Radius of the Observable universe

[Alan Grayson]

It's claimed to be 46 billion LY, but its age is only measured as 13.8 billion years. What I find puzzling about these numbers is that it seems this would imply the rate of expansion must have been greater than c during its lifetime. But AFAICT, the measured rate of expansion using Hubble's law never exceeded light speed before it reached its present size. Can anyone explain this apparent discrepancy? TY, AG

[John Clark]

On Mon, Sep 16, 2024 at 12:53 AM Alan Grayson <agrays...@gmail.com> wrote:

> It's claimed to be 46 billion LY, but its age is only measured as 13.8 billion years. What I find puzzling about these numbers is that it seems this would imply the rate of expansion must have been greater than c during its lifetime.

 

No. It has taken light from a star (or more likely from the CMB) 13.8 billion years to reach us but during those 13.8 billion years the star has not remained stationary relative to us, it has been accelerating away. In fact telescopic observation tells us that 9 billion years ago, when Dark Energy became more dominant than Dark Matter (plus regular matter), the acceleration has been accelerating. This *MIGHT* be because as the universe expands Dark Matter (plus regular matter) becomes more dilute but Dark Energy does not become diluted because it is an intrinsic part of space itself, so the more space you have the more Dark Energy you have.

 John K Clark    See what's on my new list at  Extropolis 

mde

[Alan Grayson]

If an object is receding for 13.8 BY, and the universe is expanding during that time, doesn't that imply a recession velocity faster than c, for the object to be on our observational horizon of 46 BLY? AG 

 

[John Clark]

On Mon, Sep 16, 2024 at 10:40 AM Alan Grayson <agrays...@gmail.com> wrote:

>> It has taken light from a star (or more likely from the CMB) 13.8 billion years to reach us but during those 13.8 billion years the star has not remained stationary relative to us, it has been accelerating away. In fact telescopic observation tells us that 9 billion years ago, when Dark Energy became more dominant than Dark Matter (plus regular matter), the acceleration has been accelerating. This *MIGHT* be because as the universe expands Dark Matter (plus regular matter) becomes more dilute but Dark Energy does not become diluted because it is an intrinsic part of space itself, so the more space you have the more Dark Energy you have.

> If an object is receding for 13.8 BY, and the universe is expanding during that time, doesn't that imply a recession velocity faster than c, for the object to be on our observational horizon of 46 BLY? AG 

Yes and that is a clear violation of Special Relativity, however that was not Einstein's last word on the subject, that came 10 years later with General Relativity.  Einstein still says matter, energy and information cannot travel through space faster than light, BUT space itself is free to expand at any speed, including much faster than light.

 John K Clark    See what's on my new list at  Extropolis  

mfl

[Jesse Mazer]

The Scientific American article "Misconceptions About The Big Bang" by Charles Lineweaver and Tamara Davis at https://www.mso.anu.edu.au/~charley/papers/LineweaverDavisSciAm.pdf (distilled from their more technical review 'Expanding Confusion' at https://arxiv.org/abs/astro-ph/0310808 ) covers this question on p. 42-43, along with other common misconceptions:

"Running to Stay Still

the idea of seeing faster-than-light galaxies may sound mystical, but it is made possible by changes in the expansion rate. Imagine a light beam that is farther than the Hubble distance of 14 billion light-years and trying to travel in our direction. It is moving toward us at the speed of light with respect to its local space, but its local space is receding from us faster than the speed of light. Although the light beam is traveling toward us at the maximum speed possible, it cannot keep up with the stretching of space. It is a bit like a child trying to run the wrong way on a moving sidewalk. Photons at the Hubble distance are like the Red Queen and Alice, running as fast as they can just to stay in the same place.

One might conclude that the light beyond the Hubble distance would never reach us and that its source would be forever undetectable. But the Hubble distance is not fixed, because the Hubble constant, on which it depends, changes with time. In particular, the constant is proportional to the rate of increase in the distance between two galaxies, divided by that distance. (Any two galaxies can be used for this calculation.) In models of the universe that fit the observational data, the
denominator increases faster than the numerator, so the Hubble constant decreases. In this way, the Hubble distance gets larger. As it does, light that was initially just outside the Hubble distance and receding from us can come within the Hubble distance. The photons then find themselves in a region of space that is receding slower than the speed of light. Thereafter they can approach us.

The galaxy they came from, though, may continue to recede superluminally. Thus, we can observe light from galaxies that have always been and will always be receding faster than the speed of light. Another way to put it is that the Hubble distance is not fixed and does not mark the edge of the observable universe.

What does mark the edge of observable space? Here again there has been confusion. If space were not expanding, the most distant object we could see would now be about 14 billion light-years away from us, the distance light could have traveled in the 14 billion years since the big bang. But because the universe is expanding, the space traversed by a photon expands behind it during the voyage. Consequently, the current distance to the most distant object we can see is about three times farther, or 46 billion light-years."

On Mon, Sep 16, 2024 at 12:53 AM Alan Grayson <agrays...@gmail.com> wrote:

It's claimed to be 46 billion LY, but its age is only measured as 13.8 billion years. What I find puzzling about these numbers is that it seems this would imply the rate of expansion must have been greater than c during its lifetime. But AFAICT, the measured rate of expansion using Hubble's law never exceeded light speed before it reached its present size. Can anyone explain this apparent discrepancy? TY, AG

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[Alan Grayson]

But when we approach the observable event horizon, the spatial expansion is still increasing less than c, so I don't see how the observational event horizon is 46 BLY. AG 

l

[Alan Grayson]

On Monday, September 16, 2024 at 12:17:45 PM UTC-6 Jesse Mazer wrote:

The Scientific American article "Misconceptions About The Big Bang" by Charles Lineweaver and Tamara Davis at https://www.mso.anu.edu.au/~charley/papers/LineweaverDavisSciAm.pdf (distilled from their more technical review 'Expanding Confusion' at https://arxiv.org/abs/astro-ph/0310808 ) covers this question on p. 42-43, along with other common misconceptions:

"Running to Stay Still

the idea of seeing faster-than-light galaxies may sound mystical, but it is made possible by changes in the expansion rate. Imagine a light beam that is farther than the Hubble distance of 14 billion light-years and trying to travel in our direction. It is moving toward us at the speed of light with respect to its local space, but its local space is receding from us faster than the speed of light. Although the light beam is traveling toward us at the maximum speed possible, it cannot keep up with the stretching of space. It is a bit like a child trying to run the wrong way on a moving sidewalk. Photons at the Hubble distance are like the Red Queen and Alice, running as fast as they can just to stay in the same place.

One might conclude that the light beyond the Hubble distance would never reach us and that its source would be forever undetectable. But the Hubble distance is not fixed, because the Hubble constant, on which it depends, changes with time. In particular, the constant is proportional to the rate of increase in the distance between two galaxies, divided by that distance. (Any two galaxies can be used for this calculation.) In models of the universe that fit the observational data, the
denominator increases faster than the numerator, so the Hubble constant decreases. In this way, the Hubble distance gets larger. As it does, light that was initially just outside the Hubble distance and receding from us can come within the Hubble distance. The photons then find themselves in a region of space that is receding slower than the speed of light. Thereafter they can approach us.

The galaxy they came from, though, may continue to recede superluminally. Thus, we can observe light from galaxies that have always been and will always be receding faster than the speed of light. Another way to put it is that the Hubble distance is not fixed and does not mark the edge of the observable universe.

I don't think this is the consensus view, which is that the Hubble constant IS constant, and galaxies beyond our event horizon will never be seen, if the universe in their region is expanding faster than c. AG 

What does mark the edge of observable space? Here again there has been confusion. If space were not expanding, the most distant object we could see would now be about 14 billion light-years away from us, the distance light could have traveled in the 14 billion years since the big bang. But because the universe is expanding, the space traversed by a photon expands behind it during the voyage. Consequently, the current distance to the most distant object we can see is about three times farther, or 46 billion light-years."

But within the observable universe, space is expanding at a rate less than c. Correct? So the 46 BLY distance doesn't seem right. AG

[John Clark]

On Mon, Sep 16, 2024 at 7:31 PM Alan Grayson <agrays...@gmail.com> wrote:

> when we approach the observable event horizon, the spatial expansion is still increasing less than c,

Yes. Obviously you will not be able to see any stars receding from you faster than c. 

> so I don't see how the observational event horizon is 46 BLY. AG 

If you are 5 billion light years away from me then in one direction you will be able to see stars that I cannot see, but in the opposite direction I will be able to see stars that you cannot see. And If you see the light from a star that has travel for 13.8 billion years before entering your telescope and then you get into a spaceship that travels at 99.999% the speed of light for 13.8 billion years, you will still be nowhere near that star, and if you travel for 46 billion years you will STILL be nowhere near that star, although you will be able to see many stars that I cannot see, but you will not be able to see any of the stars that I, who has remained on the Earth, can currently see.  

> the observable universe, space is expanding at a rate less than c. Correct? So the 46 BLY distance doesn't seem right. 

Even if you forget about General Relativity, and even if you forget that the universe is accelerating, we've known for a century that the universe is expanding, so if you're looking at a star as it was 13.8 billion years ago then even Newton would say that by now that star is much further away than 13.8 billion light years.  

 John K Clark    See what's on my new list at  Extropolis 

mfa

[Alan Grayson]

I agree. But my problem is that 46 BLY seems too large. It seems to imply the expansion rate is greater than c in the observable universe. AG

fa

[Jesse Mazer]

On Mon, Sep 16, 2024 at 7:41 PM Alan Grayson <agrays...@gmail.com> wrote:

On Monday, September 16, 2024 at 12:17:45 PM UTC-6 Jesse Mazer wrote:

The Scientific American article "Misconceptions About The Big Bang" by Charles Lineweaver and Tamara Davis at https://www.mso.anu.edu.au/~charley/papers/LineweaverDavisSciAm.pdf (distilled from their more technical review 'Expanding Confusion' at https://arxiv.org/abs/astro-ph/0310808 ) covers this question on p. 42-43, along with other common misconceptions:

"Running to Stay Still

the idea of seeing faster-than-light galaxies may sound mystical, but it is made possible by changes in the expansion rate. Imagine a light beam that is farther than the Hubble distance of 14 billion light-years and trying to travel in our direction. It is moving toward us at the speed of light with respect to its local space, but its local space is receding from us faster than the speed of light. Although the light beam is traveling toward us at the maximum speed possible, it cannot keep up with the stretching of space. It is a bit like a child trying to run the wrong way on a moving sidewalk. Photons at the Hubble distance are like the Red Queen and Alice, running as fast as they can just to stay in the same place.

One might conclude that the light beyond the Hubble distance would never reach us and that its source would be forever undetectable. But the Hubble distance is not fixed, because the Hubble constant, on which it depends, changes with time. In particular, the constant is proportional to the rate of increase in the distance between two galaxies, divided by that distance. (Any two galaxies can be used for this calculation.) In models of the universe that fit the observational data, the
denominator increases faster than the numerator, so the Hubble constant decreases. In this way, the Hubble distance gets larger. As it does, light that was initially just outside the Hubble distance and receding from us can come within the Hubble distance. The photons then find themselves in a region of space that is receding slower than the speed of light. Thereafter they can approach us.

The galaxy they came from, though, may continue to recede superluminally. Thus, we can observe light from galaxies that have always been and will always be receding faster than the speed of light. Another way to put it is that the Hubble distance is not fixed and does not mark the edge of the observable universe.

I don't think this is the consensus view, which is that the Hubble constant IS constant, and galaxies beyond our event horizon will never be seen, if the universe in their region is expanding faster than c. AG 

Davis and Lineweaver are just reviewing the current consensus view in that article and paper, not suggesting any new physics. In general relativity's cosmological solutions there is a time-dependent "Hubble parameter" whose value at any given cosmological time is called the "Hubble constant" at that time, but which can change over the long term (see the first paragraph of https://lambda.gsfc.nasa.gov/education/graphic_history/hubb_const.html for example). Astrophysicist Ethan Siegel mentions in an article at https://bigthink.com/starts-with-a-bang/hubble-constant-changes-time/ that even in models that don't have accelerating expansion due to the cosmological constant, the Hubble constant still need not be constant in time. He explains this by looking at the first Friedmann equation governing an expanding universe, where a term equivalent to the definition of the Hubble constant is on the left side of the equality and the right side has terms for energy density, global curvature of space, and the cosmological constant. So, in an expanding universe that's spatially flat and has zero cosmological constant, if the energy density is changing as matter/energy becomes more spread out, the term equivalent to the Hubble constant must be changing as well. From the article:

"Even if you had a flat Universe (which means you can eliminate the second term on the right-hand side) and a Universe without a cosmological constant (which would mean eliminating the third term on the right-hand side, too), you’d understand immediately that the Hubble “constant” cannot be a constant in time.

...

In all cases except for a cosmological constant (i.e., dark energy, to the best of our understanding), the energy density changes as the Universe expands.

If the energy density changes, that means the expansion rate changes, too. The Hubble constant is only a constant everywhere in space, as we measure it right now. It’s not a constant in the sense that it changes over time."

Siegel has another article covering a lot of the same issues at https://www.forbes.com/sites/startswithabang/2018/06/29/surprise-the-hubble-constant-changes-over-time/ where he also mentions that it got the name "Hubble constant" because "for generations, the only distances we could measure were close enough that H appeared to be constant, and we've never updated this".

 

What does mark the edge of observable space? Here again there has been confusion. If space were not expanding, the most distant object we could see would now be about 14 billion light-years away from us, the distance light could have traveled in the 14 billion years since the big bang. But because the universe is expanding, the space traversed by a photon expands behind it during the voyage. Consequently, the current distance to the most distant object we can see is about three times farther, or 46 billion light-years."

But within the observable universe, space is expanding at a rate less than c. Correct? So the 46 BLY distance doesn't seem right. AG

Galaxies within the observable universe can be receding faster than c, as mentioned in that Davis/Lineweaver quote earlier, and in their review paper at https://arxiv.org/pdf/astro-ph/0310808 in section 3.3. If this seems like an intuitive contradiction it may help to be more precise about how cosmologists define the term "observable universe": the radius of the observable universe is defined in terms of the *current* proper distance (see https://en.wikipedia.org/wiki/Comoving_and_proper_distances#Uses_of_the_proper_distance on the meaning of 'proper distance' in cosmology) of the most distant objects (at rest relative to the cosmic microwave background radiation) such that if they emitted light towards us at some point in the *past*, the light would have been able to reach us by now. This doesn't necessarily mean that if a galaxy in the observable universe emits light *today* that the light will ever be able to reach us.

One way of visualizing this definition more easily is using the "comoving distance", which is equal to the proper distance at the current time but which is adjusted so that the comoving distance of all objects at rest relative to the CMBR is fixed, i.e. if a galaxy has a proper distance of 9 billion light years today then it had a comoving distance of 9 billion light years in the distant past, say a billion years after the Big Bang, even though its proper distance at that time was much smaller (the 'scale factor' in cosmological equations gives the proportionality between the proper distance to the comoving distance). If you have a graph of various galaxies plotted in terms of the comoving distance, then the size of the observable universe is just the maximum size of our past light cone on this graph--see the last two of the three graphs Fig. 1 on p. 3 of that Davis/Lineweaver paper at https://arxiv.org/pdf/astro-ph/0310808 where the lines labeled "light cone" show our current past light cone which defines the size of the observable universe (the third graph is visually simplest because they use a "conformal" time coordinate which has a varying relation to ordinary proper time, in such a way that all light ray worldlines are 45 degree angles just like in special relativity graphs--on that third graph the left axis shows the conformal time, the right axis shows the proper time). The two graphs with comoving distance also show that the maximum size of our past light cone is identical to the *current* size of our "particle horizon", which is just the future light cone of our location at a point arbitrarily near the Big Bang. So the observable universe can also be defined in terms of the particle horizon (i.e. the current distance to the furthest galaxy that could receive a light signal from our location emitted at some point in the past).

And like I said above, one consequence of these definitions is that just because a galaxy is currently within the observable universe, that does not rule out the possibility that light emitted from the galaxy *today* will never be able to reach us. This is shown by the third conformal graph in Fig. 1, where the definition of conformal time is such that an infinite future proper time is only a finite interval of the conformal time, so the top of the graph shows the maximum distance any given light ray will reach at a proper time of infinity. This means we will never see any events outside our past light cone at infinity, which is labeled our "event horizon" on the graph. If you think of the vertical dotted lines on the graph as worldlines of particular galaxies, you can see there that some of them were at one point within our past "light cone" which has an apex at the current time, but their current location in spacetime (where their worldlines intersect with the horizontal 'now' line) is outside the "event horizon", our past light cone whose apex is at infinite future proper time. So, we will never receive light from those galaxies as they are today, but since we can receive light from them that they emitted in the distant past, their current location is considered part of the "observable universe".

Jesse

 

 

On Mon, Sep 16, 2024 at 12:53 AM Alan Grayson <agrays...@gmail.com> wrote:

It's claimed to be 46 billion LY, but its age is only measured as 13.8 billion years. What I find puzzling about these numbers is that it seems this would imply the rate of expansion must have been greater than c during its lifetime. But AFAICT, the measured rate of expansion using Hubble's law never exceeded light speed before it reached its present size. Can anyone explain this apparent discrepancy? TY, AG

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[Alan Grayson]

On Tuesday, September 17, 2024 at 10:12:53 AM UTC-6 Jesse Mazer wrote:

On Mon, Sep 16, 2024 at 7:41 PM Alan Grayson <agrays...@gmail.com> wrote:

On Monday, September 16, 2024 at 12:17:45 PM UTC-6 Jesse Mazer wrote:

The Scientific American article "Misconceptions About The Big Bang" by Charles Lineweaver and Tamara Davis at https://www.mso.anu.edu.au/~charley/papers/LineweaverDavisSciAm.pdf (distilled from their more technical review 'Expanding Confusion' at https://arxiv.org/abs/astro-ph/0310808 ) covers this question on p. 42-43, along with other common misconceptions:

"Running to Stay Still

the idea of seeing faster-than-light galaxies may sound mystical, but it is made possible by changes in the expansion rate. Imagine a light beam that is farther than the Hubble distance of 14 billion light-years and trying to travel in our direction. It is moving toward us at the speed of light with respect to its local space, but its local space is receding from us faster than the speed of light. Although the light beam is traveling toward us at the maximum speed possible, it cannot keep up with the stretching of space. It is a bit like a child trying to run the wrong way on a moving sidewalk. Photons at the Hubble distance are like the Red Queen and Alice, running as fast as they can just to stay in the same place.

One might conclude that the light beyond the Hubble distance would never reach us and that its source would be forever undetectable. But the Hubble distance is not fixed, because the Hubble constant, on which it depends, changes with time. In particular, the constant is proportional to the rate of increase in the distance between two galaxies, divided by that distance. (Any two galaxies can be used for this calculation.) In models of the universe that fit the observational data, the
denominator increases faster than the numerator, so the Hubble constant decreases. In this way, the Hubble distance gets larger. As it does, light that was initially just outside the Hubble distance and receding from us can come within the Hubble distance. The photons then find themselves in a region of space that is receding slower than the speed of light. Thereafter they can approach us.

The galaxy they came from, though, may continue to recede superluminally. Thus, we can observe light from galaxies that have always been and will always be receding faster than the speed of light. Another way to put it is that the Hubble distance is not fixed and does not mark the edge of the observable universe.

I don't think this is the consensus view, which is that the Hubble constant IS constant, and galaxies beyond our event horizon will never be seen, if the universe in their region is expanding faster than c. AG 

Davis and Lineweaver are just reviewing the current consensus view in that article and paper, not suggesting any new physics. In general relativity's cosmological solutions there is a time-dependent "Hubble parameter" whose value at any given cosmological time is called the "Hubble constant" at that time, but which can change over the long term (see the first paragraph of https://lambda.gsfc.nasa.gov/education/graphic_history/hubb_const.html for example). Astrophysicist Ethan Siegel mentions in an article at https://bigthink.com/starts-with-a-bang/hubble-constant-changes-time/ that even in models that don't have accelerating expansion due to the cosmological constant, the Hubble constant still need not be constant in time. He explains this by looking at the first Friedmann equation governing an expanding universe, where a term equivalent to the definition of the Hubble constant is on the left side of the equality and the right side has terms for energy density, global curvature of space, and the cosmological constant. So, in an expanding universe that's spatially flat and has zero cosmological constant, if the energy density is changing as matter/energy becomes more spread out, the term equivalent to the Hubble constant must be changing as well. From the article:

"Even if you had a flat Universe (which means you can eliminate the second term on the right-hand side) and a Universe without a cosmological constant (which would mean eliminating the third term on the right-hand side, too), you’d understand immediately that the Hubble “constant” cannot be a constant in time.

...

In all cases except for a cosmological constant (i.e., dark energy, to the best of our understanding), the energy density changes as the Universe expands.

If the energy density changes, that means the expansion rate changes, too. The Hubble constant is only a constant everywhere in space, as we measure it right now. It’s not a constant in the sense that it changes over time."

Siegel has another article covering a lot of the same issues at https://www.forbes.com/sites/startswithabang/2018/06/29/surprise-the-hubble-constant-changes-over-time/ where he also mentions that it got the name "Hubble constant" because "for generations, the only distances we could measure were close enough that H appeared to be constant, and we've never updated this".

 

What does mark the edge of observable space? Here again there has been confusion. If space were not expanding, the most distant object we could see would now be about 14 billion light-years away from us, the distance light could have traveled in the 14 billion years since the big bang. But because the universe is expanding, the space traversed by a photon expands behind it during the voyage. Consequently, the current distance to the most distant object we can see is about three times farther, or 46 billion light-years."

But within the observable universe, space is expanding at a rate less than c. Correct? So the 46 BLY distance doesn't seem right. AG

Galaxies within the observable universe can be receding faster than c, as mentioned in that Davis/Lineweaver quote earlier, and in their review paper at https://arxiv.org/pdf/astro-ph/0310808 in section 3.3. If this seems like an intuitive contradiction it may help to be more precise about how cosmologists define the term "observable universe": the radius of the observable universe is defined in terms of the *current* proper distance (see https://en.wikipedia.org/wiki/Comoving_and_proper_distances#Uses_of_the_proper_distance on the meaning of 'proper distance' in cosmology) of the most distant objects (at rest relative to the cosmic microwave background radiation) such that if they emitted light towards us at some point in the *past*, the light would have been able to reach us by now. This doesn't necessarily mean that if a galaxy in the observable universe emits light *today* that the light will ever be able to reach us.

One way of visualizing this definition more easily is using the "comoving distance", which is equal to the proper distance at the current time but which is adjusted so that the comoving distance of all objects at rest relative to the CMBR is fixed, i.e. if a galaxy has a proper distance of 9 billion light years today then it had a comoving distance of 9 billion light years in the distant past, say a billion years after the Big Bang, even though its proper distance at that time was much smaller (the 'scale factor' in cosmological equations gives the proportionality between the proper distance to the comoving distance). If you have a graph of various galaxies plotted in terms of the comoving distance, then the size of the observable universe is just the maximum size of our past light cone on this graph--see the last two of the three graphs Fig. 1 on p. 3 of that Davis/Lineweaver paper at https://arxiv.org/pdf/astro-ph/0310808 where the lines labeled "light cone" show our current past light cone which defines the size of the observable universe (the third graph is visually simplest because they use a "conformal" time coordinate which has a varying relation to ordinary proper time, in such a way that all light ray worldlines are 45 degree angles just like in special relativity graphs--on that third graph the left axis shows the conformal time, the right axis shows the proper time). The two graphs with comoving distance also show that the maximum size of our past light cone is identical to the *current* size of our "particle horizon", which is just the future light cone of our location at a point arbitrarily near the Big Bang. So the observable universe can also be defined in terms of the particle horizon (i.e. the current distance to the furthest galaxy that could receive a light signal from our location emitted at some point in the past).

And like I said above, one consequence of these definitions is that just because a galaxy is currently within the observable universe, that does not rule out the possibility that light emitted from the galaxy *today* will never be able to reach us. This is shown by the third conformal graph in Fig. 1, where the definition of conformal time is such that an infinite future proper time is only a finite interval of the conformal time, so the top of the graph shows the maximum distance any given light ray will reach at a proper time of infinity. This means we will never see any events outside our past light cone at infinity, which is labeled our "event horizon" on the graph. If you think of the vertical dotted lines on the graph as worldlines of particular galaxies, you can see there that some of them were at one point within our past "light cone" which has an apex at the current time, but their current location in spacetime (where their worldlines intersect with the horizontal 'now' line) is outside the "event horizon", our past light cone whose apex is at infinite future proper time. So, we will never receive light from those galaxies as they are today, but since we can receive light from them that they emitted in the distant past, their current location is considered part of the "observable universe".

Jesse

I don't get it, but I'll keep trying. The claim seems to be that a star can be receding from an observer at velocity greater than c, and still be in his observable universe, and this is intelligible by changing the definition of observable universe and Hubble's constant. Is this the claim? TY, AG

[Jesse Mazer]

One could say the definition of Hubble's constant changed, since they initially did think it was constant but then theoretical modeling in general relativity and more distant observations favored the idea of a parameter that could change with time. But I don't think the definition of "observable universe" has changed, I think it always referred to any region of the universe that we can see today, even if we're seeing light that was emitted in the distant past when the proper distance was smaller. Do you just mean it doesn't match the intuitive meaning you would attach to the term? And if so, do you have an alternate preferred definition, like those regions where if a light beam was emitted today we'd be able to see it eventually, even if not for billions of years in the future?

Jesse

 

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[Alan Grayson]

I'm satisfied leaving the definition of Observable Universe fixed, but I can't see how anything can recede at velocity > c and remain within our Observable Universe. And the measured radius of 46 BLY seems too large if the velocity of recession is < c. I will look at your links. AG 

[Alan Grayson]

I think you're claiming that the expansion rate, and hence Hubble's constant, could change over time. So a galaxy we could not see at some time in our past since it WAS receding faster than c, will come into view, that is, be within our Observable Universe if the rate of expansion slows. Correct? I don't find this a radical idea, but nevertheless, a radius of 46 BLY seems way to large, compared to the universe's age of 13.8 BY. AG

[John Clark]

On Wed, Sep 18, 2024 at 2:01 AM Alan Grayson <agrays...@gmail.com> wrote:

>  I can't see how anything can recede at velocity > c and remain within our Observable Universe. 

We can observe a very distant galaxy even though it is now moving away from us faster than the speed of light because we are not observing the galaxy as it is now, we are observing it as it was 13 billion years ago; and back then it was NOT moving away from us faster than the speed of light. Thus even though we can see the galaxy we could NEVER travel to it, not even if we could move at the speed of light, not even in an infinite number of years. You can object to the definition of "observable universe" if you want to but remember we can NOT observe ANYTHING as it is now. It takes a finite amount of time for light to go from the tip of your nose to your eye, so even that observation is in the past.

Our observational horizon is shrinking, in about 1 trillion years we will not be able to see any galaxies except those in our local group, and they would probably all have merged into a single large globular galaxy by then. So if there are any astronomers around in 1 trillion years they will incorrectly conclude what astronomers in the early 20th century concluded, the entire universe consists of just one galaxy surrounded by an infinity of nothingness. That is to say surrounded by an infinite boundless homogeneity. 

 John K Clark    See what's on my new list at  Extropolis 

ius

[Alan Grayson]

On Wednesday, September 18, 2024 at 6:14:21 AM UTC-6 John Clark wrote:

On Wed, Sep 18, 2024 at 2:01 AM Alan Grayson <agrays...@gmail.com> wrote:

>  I can't see how anything can recede at velocity > c and remain within our Observable Universe. 

We can observe a very distant galaxy even though it is now moving away from us faster than the speed of light because we are not observing the galaxy as it is now, we are observing it as it was 13 billion years ago; and back then it was NOT moving away from us faster than the speed of light.

 

It sure the hell was moving away faster than light speed due to Inflation. In fact, it was Inflation that caused the UNobservable universe to come into existence. AG

 

Thus even though we can see the galaxy we could NEVER travel to it, not even if we could move at the speed of light, not even in an infinite number of years. You can object to the definition of "observable universe" if you want to but remember we can NOT observe ANYTHING as it is now. It takes a finite amount of time for light to go from the tip of your nose to your eye, so even that observation is in the past.

Our observational horizon is shrinking, in about 1 trillion years we will not be able to see any galaxies except those in our local group, and they would probably all have merged into a single large globular galaxy by then. So if there are any astronomers around in 1 trillion years they will incorrectly conclude what astronomers in the early 20th century concluded, the entire universe consists of just one galaxy surrounded by an infinity of nothingness. That is to say surrounded by an infinite boundless homogeneity. 

 us

[Jesse Mazer]

But according to that definition, if some object at rest relative in comoving coordinates (i.e. its motion away from us is purely due to expansion of space, so it's at rest in the local CMBR frame), then if it was ever observable at any point in the past, it will be considered part of the "observable universe" forever, even if there is some time after which we can no longer observe any more light from it. Again, "observable universe" just means regions that can be observed by us at *some* time in their history.

Jesse

 

 

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[Alan Grayson]

I think observable universe means what we can observe now, which according to theory will decrease in the future. But your definition suggests any galaxy that might have been observed in the past, will continue to be part of the observable universe even if it goes out of view. I don't think this is correct. AG 

[Alan Grayson]

While it's true that some galaxies we can now view, have already passed beyond our horizon, these will wink out, and the remainder will remain within our event horizon until they also eventually wink out, as long as the universe expands. AG 

[Jesse Mazer]

Do you mean our "event horizon" in the sense I talked about earlier of our past light cone at a time of +infinity, as opposed to our past light cone today? Either way, if part of a galaxy's worldline is within our past light cone at a given time, in relativistic terms we could still be getting some kind of causal signal from it, even if in practice the light (or other causal signals moving at the speed of light like gravitational waves) from sufficiently distant galaxies may be too redshifted to detect with current instruments. Redshift approaches infinity as you approach the Big Bang in terms of when a given signal was emitted, but in the distant future even signals emitted long after the Big Bang will have very large but finite redshifts, so you'd need to be able to detect very long radio waves to "see" them, and if you can't the galaxy has effectively winked out of view.

Jesse

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[Cosmin Visan]

Universe doesn't exist. "Universe" is just an idea in consciousness.

[Alan Grayson]

For me, the Observable universe means just that; the universe we can observe. How that fits into the constraints you define above, I am not sure. But I can say that some galaxies we can observe today have already crossed our horizon, and we are observing their last emissions just before crossing our horizon. But eventually they will wink out if the universe keeps expanding, as will all other galaxies not in our local group. I have no idea why you claim the red shift approaches infinity as we approach the BB, and I don't believe it. And I still don't know why the observed universe has such a large radius, of 46 BLY, which seems to imply the expansion rate must have exceeded light speed during the lifetime of the universe, allegedly 13.8 BY.  AG

[Alan Grayson]

Further, since the expansion of observable universe has slowed due to gravity since Inflation (ignoring the increase in the rate of expansion discovered in 1998), and was never receding faster than c, ISTM the radius of the observable universe has an upper bound of twice the age of the universe, or about 2x13.8 light years. But obviously this upper bound is way too low compared to the claim that it is 46 BLY. I have no idea how to resolve this discrepancy other than to conjecture that the universe must be much older than 13.8 BLY. Is this what observations of the James Webb Space Telescope suggests, with observations of fully formed galaxies in the very early universe? AG

[Quentin Anciaux]

Inflation lasted 10^-32 seconds... inflation is not the cause of recessional velocity > c, it's space expansion, not inflation, as long as it is *uniform* (the point you seem unable to grasp), object will sooner or later recess from each other > c.

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[Quentin Anciaux]

Also inflation happened *before* the hot big bang.

[Alan Grayson]

On Saturday, September 21, 2024 at 3:00:16 AM UTC-6 Quentin Anciaux wrote:

Inflation lasted 10^-32 seconds... inflation is not the cause of recessional velocity > c, it's space expansion, not inflation, as long as it is *uniform* (the point you seem unable to grasp), object will sooner or later recess from each other > c.

Inflation causes space to expand hugely, perhaps as many as 100 doublings in an incredibly short time duration. I never claimed otherwise. And as long as space is expanding, objects will recede sooner or later from each other at velocities > c. I don't see any difference in our respective pov's. AG

[Alan Grayson]

On Saturday, September 21, 2024 at 3:06:40 AM UTC-6 Alan Grayson wrote:

On Saturday, September 21, 2024 at 3:00:16 AM UTC-6 Quentin Anciaux wrote:

Inflation lasted 10^-32 seconds... inflation is not the cause of recessional velocity > c, it's space expansion, not inflation, as long as it is *uniform* (the point you seem unable to grasp), object will sooner or later recess from each other > c.

Inflation causes space to expand hugely, perhaps as many as 100 doublings in an incredibly short time duration. I never claimed otherwise. And as long as space is expanding, objects will recede sooner or later from each other at velocities > c. I don't see any difference in our respective pov's. AG

Maybe, after Inflation, some law of inertia applies, and space will continue to expand at the last rate defining its expansion. I don't think the cause of this rate of expansion is known.  AG

[Alan Grayson]

On Saturday, September 21, 2024 at 3:17:07 AM UTC-6 Alan Grayson wrote:

On Saturday, September 21, 2024 at 3:06:40 AM UTC-6 Alan Grayson wrote:

On Saturday, September 21, 2024 at 3:00:16 AM UTC-6 Quentin Anciaux wrote:

Inflation lasted 10^-32 seconds... inflation is not the cause of recessional velocity > c, it's space expansion, not inflation, as long as it is *uniform* (the point you seem unable to grasp), object will sooner or later recess from each other > c.

Inflation causes space to expand hugely, perhaps as many as 100 doublings in an incredibly short time duration. I never claimed otherwise. And as long as space is expanding, objects will recede sooner or later from each other at velocities > c. I don't see any difference in our respective pov's. AG

Maybe, after Inflation, some law of inertia applies, and space will continue to expand at the last rate defining its expansion. I don't think the cause of this rate of expansion is known.  AG

I think you make a good point, distinguishing between Inflation, which hugely increases the size of the universe, and the continuing expansion of space. But why is space continuing to expand? AG 

[Cosmin Visan]

@Alan. You can never observe any universe. All that you can ever observe is yourself. Now ask yourself: Why do you confuse the observation of yourself with the observation of a presumed "universe" ?

[Alan Grayson]

On Sunday, September 22, 2024 at 5:28:40 PM UTC-6 Cosmin Visan wrote:

@Alan. You can never observe any universe. All that you can ever observe is yourself. Now ask yourself: Why do you confuse the observation of yourself with the observation of a presumed "universe" ?

I'm not confused. I can change the label anytime I want, of what I am observing. And so can you.  AG 

[Cosmin Visan]

@Alan. There is no label. Is just you. You are only ever observing yourself.

[Alan Grayson]

On Tuesday, September 24, 2024 at 4:28:06 AM UTC-6 Cosmin Visan wrote:

@Alan. There is no label. Is just you. You are only ever observing yourself.

I know that. Now what? AG 

[Cosmin Visan]

@Alan. Now we start talking about reality instead of fairy tales. Like for example how does consciousness imagine the world.

[Alan Grayson]

On Tuesday, September 24, 2024 at 4:37:56 AM UTC-6 Cosmin Visan wrote:

@Alan. Now we start talking about reality instead of fairy tales. Like for example how does consciousness imagine the world.

You're the expert. You tell me. AG 

[Cosmin Visan]

See my papers, like "How Self-Reference Builds the World": https://philpeople.org/profiles/cosmin-visan