# Speed of Space Expansion

## Main Question or Discussion Point

I was wondering how fast does the space expand, but I wasn't able to find answer anywhere around the internet, the only thing I was able to get is the rate of expansion - 72 km/sec/Megaparsec, but I dont know if I can derive the speed of expansion from that. Also I obtained information that the speed of expansion is increasing.

So how fast does the universe expand? Are there any good theories?

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I was wondering how fast does the space expand, but I wasn't able to find answer anywhere around the internet, the only thing I was able to get is the rate of expansion - 72 km/sec/Megaparsec, but I dont know if I can derive the speed of expansion from that. Also I obtained information that the speed of expansion is increasing.

So how fast does the universe expand? Are there any good theories?
Consider three galaxies, A, B, and C.

The centers of A, B, and C are aligned along a common axis.

A is 1 Megaparsec from B and 2 Megaparsecs from C. B is 1 Megaparsec from C.

Therefore:

A recedes from B at 72 km/sec, A recedes from C at 144 km/second, and B recedes from C at 72 km/sec.

Consider three galaxies, A, B, and C.

The centers of A, B, and C are aligned along a common axis.

A is 1 Megaparsec from B and 2 Megaparsecs from C. B is 1 Megaparsec from C.

Therefore:

A recedes from B at 72 km/sec, A recedes from C at 144 km/second, and B recedes from C at 72 km/sec.
Yes thanks, I got that. But I was interested if there are any "boundaries" to spacetime.
Imagine spacetime - universe as a giant bubble. Since the big bang the bubble was getting bigger and bigger and the expansion kept accelerating. What would be the speed at which the edge of the *bubble* is expanding?
Or is it irrelevant and do I picture the space wrong? Is it all absolutely relative on where am I looking from?

DaveC426913
Gold Member
What would be the speed at which the edge of the *bubble* is expanding?
Or is it irrelevant and do I picture the space wrong?
There is no edge. The universe is, as far as we know, unbounded.

Have you heard of the expanding balloon analogy?

Speed of Space Expansion And "Star Stuff"

Speed of space expansion and star stuff.

The speed of space expansion is not just the space expanding (if space CAN expand, at all)
but is it not also about all the materials within it traveling outward from the center?

And isn't that what Tachyonie was referring to? Kind regards.......... th' humbled1

russ_watters
Mentor
There is no center, so no, it's just space expanding, increasing the distance between all objects above a certain scale.

There is no center, so no, it's just space expanding, increasing the distance between all objects above a certain scale.
I've been wondering about that scale effect. Is the scale certain enough to be given a measurement? Is there an upper and lower bound? Is the change locally just too small to be measured?

George Jones
Staff Emeritus
Gold Member
I've been wondering about that scale effect. Is the scale certain enough to be given a measurement? Is there an upper and lower bound? Is the change locally just too small to be measured?
See https://www.physicsforums.com/showthread.php?t=206604", particularly posts #11 and #15.

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Thanks. I think I see that the answer to my question is that changes at great distance over human timescales are too small to be measured by current spectographic technology. Changes at smaller distances will be even less, so even further beyond our ability to measure by spectrographic methods.

With a spectrograph, we are looking at variations in the wavelength of light....and these wavelengths are very short, making it very hard to see any differences. One might expect then to be able to see larger, more locally evident changes in events with much longer wavelengths. I am speculating that the Pioneer anomalie could be such an event. the wavelength is established by the distance traveled...so we are able to look at changes in wavelengths on the order of the diameter of the solar system. This is still not extremely large in cosmological terms, but is many many orders of magnitude larger than a wavelength of light in a spectrogram.

I wish I could calculate a rate of expansion from the Pioneer data, and compare it to the limits provided by the spectroscopic data. Is the Pioneer data in a range that might be considered as an effect of universal expansion?

George Jones
Staff Emeritus
Gold Member
I wish I could calculate a rate of expansion from the Pioneer data, and compare it to the limits provided by the spectroscopic data. Is the Pioneer data in a range that might be considered as an effect of universal expansion?
I know next to nothing about the Pioneer data. https://www.physicsforums.com/showpost.php?p=1574979&postcount=23" is a post about it, and I'm sure other posts/threads about the Pioneer stuff exist here at Physics Forums.

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Wallace
I wish I could calculate a rate of expansion from the Pioneer data, and compare it to the limits provided by the spectroscopic data. Is the Pioneer data in a range that might be considered as an effect of universal expansion?
The global expansion of the Universe plays no role on scales such as the solar system, not because the distance is small but because there is no expansion. The global expansion Hubble law is an approximation based on the Universe being the same everywhere (homogeneous) so breaks down on scales where this is not the case (such as galaxies).

In any case the Pioneer anomaly is an unexpected small acceleration towards the Sun, whereas if the Hubble law was valid in solar system scales you would expected it to be an acceleration away.

DaveC426913
Gold Member
The global expansion of the Universe plays no role on scales such as the solar system, not because the distance is small but because there is no expansion.
I thought it was simply because the expansion is much weaker than gravity and so is easily overwhelmed by all interactions except those especially weak forces across intergalactic distances.

Wallace
I thought it was simply because the expansion is much weaker than gravity and so is easily overwhelmed by all interactions except those especially weak forces across intergalactic distances.
You can't compare 'the expansion of space' or even just 'the expansion' with gravity since one is a force and the other is not! If anything, gravity causes the expansion of space, since the things we ascribe to 'the expansion of space' are a result of the solution of the GR equations in an expanding Universe.

The Universe expands because it did so in the past and this produces an effect we dub the expansion of space. If you think about it the other way around (space expands causing things to move apart) misconception will greet you at every turn.

You can't compare 'the expansion of space' or even just 'the expansion' with gravity since one is a force and the other is not! If anything, gravity causes the expansion of space, since the things we ascribe to 'the expansion of space' are a result of the solution of the GR equations in an expanding Universe.
So are you saying that gravity is a force that you cannot compare with the expansion of space because gravity causes it? And that makes total sense to you?

The solution of the GR equations? Which solution are you refering to?

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Wallace
So are you saying that gravity is a force that you cannot compare with the expansion of space because gravity causes it? And that makes total sense to you?
The 'expansion of space' is merely a property of particular co-ordinate system. So yes, I do think comparing the effects of a force to the effects of a co-ordinate system is something that you cannot do. Clearly this makes no sense.

The solution of the GR equations? Which solution are you refering to?
The FRW metric and the resultant Friedman equations. The point is that 'the expansion of space' is merely a description of the the FRW solution in co-moving co-ordinates, not a 'real' physical effect. Cosmologists of course know this and use the term expansion of space very loosely, but it gets misinterpreted very often as having some great significance.

The 'expansion of space' is merely a property of particular co-ordinate system.
Depends what you mean by coordinate system here, in the FRW models it is caused because a scaling factor is introduced.

One would think that people get a hint when they find out that objects recess faster than the speed of light in such models, unfortunately they don't.

Not surprising when one takes the liberty of morphing a 4 dimensional curved spacetime into a curved 3 dimensional space with linear time.

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Wallace