So, and would someone here correct me if I haven't got this perfectly right, the very core of only the most important part of his whole argument can be very briefly summarized as:
The reason why the universe cannot either be or ever have once been expanding over the speed of light:
The expansion of the universe is defined by the Hubble constant, which is in units of 1/time and NOT in speed units which are always in units of distance/time. Therefore, that expansion isn't expressed as speed. That is because the expansion of the universe cannot be expressed as speed because it has no definable speed. Therefore, the universe cannot be expanding over the speed of light because it is not expanding at ANY speed, let alone over the speed of light.
Originally posted by humy His argument sounds good to me.
So, and would someone here correct me if I haven't got this perfectly right, the very core of only the most important part of his whole argument can be very briefly summarized as:
[b]The reason why the universe cannot either be or ever have once been expanding over the speed of light:
The expansion of the universe is ...[text shortened]... the speed of light because it is not expanding at ANY speed, let alone over the speed of light.[/b]
Do we need another definition of 'speed' then? He seems to admit there are galaxies we cannot see because light from them has not and cannot reach us because they are too far away so what new definition of 'speed' do we come up with to define this situation?
Originally posted by sonhouse http://www.preposterousuniverse.com/blog/2015/10/13/the-universe-never-expands-faster-than-the-speed-of-light/
Anyone agree, disagree?
Btw, he seems to be the real deal, theoretical physicist at Cal Tech.
I have attempted to explain it to you several times.
I agree with him. Speed is relative. Expansion can never be measured in terms of speed alone. To measure expansion in terms of speed you must specify two points that you are referring to.
Originally posted by sonhouse Do we need another definition of 'speed' then?
No, you just have to realise that expansion is not measured in units of speed. It is measured in units of speed per unit distance.
I posted this last time this came up:
http://www.space.com/17884-universe-expansion-speed-hubble-constant.html
Space itself is pulling apart at the seams, expanding at a rate of 74.3 plus or minus 2.1 kilometers (46.2 plus or minus 1.3 miles) per second per megaparsec (a megaparsec is roughly 3 million light-years).
Originally posted by sonhouse Do we need another definition of 'speed' then? He seems to admit there are galaxies we cannot see because light from them has not and cannot reach us because they are too far away so what new definition of 'speed' do we come up with to define this situation?
since the expansion of the universe has no speed units but has different units, the speed of galaxies receding away from us cannot define the speed of the expansion of the universe and, no, we don't need another definition of 'speed' to define the expansion of the universe nor for speed of galaxies receding away from us.
We already have units for expansion of the universe (1/time);
We already have units for the movement of galaxies receding away from us (distance/time);
So we already have satisfactory units for both; we don't need new ones for either one of them.
Originally posted by humy ...
The expansion of the universe is defined by the Hubble constant, which is in units of 1/time and NOT in speed units which are always in units of distance/time. Therefore, that expansion isn't expressed as speed. That is because the expansion of the universe cannot be expressed as speed because it has no definable speed. Therefore, the universe cannot be ex ...[text shortened]... the speed of light because it is not expanding at ANY speed, let alone over the speed of light..
I should just add a tiny bit more to that of:
...and the speed of the galaxies receding away from us, even the speed of the most distant galaxies, has nothing to do with it; that is just that, the speed of galaxies, not 'speed' of the expansion of the universe.
Originally posted by twhitehead I have attempted to explain it to you several times.
I agree with him. Speed is relative. Expansion can never be measured in terms of speed alone. To measure expansion in terms of speed you must specify two points that you are referring to.
What if those two points were situated at the center of two widely spaced galaxies?
Originally posted by sonhouse What if those two points were situated at the center of two widely spaced galaxies?
It wouldn't make any difference whatsoever. The expansion as far as we know is more or less uniform, so you can put your points wherever you like. Just measure the distance between them and the expansion will be 74.3 plus or minus 2.1 kilometers times the number of megaparsecs between them.
Originally posted by twhitehead It wouldn't make any difference whatsoever. The expansion as far as we know is more or less uniform, so you can put your points wherever you like. Just measure the distance between them and the expansion will be 74.3 plus or minus 2.1 kilometers times the number of megaparsecs between them.
So the problem with relativistic 'speed' is the flow of time?
Originally posted by sonhouse What if those two points were situated at the center of two widely spaced galaxies?
then those two points would be moving relatively away from each other at the relative speed of those two galaxies, not any kind of 'speed' of the expansion of the universe.
Originally posted by sonhouse So the problem with relativistic 'speed' is the flow of time?
No, there is no problem with relativistic speed. Seriously, I don't know what you are getting at or why you find this so hard to understand. I am not really taking time into account at all. I am just taking about the expansion of space as it is now.
Draw some dots on a balloon. Now blow it up. The dots move away from each other. How fast any pair of dots move away from each other is related to how far apart they are. So you cannot simply say 'the dots are moving away from each other at this speed'. You have to say 'they are moving away from each other at this speed for every inch of separation.'
On a balloon, there will be a maximum such velocity for dots on opposite sides of the balloon. If the universe is finite, there will also be a maximum velocity, but we have no hope of knowing what that is without knowing the size of the universe and if it is infinite then there is no maximum.
Originally posted by twhitehead Draw some dots on a balloon. Now blow it up. The dots move away from each other. How fast any pair of dots move away from each other is related to how far apart they are. So you cannot simply say 'the dots are moving away from each other at this speed'. You have to say 'they are moving away from each other at this speed for every inch of separation.'
O ...[text shortened]... that is without knowing the size of the universe and if it is infinite then there is no maximum.
If our universe started with the big bang then how could it be infinite? Wouldn't it need to be infinite to begin with?
Originally posted by lemon lime If our universe started with the big bang then how could it be infinite? Wouldn't it need to be infinite to begin with?
08 Apr '16 15:5210 edits
Interesting take on Cosmology: Sean Carroll:
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