[Azhar (OP)]

Hello,

I've read quite few times that nothing can travel at a speed greater than speed of light. I think this is an upper limit proposed by Einstein's theory of special relativity.

However I have also read that universe if expanding at speed greater than speed of light.  I think we call it dark energy.  My question is, these two statements seem to be at odds with each other.  How do you reconcile these?

Thanks in advance.

[jeffreyH]

Even though the universe is expanding that does not mean that the speed of light is increasing. There is just more space for the photons to travel through. This in turns means that the galaxies are moving away without actually increasing their velocities. You have to distinguish between regions where gravity dominates and those where dark energy dominates.

[Azhar (OP)]

Thank you for the response. I'm not sure my question is answered though.

If nothing can travel at a speed greater than light then how can the statement "galaxies are moving away at a speed greater than speed of light" be true? The second statement seem to contradict the first statement and I'm curious to know which one of them is true.

Thanks.

[chris]

galaxies are moving away at a speed greater than speed of light

I agree that this can be a bit confusing. What @jeffreyH is saying is that the space itself has expanded, making the galaxies become further away; they've not actually moved though; they are just separated from us by more intervening space, making them appear to be retreating from us; granted they are moving, but some of the apparent movement is the expansion of the Universe.

[Azhar (OP)]

galaxies are moving away at a speed greater than speed of light

I agree that this can be a bit confusing. What @jeffreyH is saying is that the space itself has expanded, making the galaxies become further away; they've not actually moved though; they are just separated from us by more intervening space, making them appear to be retreating from us; granted they are moving, but some of the apparent movement is the expansion of the Universe.

So does this mean that when they say "galaxies are moving away with speed greater than speed of light" what they mean is that the light that SHOULD HAVE ALREADY been able to reach those galaxies since it had been travelling towards them, HAS NOT reached them becauses galaxies have moved away (expanded space)?

I feel like the language used to describe this seemingly contradictory nature of things is a bit confusing for a layman like myself. To me if nothing cannot travel faster than speed of light then space shouldn't be able to expand at speed greater than speed of light. No?

[evan_au]

I feel like the language used to describe this seemingly contradictory nature of things is a bit confusing for a layman like myself.

An analogy often used to describe the expansion of the universe is to imagine that the galaxies are like little dots on the surface of a balloon. As you inflate the balloon, the dots move away from each other - but the dots are still where you originally put them.

And the dots that were originally farther apart are moving away from each other faster than dots that are nearby.

Like all analogies, it has severe limitations (like the surface of a balloon is 2-dimensional, while our familiar space is 3-dimensional), But if you imagine that light travels at a finite speed across the balloon surface, there is a certain distance beyond which light from these galaxy dots will never reach us, because they are expanding away from us faster than light can travel.

For a more in-depth introduction, see: https://en.wikipedia.org/wiki/Metric_expansion_of_space 

[Bill S]

I've read quite few times that nothing can travel at a speed greater than speed of light.

This is probably a case of "sloppy" language.  What SR says is that nothing can be accelerated from subluminal to superluminal speed.  (Nit-pickers of the world unite. )

[Azhar (OP)]

I've read quite few times that nothing can travel at a speed greater than speed of light.

This is probably a case of "sloppy" language.  What SR says is that nothing can be accelerated from subluminal to superluminal speed.  (Nit-pickers of the world unite. )

This right there actually explains it. Nit-picking is good

Physics is all about getting close to as much accuracy as possible.

[Bill S]

What SR says is that nothing can be accelerated from subluminal to superluminal speed.

Of course, this doesn't answer your original question.  To find that answer you probably have to think that the galaxies; or rather, the galaxy groups; are not accelerating relative to space.  Oops! is that a bit Newtonian? 

[hamdani yusuf]

I've read quite few times that nothing can travel at a speed greater than speed of light.

This is probably a case of "sloppy" language.  What SR says is that nothing can be accelerated from subluminal to superluminal speed.  (Nit-pickers of the world unite. )

Due to acceleration of universe's expansion rate, some galaxies with current recession speed faster than the speed of light, previously have subluminal speed.

[opportunity]

Azhar writes:

Hello,

I've read quite few times that nothing can travel at a speed greater than speed of light. I think this is an upper limit proposed by Einstein's theory of special relativity.

However I have also read that universe if expanding at speed greater than speed of light.  I think we call it dark energy.  My question is, these two statements seem to be at odds with each other.  How do you reconcile these?

Thanks in advance.



You've got a cryptic question here, and I am going to focus on your initial question if that's ok with the forum members having read all their replies.

The idea of how we understand the stars relates to two key things, the red shift effect and the cosmic background microwave radiation. From these two things it was logical to consider a primary source event in time validating the CMB and red shift effect. In applying the equations of the Planck scale, those energy realms, it was considered necessary to "consider" the idea of dark energy and dark matter to explain the Planck scale energy equations on the universal realm. Correct me anyone if I am wrong.

When we look at the red shift effect, the CMB, and put it to a big bang that then requires a menue of new ideas that we can't prove but write well, what about quantum entanglement? How does quantum entanglement work through the universe........immediate interaction despite metrics of time and space? If there's gold in physics theory it's explaining quantum entanglement "first" as concepts of time and space. I say thins because you're referring to faster than light travel.......that's quantum entanglement. How is that recognised beyond "here to the moon"?

A true answer to your question therefore, despite the dispersions I'm creating here, needs to focus on other fundamentals at play seemingly being ignored, like quantum entanglement over vast distances, faster than light travel.

[Bill S]

A true answer to your question therefore, despite the dispersions I'm creating here, needs to focus on other fundamentals at play seemingly being ignored, like quantum entanglement over vast distances, faster than light travel.

I don't see how you link quantum entanglement to the recession rates of galaxy groups.

[Bill S]

Due to acceleration of universe's expansion rate, some galaxies with current recession speed faster than the speed of light, previously have subluminal speed.

True; but are the galaxies actually being accelerated? 

There's quite a good explanation here:

http://curious.astro.cornell.edu/physics/104-the-universe/cosmology-and-the-big-bang/expansion-of-the-universe/1066-can-two-galaxies-move-away-from-each-other-faster-than-light-intermediate

[opportunity]

A true answer to your question therefore, despite the dispersions I'm creating here, needs to focus on other fundamentals at play seemingly being ignored, like quantum entanglement over vast distances, faster than light travel.

I don't see how you link quantum entanglement to the recession rates of galaxy groups.

Neither do I....but quantum entanglement can represent not just quanta but groups of quanta.....who says it can't?

Besides, recessions rates of galaxy groups is red-shift basis. Then we have dark matter, dark energy. My point is "where's quantum entanglement on a grand scale"? Why limit the idea to a lab, why not factor it in to the universe?

[Azhar (OP)]

Can this explain it?

newbielink:https://phys.org/news/2015-10-galaxies-faster.html [nonactive]

[Azhar (OP)]

Actually this gives a much better answer: newbielink:http://curious.astro.cornell.edu/physics/104-the-universe/cosmology-and-the-big-bang/expansion-of-the-universe/1066-can-two-galaxies-move-away-from-each-other-faster-than-light-intermediate [nonactive]

[evan_au]

In applying the equations of the Planck scale, those energy realms, it was considered necessary to "consider" the idea of dark energy and dark matter to explain the Planck scale energy equations on the universal realm. Correct me anyone if I am wrong.

I'll have a go...

The idea of Dark Matter was originally presented in terms of the rotation rates of galaxies. With spectroscopy, it is possible to measure the rotation velocity of a galaxy at various distances from the center (measured in thousands of light-years). There seemed to be more matter there than could be seen.

It required no consideration of Planck scales (the very small). It merely used Newton's laws of motion (even General Relativity is unnecessary to spot this one...)
See: https://en.wikipedia.org/wiki/Galaxy_rotation_curve

A hint of Dark Energy is implicit in the equations of Einstein's general relativity, studying how matter moves in the presence of mass. An undefined constant is present in these equations - Dark Energy puts a value to that constant (not the first value that has been put on that constant, by the way).

The discovery of Dark Energy came about through the study of distant supernova, and careful searches and spectrum measurements of these distant events. The smallest thing in these measurements is the large telescopes used for the search, followed by an exploding massive star. There was no consideration of the Planck Scale.
See: https://en.wikipedia.org/wiki/Dark_energy#Supernovae

[Colin2B]

I don't see how you link quantum entanglement to the recession rates of galaxy groups.

My point is "where's quantum entanglement on a grand scale"? Why limit the idea to a lab, why not factor it in to the universe?

If we find reasonable evidence then yes, but at the moment you are introducing unrelated factors without any reasonable evidence. Making broad brush comments like this does nothing to move discussion forward but simply causes confusion for the person asking the question and starts to irritate your fellow posters.

Please read post above by @evan_au

[Bill S]

Could this throw some light on dark matter in the early Universe?

https://www.nature.com/articles/nature25792?WT.ec_id=NATURE-20180302&spMailingID=56095976&spUserID=MjgzNDMxNjU2ODES1&spJobID=1360109627&spReportId=MTM2MDEwOTYyNwS2

[Colin2B]

@Bill S - there is a popsci write up here http://www.bbc.co.uk/news/science-environment-43230729
I just love the Heath Robinson radio spectrometer. That’s real science