approx. $22*10^{12} km$, which is a grand total of 2 light years and change. This in turn means 1/750 000 000 change in distance and a corresponding change in the angular diameter of a diffuse object whose size is measured in arcseconds or fractions thereof.
This is not the kind of precision that's currently achievable.View attachment 246661

Thanks, Bandersnatch. Refreshing to get a calm and to-the-point response. Do you think current technology could make accurate enough measurements, and look at enough galaxies (there are, after all, quite a few of them) over that time period, to detect such tiny changes?

 

[hkyriazi]

Hi hkyriazi:

I am very surprised that no one has brought up the point I make below.

Galaxies do not expand in the manner that astronmers know that the universe expands. That is, stars in a galaxy to not move apart (due according to the phenomenon of the Hubble constant) with a speed proportional to distance. Objects in the universe that are gravitationally bound to each other do not speed away from each other proportionately to distance.

Regards,
Buzz

I'd assumed that galaxies are fairly stable in size, otherwise it'd be a confounding variable. The size change I was asking about is that due to them (galaxies not in our local cluster) being continually further away from us with time.

 

[Bandersnatch]

Do you think current technology could make accurate enough measurements, and look at enough galaxies (there are, after all, quite a few of them) over that time period, to detect such tiny changes?

I don't think so. If I'm juggling the numbers right, it's like trying to discern which object is larger - a 100 000 light-year diameter galaxy, or a 100 000.0001 light-year one, all from a distance of 150 000 000 light-years. That's a $10^{-9}$ of an arc second difference (or thereabouts). The diffraction limit on resolution of Earth-spanning radio interferometry is on the order of $10^{-6}$ arcsec.
How many galaxies you look at shouldn't matter.

 

[Vanadium 50]

it's like trying to discern which object is larger - a 100 000 light-year diameter galaxy, or a 100 000.0001 light-year one

More to the point, galaxies don't have a boundary that sharp - a part per billion. Heck, the sun doesn't have a boundary that sharp (around a meter).

 

[Vanadium 50]

Arguably, even the Earth doesn't have a boundary that sharp! (A quarter-inch)

 

[Bandersnatch]

More to the point, galaxies don't have a boundary that sharp - a part per billion. Heck, the sun doesn't have a boundary that sharp (around a meter).

I was thinking, if you had that kind of resolving power, you could pick some arbitrary features in the galaxy to serve as the boundary. Individual stars even.

 

[metastable]

I suppose I am. A Doppler shift is one interpretation of the redshift/distance relationship. Others are possible.

Others are logically possible, but they have been considered over decades and ruled out based on observational evidence.

By "others" are we referring to gravitational redshift? @PeterDonis are you saying all other logically possible interpretations have been ruled out?

https://www.physicsforums.com/threa...e-moving-away-from-a-black-hole.973395/page-2
@PeterDonis ^I will refer to the above thread in which you offered the following observation:

If the separation distance is not so small, then the ships will cover a large enough region of spacetime during the flight time of any particular photon that they cannot be treated as all being at rest in a single inertial frame during that flight time. In that case, gravitational redshift will be observable.

 

[Vanadium 50]

you could pick some arbitrary features in the galaxy to serve as the boundary. Individual stars even.

I think stars are moving 1-2 orders of magnitude too fast for that.

 

[hkyriazi]

Amazing. You are seriously suggesting that all of the thousands of scientists who have confirmed it in various way have no idea what they are doing but YOU know the truth. You REALLY might want to rethink that.

If I'm wrong, please do correct me, but from my study of the question, all the other "confirmations" are based on hypotheses that are simply *consistent* with it--for example, the baryon acoustic oscillations hypothesis to explain the CMB heterogeneity, and hot nucleosynthesis. I'm looking for direct, rather than circumstantial (interpretation-dependent), evidence of a universal expansion.

 

[hkyriazi]

How many galaxies you look at shouldn't matter.

How so? If one does a plot of, say, 1,000 galaxy subtended angles measured over time, one might expect to see a trend (significant correlation) even if individual measurements are too imprecise to say.

 

[hkyriazi]

By "others" are we referring to gravitational redshift? @PeterDonis are you saying all other logically possible interpretations have been ruled out?

FWIW, I assumed he meant Zwicky's "tired light" hypothesis, from 1929 IIRC.

 

[George Jones]

FWIW, I assumed he meant Zwicky's "tired light" hypothesis, from 1929 IIRC.

And "tired light" theories are disfavoured by observations. Gravitational time dilation in expanding universe models predicts that the rate at which photons leave a distant source is larger than the rate at which photons are received here. This leads to observable effects that are not predicted by tired light theories.

Steven Weinberg, in his 2008 grad-level book "Cosmology", writes
"For instance, one important difference between "tired light" theories and the conventional big bang theory is that in the conventional theory all rates at the source are decreased by a factor (1+z)−1, while in tired light theories there is no such slowing down. One rate that is slowed down at large redshifts in the conventional theory is the rate at which photons are emitted by the source. This is responsible for one of two factors of (1+z)−1 in ... apparent luminosity, the other factor being due to the reduction of energy of individual photons. On the other hand, if the rate of photon emission is not affected by the redshift, then in a static Euclidean universe in which photons lose energy as they travel to us, the apparent luminosity of distant source ... will be given by ... only a single factor of 1+z in the denominator.

Lubin and Sandage have used the Hubble Space Telescope to compare the surface brightness of galaxies in three distant clusters ... quite inconsistent with the behavior ... expected in a universe with 'tired light'. ...

In the standard big bang cosmology all rates observed from a distant source are slowed by a factor (1+z)−1, not just the rate at which photons are emitted.This slowing has been confirmed for the rate of decline of light from some of the Type Ia supernovae used by the Supernova Cosmology Project ... "

 

[Vanadium 50]

How so? If one does a plot of, say, 1,000 galaxy subtended angles measured over time, one might expect to see a trend (significant correlation) even if individual measurements are too imprecise to say.

Is that like "we lose a little on every sale but we make it up on volume?"

You need to work out the numbers rather than just spitballing it and making us refute these claims. Using Bandersnatch's numbers, which I believe are too optimistic by a factor of at least 10 and more likely 100, you need ten million galaxies to do even a 30% measurement. And this ignores the problem that galaxies don't have the sharp edge you need.

The HUDF has 0.1% of that and took 4 months to image. So you need 4000 months - about 3 centuries - to collect enough data. Then you need to do it again.

 

[Buzz Bloom]

How so? If one does a plot of, say, 1,000 galaxy subtended angles measured over time, one might expect to see a trend (significant correlation) even if individual measurements are too imprecise to say.

Hi hkyriazi:

Again I wonder why no one has made the comment I make below.

The expansion of the universe does not change any angle between observed distant objects moving away as the universe expands. The only angle changes would be with respect to non-expansion velocities of objects whose kinetic energy motion relative to comoving coordinates have a component transvese to the line of sight.

Regards,
Buzz

 

[phinds]

The expansion of the universe does not change any angle between observed distant objects moving away as the universe expands.

But no one here is talking about the angle BETWEEN objects, we're talking about the angle subtended BY the objects (from one edge to the other)

 

[PeterDonis]

By "others" are we referring to gravitational redshift?

AFAIK nobody has ever tried to explain the redshifts of distant galaxies as gravitational redshift. The reason why nobody would have ever tried is simple: it's obviously a nonstarter. For a system whose spin is slow enough (and galaxies certainly meet that criterion), any gravitational redshift larger than a fairly small amount cannot be produced by an object in a free-fall orbit. (For a non-rotating system, i.e., Schwarzschild spacetime geometry, the limit is $1 + z = \sqrt{3}$, or about $z = 0.73$.) We observe galaxies with redshifts much larger than this limit, which rules out gravitational redshift as an explanation.

are you saying all other logically possible interpretations have been ruled out?

The previous thread of yours that you linked to is not a valid reference; if you link to it again here you will receive a warning. You've had several threads now to discuss that argument and that's enough. Don't hijack someone else's thread with your personal speculation.

I will refer to the above thread in which you offered the following observation

You are quoting me out of context and claiming that what I said in that thread supports what you are trying to say here, when that is not actually the case. Do not make such a claim again or you will receive a warning and a temp ban.

 

[PeterDonis]

Gravitational time dilation in expanding universe models

I don't think "gravitational time dilation" is a good term to use here, since an expanding universe is not stationary and gravitational time dilation in the standard sense of that term is only well-defined in a stationary spacetime.

 

[Buzz Bloom]

But no one here is talking about the angle BETWEEN objects, we're talking about the angle subtended BY the objects (from one edge to the other)

Hi phinds:

I apologize. My Bad. I sort of had the idea in your post for a while, but as I read further, I became confused about what the discussion was about.

Now that I get it, I suggest it might be possible that a measurement across a combination of galaxies in relatively nearby cluster group (but not one that we are gravitationally connected with) might have a large enough angle to notice a angular shrinking over say a hundred years. I would like to research this but the three sources I was able to find quickly are somewhat daunting, and it will take me some time to try to check this out. The main problem is that I have to confirm from the data in the source that the group is far enough away to not have a significant gravitational effect on the Milky Way, and not so far away for the change in angle to be too small.

https://en.wikipedia.org/wiki/List_of_galaxy_groups_and_clusters​

https://sites.psu.edu/astrowright/ruprecht-147-the-oldest-nearby-galactic-cluster/​

http://www.atlasoftheuniverse.com/nearsc.html​

Any advise?

Regards,
Buzz

 

[phinds]

... I suggest it might be possible that a measurement across a combination of galaxies in relatively nearby cluster group

Interesting idea. You should check it against the calculations in posts 10 and 15. I think it likely that the numbers will show that it won't work, but I certainly can't say that for sure.

 

[Justin Hunt]

Forgive my ignorance, but wouldn't "gravitational time dilation" be a factor considering the universe billions of years ago was more dense then universe of today?

But, as for a direct observation, I would imagine a change in red shift over time would be a much better value. An object approaching the edge of the observable universe should have it's red shift increase towards infinity as its light intensity approaches zero. This effect should gradually become more extreme the closer and object gets to the edge of the observable universe. The best part of this type of observation would also be that we could use it to determine how much of the red shift value is due to universe expansion and could help refine the hobble value H0.

 

[phinds]

Forgive my ignorance, but wouldn't "gravitational time dilation" be a factor considering the universe billions of years ago was more dense then universe of today?

No. I don't follow why you think it would.

But, as for a direct observation, I would imagine a change in red shift over time would be a much better value.

yes

An object approaching the edge of the observable universe should have it's red shift increase towards infinity as its light intensity approaches zero.

No, it would not.

This effect should gradually become more extreme the closer and object gets to the edge of the observable universe.

since such an effect (approaching infinity) doesn't exist, that makes no sense.

You seem to have some sort of belief that there is something magical about the "edge" of the observable universe. There is not. The objects at the remotest observable point in the observable universe are receding from us at about 3c and their red shift (about 1100) is not approaching infinity except in the most trivial sense that 10 is bigger than 9

 

[Justin Hunt]

@phinds In order for a distance galaxy to be observed, a photon released from said galaxy has to enter a region of space that is not moving away from us >C due to the expansion of space. This is why we have an observable universe that can change in size depending on the rate of expansion. Considering Scientists believe the universe is expanding at an accelerated rate, out observable universe will continue to shrink.

like traveling through a black holes event horizon, there is nothing special about it for the the object. But, observing said object here on Earth there is a distinction.

 

[George Jones]

Considering Scientists believe the universe is expanding at an accelerated rate, out observable universe will continue to shrink.

Actually, this is a myth. If an object is observable (i.e., on our past lightcone), the object will always be on our past lightcone, even in universes that have accelerating expansion. As time passes, some objects that are not our past lightcone move onto our past lightcone, but, once on, no object moves off our past lighcone, i.e., the amount of stuff in the observable universe increases with time.

 

[phinds]

@phinds In order for a distance galaxy to be observed, a photon released from said galaxy has to enter a region of space that is not moving away from us >C due to the expansion of space.

Sure. So what?

This is why we have an observable universe that can change in size depending on the rate of expansion.

No, it is not the reason.

Considering Scientists believe the universe is expanding at an accelerated rate, out observable universe will continue to shrink.

Not only is that wrong, it is backwards. The OU will continue to increase at a very small rate, both in terms of its size and in terms of the amount of stuff in it. It WILL eventually red-shift beyond our ability to detect it, but so what?

like traveling through a black holes event horizon, there is nothing special about it for the the object. But, observing said object here on Earth there is a distinction.

Again, so what? No one is disputing gravitational time dilation if you go near a black hole.

 

[Bandersnatch]

Forgive my ignorance, but wouldn't "gravitational time dilation" be a factor considering the universe billions of years ago was more dense then universe of today?

If everything is equally denser, then there is no gravity well to climb out of.

But, as for a direct observation, I would imagine a change in red shift over time would be a much better value. An object approaching the edge of the observable universe should have it's red shift increase towards infinity as its light intensity approaches zero. This effect should gradually become more extreme the closer and object gets to the edge of the observable universe. The best part of this type of observation would also be that we could use it to determine how much of the red shift value is due to universe expansion and could help refine the hobble value H0.

This does happen, but not with the edge of the observable universe. You need - much like with black holes - an event horizon.
However, that cosmological event horizon is farther away than what is observable. Signals emitted at the event horizon will reach us in the infinite future and will be infinitely redshifted. The longer we wait, the closer from the event horizon the signals will be reaching us, and the more redshifted they'll be. But it won't be possible to observe infinite redshifts until infinite time has passed.

Furthermore, neither the edge of the observable universe nor the event horizon are at the distance where expansion reaches c (aka Hubble radius). This is due to the changing rate of expansion (i.e. the Hubble constant decreasing), which permits light signals emitted from where expansion is >c to enter regions where it is <c - or, in other words, the Hubble radius is expanding and gradually encompasses previously non-approaching light signals.
There is a nice article in the Insights section of the forum discussing this phenomenon, here: https://www.physicsforums.com/insights/inflationary-misconceptions-basics-cosmological-horizons/. This pedagogical paper covers this as well: https://arxiv.org/abs/astro-ph/0310808 (the light cone graphs are of great help, if you can read them).

 

[Bandersnatch]

The OU will continue to increase at a very small rate, both in terms of its size and in terms of the amount of stuff in it.

While true in terms of the stuff in it (i.e. comoving radius of the past light cone), in terms of its size the rate is not small at all. The size of the OU is the proper distance to the particle horizon, and this increases to infinity in the infinite future.

 

[Justin Hunt]

Except our universe is currently expanding at an accelerated rate. which is covered quite clearly in the insights article Bandersnatch provided above. The OU decreases in those situations. our universe had massive inflation at the beginning and then most of our past history has had the rate of expansion decreasing over time. some time ago until the present, however, the expansion has been accelerating again. It well may be the case that the OU is currently still increasing, I am not expert on this, but if our universe continues to expand at an accelerated rate, the OU will begin shrinking at some point until it is a very lonely universe.

 

[phinds]

Except our universe is currently expanding at an accelerated rate. which is covered quite clearly in the insights article Bandersnatch provided above. The OU decreases in those situations. our universe had massive inflation at the beginning and then most of our past history has had the rate of expansion decreasing over time. some time ago until the present, however, the expansion has been accelerating again. It well may be the case that the OU is currently still increasing, I am not expert on this, but if our universe continues to expand at an accelerated rate, the OU will begin shrinking at some point until it is a very lonely universe.

You are mistaking what CAN be seen with what COULD be seen. The "edge" of the OU is that place where light COULD reach us (and may well do so) but which may have been red-shifted beyond our ability to detect it.

You certainly ARE correct in saying that it's going to be lonely place, since only the local group (at most) will still be visible at some point in the distant future.

 

[Buzz Bloom]

since only the local group (at most) will still be visible at some point in the distant future.

Hi phinds:

Are you saying that galactic groups/clusters in the Virgo Supercluster other than the Local Group are not gravitationally bound to the Local Group?

https://en.wikipedia.org/wiki/Virgo_Supercluster​

I would much appreciate a source clarifying this. The Wikipedia article above does not make this clear.

Regards,
Buzz

 

[phinds]

Hi phinds:

Are you saying that galactic groups/clusters in the Virgo Supercluster other than the Local Group are not gravitationally bound to the Local Group?

https://en.wikipedia.org/wiki/Virgo_Supercluster​

I would much appreciate a source clarifying this. The Wikipedia article above does not make this clear.

Regards,
Buzz

Not positive, but my understanding is that superclusters ARE affected by expansion so only the local group will remain (again, if that).

 

[Bandersnatch]

Except our universe is currently expanding at an accelerated rate. which is covered quite clearly in the insights article Bandersnatch provided above. The OU decreases in those situations.

It doesn't. It doesn't follow that accelerated expansion causes reduction of OU.
To be clear, OU decreasing would mean that the farthest emitters we see today were beyond the farthest emitters observable tomorrow. This never happens in the LCDM model, regardless of accelerated expansion. The fig.1 graphs in the second link show this is the case.
Perhaps you have something else in mind, that you identifying the OU with, and that causes the confusion?

 

[Bandersnatch]

You are mistaking what CAN be seen with what COULD be seen. The "edge" of the OU is that place where light COULD reach us (and may well do so) but which may have been red-shifted beyond our ability to detect it.

No, it's the other way around. The OU is what can be seen right now. It's our past light cone and the edge is its base.
What could reach us (eventually, given enough time, and arbitrary detection abilities) is the event horizon - our past light cone in the infinite future.

 

[Justin Hunt]

@Bandersnatch I suppose I was really talking about the Hubble sphere, but the Hubble sphere has a direct effect on the OU. The OU is just much harder to calculate.

"
Fig. 4 Time sequence of the Hubble sphere (red circle) expanding to overtake a receding galaxy (blue dot) comoving with the expansion in a decelerating universe.

Reference https://www.physicsforums.com/insights/inflationary-misconceptions-basics-cosmological-horizons/
"

"
Fig. 7 Time sequence of the Hubble sphere (red circle) expanding in an inflating universe. The receding galaxy (blue dot), comoving with the expansion, is pulled outside the Hubble sphere. What does this mean for our observable universe? In the decelerating universe, all objects eventually become visible simply because they are slowing down in their recession and the light they emit is able to find its way into our telescopes. Things are not so simple during inflation: it is easy to imagine a situation where the light emitted towards Earth by a distant galaxy gets overwhelmed by the accelerated expansion, dashing any hope of forward progress.

Reference https://www.physicsforums.com/insights/inflationary-misconceptions-basics-cosmological-horizons/
"

Due to technology limitations, even in the first case where we could theoretically see the entire universe at some point, what we see would still be limited to our technology. But, in the second case, objects do move from being inside our OU to outside our OU.

 

[George Jones]

But, in the second case, objects do move from being inside our OU to outside our OU.

Again, this is absolutely incorrect.

 

[Justin Hunt]

did you read the article?
"
Fig. 9 Illustration of the event horizon, ℓ, during de Sitter expansion. The gray circle is the Hubble sphere and the red circle is the event horizon. The leftmost frame is today: a photon is just emitted (yellow dot), the Hubble sphere and event horizon coincide. Subsequent frames show times in the future, as the shell of photons moves outwards. The Hubble sphere does not expand since d˙H=0 during de Sitter expansion. The event horizon, the comoving distance (7 units) that coincides with the Hubble scale today (leftmost frame), moves outwards with the expansion. Even if we included many more frames into the future, the photons never reach the receding event horizon. The boundary set by ℓ carves out a region of spacetime containing events that will never influence us: those events beyond ℓ occurring now and forever into the future. For this reason, ℓ is known as our future event horizon, or simply, our event horizon. Contrast this with the particle horizon, our past event horizon, consisting of those events we will never influence (cf. Figure 6 and surrounding discussion). We can draw something like Figure 6 to depict the event horizon, but we can’t ignore the expansion because the event horizon exists only by virtue of the inflating

Reference https://www.physicsforums.com/insights/inflationary-misconceptions-basics-cosmological-horizons/

"

Things outside the event horizon can not be seen.

"
The accelerated expansion “pushes” all comoving objects outwards towards the event horizon, a conveyor belt to the point of no return. What all of this means, of course, is that the inflating universe is a rather lonely place

Reference https://www.physicsforums.com/insights/inflationary-misconceptions-basics-cosmological-horizons/

"

 

[Bandersnatch]

I suppose I was really talking about the Hubble sphere, but the Hubble sphere has a direct effect on the OU. The OU is just much harder to calculate.

Sure, objects are leaving the Hubble sphere. But you can't just wave a hand and call it OU. Or conflate Hubble sphere with event horizon, as you appear to do in your later post - which are equivalent only in the case of a de Sitter expansion. When the article invokes de Sitter expansion, it presents it as an edge case - physical during inflationary epoch, but not in the current universe - which it clearly states.

But, in the second case, objects do move from being inside our OU to outside our OU.

Only in the sense that the objects move farther than they were at the time when they emitted currently observed light, including towards a distance from which no light they emit >now< could ever be observed (i.e. they move beyond the event horizon). But this is not what OU means. If you can see them, they're inside your observable universe. Their light, emitted during their past histories will remain observable, even as their current states may be forever out of reach.

Here's a graph of LCDM model in comoving coordinates (discussed in the Insights article) from the Lineweaver and Davies paper, linked earlier:

The light cone is our OU. The particle horizon is its inverted equivalent (as discussed in the Insights article).
The dotted lines can be treated as galaxies at indicated redshifts. As time progresses towards infinite future, the light cone ascends the graph towards the event horizon. Its base continually expands to encompass more comoving galaxies, increasing the observable universe.
At any given >now< time there are objects (e.g. a galaxy with redshift z=3) that are >now< beyond the current or any possible light cone in the future. But that same galaxy in the past emitted signals that are on the light cone now, and there will always be signals on light cones in the future that were emitted by that galaxy = the galaxy remains observable forever.

 

[George Jones]

@Justin Hunt , let's look at some examples from @Bandersnatch 's most recen post, using the diagram in this post, and let's assume that the universe is transparent through its history (not just after 380000 years).

The worldlines of galaxies that move with the Hubble flow are vertical lines that can be labeled by their comoving coordinate (numbers on the bottom horizontal line).

Consider galaxy P that has comoving coordinate of 21. We can currently see P, as its worldline intersects our past lightcone at a conformal time of about 31. The current conformal time is about 46, so B is currently in our observable universe, and we see it as it was in the past.The event horizon is our past lighcone in the "infinite future". B intersects this light cone at a conformal time of about 42, Consequently: we can now see B, and we can always see B. B never moves out of our observable universe. B does, however, lose causal contact with us us. At (conformal) t = 40, B can fire a death ray us which, in the future (about t = 60 ), destroys us. At t = 45, B can still fire a death ray, but this death ray will never reach us, even in the "infinite future".

Now consider galaxy Q that has a worldline labeled by comoving coordinate 50. Q's worldline does not intersect our current (t = 46) past lightcone, i.e., Q currently is not in the observable universe. Q does intersect our past light cone, starting at t = 50 for us, i.e., at t = 50, Q changes from being not in our observable universe, to being in our observable universe. For the same reasons as P, once P is in our observable universe, P always remains in our observable universe.

Bottom line: anything that is now in our observable universe remains in our observable. Some (but not all) things that are not now in observable will move into our observable universe in the future; once in, always in.

Int terms of comoving coordinte, the size of our unierse increases as time progresses.

On the diagram, wordlines with comoving coordinate greater than about 62 are never in the observable universe.

Inflation doesn't alter this picture.

 

[Justin Hunt]

Thank you for the additional detail. I am seeing your point now. A part that is still not clear to me though, is that my understanding of OU was all that can at some point be seen while Visible Universe (VU) is all that we can currently see. I am not sure if you are talking about OU above or VU. There are many galaxies in our OU that have given off light that has not had sufficent time to reach us here on Earth and so would be in the OU but not the VU. I did not see much distinction in the article between VU and OU.