# B Did anyone actually see the space moving faster than light?

#### fbs7

I keep hearing in these Science Channel programs that the reason why the Big Bang banged (instead of immediately collapsing into a black hole) is that when it banged it made space to expand faster than the speed of light. I'm always puzzled on how sure and certain the speakers look in these programs - even going like this: "the rule is that mass cannot go faster than light, but space can". Wow... which rule is that?

This idea about space expanding faster than light is fine as a hypothesis, but doesn't scientific method require scientists to actually prove it by measurements before ascertaining that as a fact? As far as I know nobody really measured that anywhere, is that true? That is, this is not a proven fact, but the best explanation for what we can know at this moment - an Occam's Razor thing?

If so, is it possible that this idea of space can expand faster than the speed of light may just be a fitting explanation that 100 years from now someone may say "Of course it can't go faster than light!!! In the early universe the speed of light was much, much faster than today!" (or whatever else) and that will then become something that the majority accepts as a new dogma?

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#### Orodruin

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I keep hearing in these Science Channel programs that the reason why the Big Bang banged (instead of immediately collapsing into a black hole) is that when it banged it made space to expand faster than the speed of light. I'm always puzzled on how sure and certain the speakers look in these programs - even going like this: "the rule is that mass cannot go faster than light, but space can". Wow... which rule is that?
Stop watching those programs if you want to learn physics and not the watered down popularised version. They are intended to attract an audience and create interest, not to teach actual physics.

Space does not have a speed of expansion, it has a rate of expansion, which has different units and therefore cannot be compared to the speed of light. What is true is that this can result in the distance between objects increasing faster than the speed of light, but that is still true today. It just depends on how far away they are.

This idea about space expanding faster than light is fine as a hypothesis, but doesn't scientific method require scientists to actually prove it by measurements before ascertaining that as a fact?
No. Actually, you can never ”prove” anything, just confirm that the observations you make agree with the predictions of your theory or not. If they do not you reject the theory as a godd description of what you observe. If they do you keep it around. It can still be a good and useful theory that describes a lot of phenomena - like Newton’s theory of gravitation.

#### PeroK

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I keep hearing in these Science Channel programs that the reason why the Big Bang banged (instead of immediately collapsing into a black hole) is that when it banged it made space to expand faster than the speed of light. I'm always puzzled on how sure and certain the speakers look in these programs - even going like this: "the rule is that mass cannot go faster than light, but space can". Wow... which rule is that?

This idea about space expanding faster than light is fine as a hypothesis, but doesn't scientific method require scientists to actually prove it by measurements before ascertaining that as a fact? As far as I know nobody really measured that anywhere, is that true? That is, this is not a proven fact, but the best explanation for what we can know at this moment - an Occam's Razor thing?

If so, is it possible that this idea of space can expand faster than the speed of light may just be a fitting explanation that 100 years from now someone may say "Of course it can't go faster than light!!! In the early universe the speed of light was much, much faster than today!" (or whatever else) and that will then become something that the majority accepts as a new dogma?
If you don't mind my saying this post sounds like trolling to me. Designed to provoke, rather than be a serious question.

#### fbs7

If you don't mind my saying this post sounds like trolling to me. Designed to provoke, rather than be a serious question.
It's a serious question, at least for me. I'm asking if there are measurements that evidence that space can indeed expand faster than the speed of light, or if that's theoretical at this moment. The reason for my curiosity is how that could ever be measured, since I suspect we don't see space doing that anywhere. Then, if it cannot be measured, I believe it comes down to making predictions, that then are proven true and provide evidence that way - if so, then we have any predictions?

The root reason for my curiosity is that I once visited the Stanford linear accelerator, and someone asked a question about string theory and the host (a postgrad student I think) said that string theory explained a lot, but hadn't (at that point, it was some 15 years ago) made predictions to prove it. So when I was watching the Science Channel program and the presenter spoke about space expanding like that, I thought... "hmm... is that really proven or is that an explanation?".

#### Orodruin

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I'm asking if there are measurements that evidence that space can indeed expand faster than the speed of light, or if that's theoretical at this moment
See #2.

I believe it comes down to making predictions, that then are proven true and provide evidence that way - if so, then we have any predictions?
This is how any theory makes predictions about anything.

#### fbs7

Space does not have a speed of expansion, it has a rate of expansion, which has different units and therefore cannot be compared to the speed of light. What is true is that this can result in the distance between objects increasing faster than the speed of light, but that is still true today. It just depends on how far away they are.
I see!! So it's not like an explosion that space is rushing away at say a million km/s from a central point. It's a rate based on distance! So if ABC are in a line at same distance, and point B is expanding away from A, then C is expanding away from B at the same rate, therefore from A point C will seem to expand away at twice the rate!

That's awesome. So whatever the rate of expansion, even very small, then if the universe is big enough then necessarily some points will be expanding away from us faster than the speed of light at this very moment, correct? So this concept is not limited to the first milissecond of the universe, but that still happens nowadays, correct?

Therefore the evidence is that, if distant objects are measured moving away at progressively faster speeds, then if the universe is infinite then at some distance space is indeed expanding away faster than the speed of light? That is, expansion faster than the speed of light is not really dependent on the big bang... it's really a consequence of having distant objects steadily increasing their red-shift!

#### CWatters

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We can observe light from galaxies that has been red shifted so much that it implies the galaxy is travelling faster than light. So either it is travelling faster than light (which we don't think is possible) or space is expanding faster than light.

#### fbs7

Stop watching those programs if you want to learn physics and not the watered down popularised version. They are intended to attract an audience and create interest, not to teach actual physics.
Haha... guilty as charged ... I love those programs specially for the beautiful graphics, although I have to say sometimes these folks say things that make a "boink" in my head. I guess there's no escaping that. I wish there were some programs that would go in more depth, though.

#### fbs7

We can observe light from galaxies that has been red shifted so much that it implies the galaxy is travelling faster than light. So either it is travelling faster than light (which we don't think is possible) or space is expanding faster than light.
Wow! That's extraordinary! So some galaxies seem to be moving faster than the speed of light! I'd never imagine such thing was possible!

A question around that, if I could... if the space around the galaxy is, from our perspective, apparently expanding faster than c... once the galaxy emits a ray of light... then from our perspective, the ray of light will still be travelling towards us at speed c, whatever the apparent speed of the galaxy, because light always travel at c, correct? That's why we can see the galaxy, even if it seems to be going faster than c?

Then what if it's done the other way around... what if space is contracting faster than the speed of light, and the galaxy emits a ray of light on our way... 1 year later, the galaxy will be closer to us than the ray of light... so the galaxy is invisible - it will suddenly reach us out of nowhere? Is that a paradox?

#### PeroK

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Wow! That's extraordinary! So some galaxies seem to be moving faster than the speed of light! I'd never imagine such thing was possible!

A question around that, if I could... if the space around the galaxy is, from our perspective, apparently expanding faster than c... once the galaxy emits a ray of light... then from our perspective, the ray of light will still be travelling towards us at speed c, whatever the apparent speed of the galaxy, because light always travel at c, correct? That's why we can see the galaxy, even if it seems to be going faster than c?

Then what if it's done the other way around... what if space is contracting faster than the speed of light, and the galaxy emits a ray of light on our way... 1 year later, the galaxy will be closer to us than the ray of light... so the galaxy is invisible - it will suddenly reach us out of nowhere? Is that a paradox?
Try this excellent insight:

https://www.physicsforums.com/insights/inflationary-misconceptions-basics-cosmological-horizons/

The short answer to your question is that an object has two parts to its velocity relative to us: its recessional velocity, determined by its distance from us and the expansion rate; and its local velocity. The velocity with which its distance from us is changing is the sum of the two.

Light always travels locally at the same speed, $c$ (*). But, if the recessional velocity is greater than $c$, then that light will be getting further away from us. Even if it's travelling in our direction.

(*) This is the generalisation of the postulate from special relativity that the speed of light in vacuum is always $c$.

#### PAllen

We can observe light from galaxies that has been red shifted so much that it implies the galaxy is travelling faster than light. So either it is travelling faster than light (which we don't think is possible) or space is expanding faster than light.
Actually, this is very misleading. In SR, there is no upper limit to red shift factor due to relative velocity. Every observed redshift would correspond, if interpreted as SR Doppler, to a velocity less than c. The Doppler corresponds to a superluminal recession rate in the context of a particular way of defining distance. This way is along hypersurface of constant proper time for comoving observers from the Big Bang event. With this definition of distance, it’s rate of growth between sufficiently separated comoving observers is greater than c. To see why this is not just quibbling, you can set up the Milne cosmology as the flat spacetime limit of FLRW cosmology. Then you have superluminal recession rates attached to comoving observers whose Doppler is exactly and unambiguously SR Doppler. Yet the recession rates are arbitrarily superluminal, while the relative velocity remains less than c.

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#### Drakkith

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I'm asking if there are measurements that evidence that space can indeed expand faster than the speed of light, or if that's theoretical at this moment. The reason for my curiosity is how that could ever be measured, since I suspect we don't see space doing that anywhere. Then, if it cannot be measured, I believe it comes down to making predictions, that then are proven true and provide evidence that way - if so, then we have any predictions?
'Space itself' cannot be observed. We can only observe how objects and disturbances of fields move and interact within space. It would be analogous to measuring the behavior of a river of invisible water by watching how leaves and other objects that have fallen in get dragged around. For the universe, we can observe the angular size of galaxies, the redshift of light, and other things that tell us the properties of space.

Then what if it's done the other way around... what if space is contracting faster than the speed of light, and the galaxy emits a ray of light on our way... 1 year later, the galaxy will be closer to us than the ray of light... so the galaxy is invisible - it will suddenly reach us out of nowhere? Is that a paradox?
Remember that light is traveling through space too, and is also affected by its properties. So the light would be 'dragged' along with the galaxy. A series of light pulses sent from the galaxy would be seen by an observer to be both blueshifted and occurring at a higher rate, and would always be ahead of the galaxy.

#### fbs7

I see! It's really mind-blowing that we could actually measure a galaxy moving faster than the speed of light - it is completely unintuitive!! I'm still flabbergasted!

Now, one last question about this. Consider a lighthouse that is turning a beam of light at 10 degrees per second; then at a distance of 10 km there's a wall; the beam will create a light circle, that will seem to move at 1.7 km/s along the wall. If the wall is farther, that circle will seem to go faster on the wall, until eventually the circle of light will seem to go faster than light. That's just a geometric trick, there's really nothing moving faster than the speed of light, it just seems to be so.

So, based on the explanations given here for universe expansion, what creates a parallel in my mind is a spring with a really really really small mass that is coiled to a length of say 10 cm, and is expanding at say 1 cm/s; if one makes a chain of these springs in a straight line, then the chain of springs is at 10cm expanding at 1 cm/s, at 20cm is expanding at 2cm/s, and so forth, until at 300,000 km it will be expanding at 10% of the speed of light.

That expansion at apparently 10% of the speed of light (and the universal expansion at faster than c), can that be considered just another geometric trick created by the long distance, just like the lighthouse? Both apparent "speeds" seem to have a different nature than say a Formula 1 car speeding on a track at 10% of the speed of light - in that case the car is really moving over the asphalt at such speed, while the springs aren't.

#### fbs7

I see. So we didn't directly measure any galaxies receding faster than the speed of light, right?

That's ok, I still stand in awe, if I understood it right, because there's a direct conclusion from other measurement, which is that we measure the red shift of a galaxy a distance D, then the red shift of another galaxy at distance 2*D, and their speeds are proportional to distance (I'm butchering math here); so if the universe is big enough, one can conclude that at some point some other galaxies are receding faster than the speed of light, even if we can't see them; is that a logical conclusion?

Of course, if someone bring evidence that the universe is small and doesn't reach that limit, or that recesion becomes asymptotic with distance, that would change.

#### PAllen

I see. So we didn't directly measure any galaxies receding faster than the speed of light, right?

That's ok, I still stand in awe, if I understood it right, because there's a direct conclusion from other measurement, which is that we measure the red shift of a galaxy a distance D, then the red shift of another galaxy at distance 2*D, and their speeds are proportional to distance (I'm butchering math here); so if the universe is big enough, one can conclude that at some point some other galaxies are receding faster than the speed of light, even if we can't see them; is that a logical conclusion?

Of course, if someone bring evidence that the universe is small and doesn't reach that limit, or that recesion becomes asymptotic with distance, that would change.
Recession rate is not a velocity at all. SR says velocity cannot exceed c, and this statement is wholly irrelevant to recession rate. To give a simple example, consider two bodies moving at .9c in opposite directions in some inertial frame in SR. Their recession rate is 1.8 c. Their relative velocity is .994c . Comparing recession rate to c as limiting velocity is a fundamental category error that is, unfortunately made by many cosmologists. Specifically, recession rate is coordinate dependent quantity that has no invariant physical significance. Further, while relative velocity of distant objects in GR is ambiguous (it depends on how velocities are compared, i.e. what path they are parallel transported along) but despite the ambiguity it is always less than c (that is, no matter what path is used to compare distant bodies motion, you always have relative velocity less than c).

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#### fbs7

That link is fascinating discussion! Quite a bit over my head, but I'm thankful for it!

#### fbs7

Recession rate is not a velocity at all. SR says velocity cannot exceed c, and this statement is wholly irrelevant to recession rate. To give a simple example, consider two bodies moving at .9c in opposite directions in some inertial frame in SR. Their recession rate is 1.8 c. Their relative velocity is .994c . Comparing recession rate to c as limiting velocity is a fundamental category error that is, unfortunately made by many cosmologists. Specifically, recession rate is coordinate dependent quantity that has no invariant physical significance. Further, while relative velocity of distant objects in GR is ambiguous (it depends on how velocities are compared, i.e. what path they are parallel transported along) but despite the ambiguity it is always less than c (that is, no matter what path is used to compare distant bodies motion, you always have relative velocity less than c).
Wow, this is a most extraordinary clarification! It shows how cosmological things are so different than everyday experience!

Let ask one more question about the nature of the recession rate. I understand that recession applies at distances between galaxies, not at distances within galaxies. Even so, we measure distance between galaxies in units of a perfect ruler called meter, which is the same ruler that we use to measure distances inside a galaxy.

So, say that I got out in a spaceship and fly into the void between galaxies. That void is expanding. And I brought with me say 1 trillion very very very light rulers, each one of them being 1m, and two balls. I put one ball free flying at one point, then a lot of these rules all linked up in a line, then another free flying ball at the other point 1 billion km away. The balls and rulers have no relative movement to each other. Then I wait 1000 years. If there was no expansion the balls would still be where I left them, at the very ends of the chain of rulers.

But universe is expanding, so I know my balls will get more distant to each other, but how about my rulers? They are linked in a chain. Will the distance between the balls still be 1 trillion counts of my rulers (ie, the rulers will get bigger), or now the distance will be say 2 trillion rulers, ie, 2 billion km, meaning the distance increased but the rulers remained the same length?

"Did anyone actually see the space moving faster than light?"

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