Actual vs apparent universal expansion rate

[StringJunky]

If the expansion of the universe is uniform throughout and is less than c, how is it that beyond the Hubble Limit (HL) a photon will never reach the observer? I can understand, at great distances, that it gives a certain measured or apparent expansion rate that may exceed c, causing a photon to redshift, but locally, around the photon, the expansion rate still matches, objectively, the expansion rate at the locality of the observer.

As it stands atm, in my current - wrong, no doubt - understanding, it seems that, beyond the HL, the expansion actually increases incrementally faster than c locally, resulting in the photon never reaching the observer i.e. it doesn’t just seem to be a measurement effect.

I know I’m missing something but I don’t know what.

[Strange]

Oddly, there is another thread on the same subject (but that is taking a decidedly non-mainstream direction).

 

Simply put, the expansion means that a photon has an increasing distance to travel as time passes. This means, for example, that a photon emitted 13 billion years ago would have been emitted approximately 4.5 billion light years away but has been "swimming against the tide" of expanding space since then and so took 13 billion light years to get here (the emitting source will be about 45 billion light years away now).

 

The further away an object is (was), the greater the delay caused by expansion is. (Because there is more space to cross, all of it expanding. Or, equivalently, because it takes longer and so more expansion happens.) At some point, the object is so far away, and the delay so great, that the photon never gets here.

[StringJunky]

<lightbulb moment> . Thanks. I thought Michel's thread was similar, and it did initiate this one, but I didn't want to confuse the flow of that one with my problem.

[Endy0816]

At some point, the object is so far away, and the delay so great, that the photon never gets here.

Are those photons thought to eventually decay?

[StringJunky]

Are those photons thought to eventually decay?

No, I don't think so, they eventually redshift out of the visible spectrum beyond a certain expanse of time/distance.

 

Leading on from Endy's question: does the photon redshift beyond detection just outside the HL distance? Is that what the HL is?

Edited July 25, 2016 by StringJunky

[Strange]

Someone (swansont?) posted a paper some time ago about estimating the lower bound on the lifetime of a photon. It is many, many times greater than the life of the universe.

 

Maybe this was it: http://backreaction.blogspot.co.uk/2013/07/how-stable-is-photon-yes-photon.html

[imatfaal]

The hubble limit is that distance at which the distance between an emitter and an observer is increasing at greater than lighter speed - this is due to expansion. But we also have accelerated expansion (it is not just expanding but the rate of the expansion is increasing) - horribly confusingly this means that we will one day see stuff that is outside our hubble limit. The limit of what we can see in an accelerating expansion universe is called the particle limit

[michel123456]

The hubble limit is that distance at which the distance between an emitter and an observer is increasing at greater than lighter speed - this is due to expansion. But we also have accelerated expansion (it is not just expanding but the rate of the expansion is increasing) - horribly confusingly this means that we will one day see stuff that is outside our hubble limit. The limit of what we can see in an accelerating expansion universe is called the particle limit

Yes but as far as I can understand, expansion is already an acceleration. Because receding speed increase with distance.

[Strange]

Yes but as far as I can understand, expansion is already an acceleration. Because receding speed increase with distance.

 

You are confusing recesional speed with expansion. Expansion is a [constant] scaling factor (ignoring accelerating expansion, for simplicity).

 

By simple arithmetic, this means that the speed at which two objects recede from one another is proportional to the distance between them. As there are no forces involved, there is no acceleration.

[imatfaal]

Yes but as far as I can understand, expansion is already an acceleration. Because receding speed increase with distance.

 

Further to Strange's answer. Expansion is technically quantified in units of s^-1 per second but that is a little confusing really the most common units are (km/s)/Mpc (kilometres per second) per Megaparsec. That is a speed for the gap between two objects to open per unit distance - ie the further away two object are the quicker the gap between them is enlarging.

 

The factor of expansion in (km/s)/Mpc is about 71 - what we mean by accelerating expansion is that this scale factor is not constant over time it is increasing. In simplistic terms the gap between two objects a megaparsec apart will be increasing at 71km/s at present - in the future the gap between two other objects also a megaparsec apart will be increasing by more than 71km/s

[StringJunky]

Yes but as far as I can understand, expansion is already an acceleration. Because receding speed increase with distance.

For the short duration of a typical observation period just consider the expansion as occurring at a constant rate.

[michel123456]

 

You are confusing recesional speed with expansion. Expansion is a [constant] scaling factor (ignoring accelerating expansion, for simplicity).

 

By simple arithmetic, this means that the speed at which two objects recede from one another is proportional to the distance between them. As there are no forces involved, there is no acceleration.

Oh yes, you are right, I am confusing recessional speed with expansion.

 

Recessional speed is increasing with distance, that is Hubble's law. In my stubborn mind, it looks terribly like an acceleration, even if there are no forces involved.

 

And I remember in this same Forum in a thread someone explaining very clearly that a geometric scale factor produces also an effect similar to acceleration. But I cannot spot the thread, it was some years ago.

[StringJunky]

 

And I remember in this same Forum in a thread someone explaining very clearly that a geometric scale factor produces also an effect similar to acceleration. But I cannot spot the thread, it was some years ago.

Might that be Imatfaal in one of my threads on the same subject where he explained with the analogy of stretching a rubber band and how the moving end seems to accelerate the further away it is from the static end?

[michel123456]

Might that be Imatfaal in one of my threads on the same subject where he explained with the analogy of stretching a rubber band and how the moving end seems to accelerate the further away it is from the static end?

Maybe, can you find it?

I remember a very long thread. It was towards the last page.

[Strange]

And I remember in this same Forum in a thread someone explaining very clearly that a geometric scale factor produces also an effect similar to acceleration. But I cannot spot the thread, it was some years ago.

 

It is basic arithmetic. Consider a number of galaxies separated by the same distance (far enough apart that the expansion of space is significant and the same between all of them).

 

At time 0, they are 1 unit apart:

A.B.C.D.E.F

 

After some time they are 2 units apart:

A..B..C..D..E..F

 

After the same time again, they are 3 units apart:

A...B...C...D...E...F

 

And so on:

A....B....C....D....E....F

 

Now, if we look at the distance between B and C, for example, it increases by 1 at every time step. But the distance between B and D increases by 2 at every step. So the distance between B and D is increasing twice as fast as the distance between B and C; i.e. the speed of separation is twice as great.

 

Choose any pairs of galaxies and you will see that apparent the speed of separation is proportional to the distance between them. Take two objects far enough apart and the speed of separation will be greater than the sped of light. No forces and no acceleration.

[michel123456]

Maybe, can you find it?

I remember a very long thread. It was towards the last page.

not this one http://www.scienceforums.net/topic/69965-recession-velocity/?hl=scaling#entry709965

Somewhere else.

[StringJunky]

not this one http://www.scienceforums.net/topic/69965-recession-velocity/?hl=scaling#entry709965

Somewhere else.

Yeah, that was the one I was thinking of .

[michel123456]

 

It is basic arithmetic. Consider a number of galaxies separated by the same distance (far enough apart that the expansion of space is significant and the same between all of them).

 

At time 0, they are 1 unit apart:

A.B.C.D.E.F

 

After some time they are 2 units apart:

A..B..C..D..E..F

 

After the same time again, they are 3 units apart:

A...B...C...D...E...F

 

And so on:

A....B....C....D....E....F

 

Now, if we look at the distance between B and C, for example, it increases by 1 at every time step. But the distance between B and D increases by 2 at every step. So the distance between B and D is increasing twice as fast as the distance between B and C; i.e. the speed of separation is twice as great.

 

Choose any pairs of galaxies and you will see that apparent the speed of separation is proportional to the distance between them. Take two objects far enough apart and the speed of separation will be greater than the sped of light. No forces and no acceleration.

In "basic" meanings, how do you call an increase of speed?

A.B.C.D.E.F

A..B..C..D..E..F

A...B...C...D...E...F

A....B....C....D....E....F

A.....B.....C.....D.....E.....F

A......B......C......D......E......F

A.......B.......C.......D.......E.......F

A........B........C........D........E........F

A.........B.........C.........D.........E.........F

 

Don't you see acceleration in this?

[StringJunky]

In "basic" meanings, how do you call an increase of speed?

Would it help if you, instead of thinking of speed, think of acceleration of increasing space since the objects themselves experience no change in velocity?

Edited July 26, 2016 by StringJunky

[Strange]

 

In "basic" meanings, how do you call an increase of speed?

A.B.C.D.E.F

A..B..C..D..E..F

A...B...C...D...E...F

A....B....C....D....E....F

A.....B.....C.....D.....E.....F

A......B......C......D......E......F

A.......B.......C.......D.......E.......F

A........B........C........D........E........F

A.........B.........C.........D.........E.........F

 

Don't you see acceleration in this?

 

 

There is an informal use of acceleration to mean an increase in speed, which is what you are referring to. But this is not the same as the meaning in physics, where it refers to a change in velocity due to a force.

 

I think it is important to keep these different concepts separate.

[michel123456]

There is an informal use of acceleration to mean an increase in speed, which is what you are referring to. But this is not the same as the meaning in physics, where it refers to a change in velocity due to a force.

 

I think it is important to keep these different concepts separate.

My point is that Hubble's law describes an increase of apparent velocity.

Formally, Newton would have called that an acceleration*.

When someone states that the expansion is accelerating, he is speaking of an acceleration of acceleration*. Where acceleration needs a force and acceleration* does not.

 

[Strange]

When someone states that the expansion is accelerating, he is speaking of an acceleration of acceleration*. Where acceleration needs a force and acceleration* does not.

 

Neither of those involve a force, as far as I know.

[michel123456]

 

Neither of those involve a force, as far as I know.

Oh i thought the secondary acceleration was linked to dark energy.

[imatfaal]

My point is that Hubble's law describes an increase of apparent velocity.

Formally, Newton would have called that an acceleration*.

When someone states that the expansion is accelerating, he is speaking of an acceleration of acceleration*. Where acceleration needs a force and acceleration* does not.

 

 

No - Newton would not have done so. The thing we are looking at (in non-accelerated expansion) is increase in speed is per unit of distance - Newton used increase in speed per unit of time (this was what he tied into a relationship with Force and mass). They are not the same.

 

What we have with accelerated expansion is increase in speed per unit of distance - which itself is increasing over time

[michel123456]

 

No - Newton would not have done so. The thing we are looking at (in non-accelerated expansion) is increase in speed is per unit of distance - Newton used increase in speed per unit of time (this was what he tied into a relationship with Force and mass). They are not the same.

 

What we have with accelerated expansion is increase in speed per unit of distance - which itself is increasing over time

I hope you are not kidding. Distance means time.