Universe expansion rate in m/s^2 ?

[roineust]

TL;DR Summary

Is it possible to tell the current average universe expansion rate i.e. acceleration of expansion in m/s^2 ?

Summary: Is it possible to tell the current average universe expansion rate i.e. acceleration of expansion in m/s^2 ?

Is it possible to tell the current average universe expansion rate i.e. acceleration of expansion in m/s^2 ?
If i say in relation to earth, does this make it easier to answer in m/s^2 ?
Why can i find this value only in m/(s*Mega-parsec)?
If Mega-parsec is a distance unit, then how can m/(s*Mega-parsec) be acceleration?
If it is possible to tell the acceleration of expansion in m/s^2 please first quote the number, before the detailed explanations.

 

[Orodruin]

No, it would make no sense to use m/s^2, these are the wrong units.

Expansion rate is a rate, not a velocity, so it is given in units that have dimensions 1/time.

Also, do not confuse expansion rate with accelerated expansion. Accelerated expansion means that the second derivative of the scale factor with respect to time is positive, this is not the same as the expansion rate increasing. Since the scale factor is dimensionless, its second derivative with respect to time has units 1/time^2.

 

[russ_watters]

Summary: Is it possible to tell the current average universe expansion rate i.e. acceleration of expansion in m/s^2 ?

You said both expansion rate and acceleration, and those are two different things, so which are you looking for?

The expansion rate is expressed in mixed distance units because it is so small (and probably because of the mixed use of redshift), but ultimately it is a percentage over time, with whatever distance and time units you want to apply. Same as you might describe a spring or a material strain rate.

You should be able to do whatever unit conversions you want...

 

[stefan r]

Why can i find this value only in m/(s*Mega-parsec)?

You look at an object at 1 mega parsec distance and it looks like it travels away at 70 km/s. You look at an object 2 mega parsecs away and it seams to move away at 140 km/s. When 13 mega parsecs out it looks like the object is moving 910 km/s.

Hubble's constant

...Is it possible to tell the current average universe expansion rate i.e. acceleration of expansion in m/s^2 ?...

That would just confuse things. In the future a distant object will have a greater distance because it is moving away from us. Assuming the Hubble constant really is a constant then the object will be moving away at a higher velocity.
No force is acting on the object. Space is expanding.

You can talk about the "acceleration of expansion" but that would be implying that Hubble's constant is not a constant. Like it is 70 km/s/parsec now but at some future time it increases to 80 km/s/parsec. There are too many published values for Hubble's constant at the moment.

 

[Orodruin]

You can talk about the "acceleration of expansion" but that would be implying that Hubble's constant is not a constant. Like it is 70 km/s/parsec now but at some future time it increases to 80 km/s/parsec. There are too many published values for Hu

This is incorrect. Accelerated expansion means that the second derivative of the scale factor is positive. This is not the same as the Hubble parameter increasing. In fact, a Universe dominated by a cosmological constant would have a constant Hubble rate. The Hubble rate generally is not a constant. The Hubble constant nomenclature is a historical leftover.

 

[roineust]

Most distant objects from us in the universe, which the rate of their velocity in relation to us is changing, because of the big bang and without any current force acting on them to create that change of velocity, at what rate does this velocity change?

 

[Bandersnatch]

There would be no change in recession velocity just due to the initial big bang impulse, without gravity of matter/dark energy acting to decelerate/accelerate (respectively).

Having said that, for a galaxy currently at approx 14 billion light-years (proper distance), which is receding at 1 c, the current acceleration is approx. 0.04c/billion years.

 

[roineust]

Thanks, i think that was the numerical value i was looking for.
Not an extreme receding rate at all 0.04 c/1By.
i.e. compared to common particle acceleration rates.

 

[Bandersnatch]

It ain't hard to beat ~0.4 nm/s^2.

 

[Orodruin]

Note that recession velocity is not an actual velocity.

 

[roineust]

Note that recession velocity is not an actual velocity.

Because there is no physical definition to anything beyond the known universe?

 

[Orodruin]

Because there is no physical definition to anything beyond the known universe?

No, because the objects involved are not located at the same place and time. Actual velocity only becomes meaningful for objects that are at the same place and at the same time (or, more operationally, sufficiently close at sufficiently similar times). For other objects, relative velocity becomes a matter subject to arbitrary conventions.

 

[roineust]

And what if there would have been discovered a way to move instantly from one place in the universe to any other place? Would that change or influence the definition of locality? and how is 'sufficient' defined in physical terms? And if i can draw on paper a form 5cm wide and say that it represents the universe, why am i not 'allowed' to ask what is the recession rate of this form in relation to the rest of the paper, not even allowed to ask what is the recession rate of the middle of the form in relation to the form edges?

 

[stefan r]

And what if there would have been discovered a way to move instantly from one place in the universe to any other place?

If you break the laws of physics many things can happen. There is a lot of really good science fiction available. I can PM you some links.

and how is 'sufficient' defined in physical terms?

If it cannot be measured it is close to not being there. There should be a some particles or small objects that are closer to the local group of galaxies than any other group of galaxies. Expansion would decrease the rate that the local group's gravity pulls them in.

 

[Bandersnatch]

Is this a distinction worth making in a B thread? Seeing how there is a standard convention of slicing space-time in cosmology, it's not like recession velocity is somehow ambiguous.

 

[roineust]

Is it true that at 1 second after the big bang, the first order derivative of the universe recession rate (is it the same as saying expansion rate?) was several hundred times faster than the speed of light? Is is true that on average it is ~7 times the speed of light (~93Bly/~14By=~7ls)?

 

[roineust]

How could it be that we are seeing galaxies from the time that the universe was very young? Wasn't the universe much smaller then (13+ Billion years ago) and hence, shouldn't the light that was emitted from these galaxies at that early stage, have already supposed to pass by us a long time ago and we are supposed to see a much later version of the universe? Does it have to do with the expansion rate at the beginning of the universe, being faster than the speed of light?

And can you please also answer my previous questions above?

 

[Nugatory]

And what if there would have been discovered a way to move instantly from one place in the universe to any other place?

Such a thing is not possible; reasoning starting from the premise that it is possible leads to contradictions as surely as would trying to reason from the premise that a prime number has an integral square root. The basic problem is the relativity of simultaneity, which you will want to understand thoroughly before you try to take on the even more counterintuitive behavior of an expanding universe. Googling for “Einstein train simultaneity” will get you started and we have many threads here that will help.

And if i can draw on paper a form 5cm wide and say that it represents the universe, why am i not 'allowed' to ask what is the recession rate of this form in relation to the rest of the paper, not even allowed to ask what is the recession rate of the middle of the form in relation to the form edges?

The universe has no edges or middle, so we can’t talk about speeds relative to these. However, we can imagine two ants moving about on the paper. Clearly it makes sense to talk about their speed relative to one another, so why can’t we do the same with two galaxies moving about in the universe?

Well, consider exactly what we’re doing with the ants. First we note where one ant is, then we find where the other ant is at the same time. We measure or calculate the distance between these two points, wait a few moments, and repeat the process. Subtract one distance from the other to find how much the distance between them changed, divide by the time between measurements, and we have their relative speed.

But note that innocent-sounding phrase “at the same time”- this process only works because we have a definition of “at the same time” that works across the entire sheet of paper. There is no corresponding way of defining “at the same time” across the entire curved and expanding universe - even the technique Einstein uses in his thought experiment will fail.

 

[phinds]

How could it be that we are seeing galaxies from the time that the universe was very young? Wasn't the universe much smaller then (13+ Billion years ago) and hence, shouldn't the light that was emitted from these galaxies at that early stage, have already supposed to pass by us a long time ago

No, the objects that emitted that light have moved away so the light had farther to travel before reaching us (and it has become redshifted as a result).

Does it have to do with the expansion rate at the beginning of the universe, being faster than the speed of light?

Yes, plus the continuing expansion

 

[timmdeeg]

Does it have to do with the expansion rate at the beginning of the universe, being faster than the speed of light?

The expansion of the universe was faster than the speed of light, not the expansion rate which has the dimension 1/s (so not a speed). The early universe expanded so fast that the distance of photons traveling towards us was increasing.

 

[timmdeeg]

Is it true that at 1 second after the big bang, the first order derivative of the universe recession rate (is it the same as saying expansion rate?) was several hundred times faster than the speed of light? Is is true that on average it is ~7 times the speed of light (~93Bly/~14By=~7ls)?

The recession speed was "several hundred times faster than the speed of light" not the universe recession rate. There is no such thing as a "universe recession rate". The expansion rate is defined as the first derivative of the scale factor divided by the scale factor, therefor its not a speed but a rate. This rate was very high in the early universe and is decreasing still today.