Question about the expanding universe

[Highburymark]

How can quasars be moving away from us faster than the speed of light?

I keep reading that the final great unsolved question is how to equate general relativity with quantum mechanics, but how much do we really understand about what's going on beyond the observable universe?

[Vox45]

From what I understand it is space itself that is expanding, so objects are not going faster than the speed of light but the speed at which some of these objects are getting away from us can be that fast. Think of it as dots on a balloon that are close together and then you inflate the baloon. The dots are not moving on the fabric of space, but space is expanding. It is a bit more complicated than that so I'll add this link that explains it much better

 

[goodricke1]

They've been discussing this over on the physicsforums, the argument being that the 'recession' of the universe is a different concept from the 'motion' of the universe. Be warned though, it's heavy going...

https://www.physicsforums.com/threads/expansion-of-the-universe-acceleration-limit.852651/

[andrew s]

18 hours ago, Highburymark said:

How can quasars be moving away from us faster than the speed of light?

I keep reading that the final great unsolved question is how to equate general relativity with quantum mechanics, but how much do we really understand about what's going on beyond the observable universe?

This is probably the best answer I have come across see attached. Beyond the observable universe all we have is extrapolations from theory. One point to note is we can see beyond the point at which the recession velocity reaches the speed of light! 

Regards Andrew

 

Cosmological Expan.pdf

[andrew s]

21 hours ago, Highburymark said:

How can quasars be moving away from us faster than the speed of light?

I keep reading that the final great unsolved question is how to equate general relativity with quantum mechanics, but how much do we really understand about what's going on beyond the observable universe?

This is probably the best answer I have come across see attached. Beyond the observable universe all we have is extrapolations from theory. One point to note is we can see beyond the point at which the recession velocity reaches the speed of light! 

Regards Andrew

[Highburymark]

Thank you for your contributions 

[Highburymark]

Interesting that the first point made in the link provided by Andrew is that many popular science books misinterpret or are wrong about the expansion of the universe. It also doesn't help that the media has a voracious appetite for stories about the cosmos - and overblows the significance of many of them. Our understanding of some of the big questions may have been transformed over the past couple of decades, but are we any closer to answering them? 

[ollypenrice]

On 29/1/2016 at 14:48, andrew s said:

This is probably the best answer I have come across see attached. Beyond the observable universe all we have is extrapolations from theory. One point to note is we can see beyond the point at which the recession velocity reaches the speed of light! 

Regards Andrew

 

Cosmological Expan.pdf

 

I'll read this with interest (or incomprehension!)

Regarding the expansion of the universe and 'normal' relative motion I see a distinction which I'd be interested to have commented upon by an expert. If a rocket leaves the earth and heads away into space the rocket has been accelerated and the earth has not (at least once its exhaust gasses no longer impact upon the earth). If I stand on roller skates and throw a cannonball away from myself I'll head backwards and the cannonball will head forwards and we'll both have been accelerated. In both cases acceleration can be seen as the precursor to the relative motion we observe.

But in the case of the universe we see the galaxies racing away from us in all directions so we have not been accelerated. If we accept the expansion of the universe as the explanation for this, rather than the explanation that we are the centre of an explosion, then all observers in all galaxies will see the same thing. None of these observers in any of these galaxies will feel they have been accelrated. Yet we are racing apart from each other.

This seems to me to distinguish between two types of motion but, although I've read a lot of cosmology and even taken a course in it, I've never seen the issue discussed in terms of acceleration. (I don't think!)

Let me stress that I do accept the expansion of the universe and reject the idea that we're in the middle of an explosion but I'm curious as to how physics feels about acceleration in this case.

Olly

 

[andrew s]

12 hours ago, ollypenrice said:

I'll read this with interest (or incomprehension!)

Olly, I assume that was an inclusive "or". I am no expert for that try the Physicsforums noted above. However, modern theorists make a clear distinction between true forces, the effect of which can be measured using an accelerometer, and pseudo-forces which can't. In the case of GR a uniform gravitational field produces a pseudo-force and the tidal forces from a non-uniform field a real force. 

I am not sure I understood your point but I think the dark energy that drives the expanding universe is likened to a pseudo-force while you rocket engine is a real one.

I realised a while ago that much of what I learnt at university doing Physics is not not considered correct any more and I have started on a process of relearning it. 

Regards Andrew

 

[saac]

Not sure how this relates to Olly's question regarding acceleration felt or otherwise by observers in the various frames of reference but I thought it worth throwing into the mix.  Regarding the expansion of the universe it is the universe itself, space time, that experiences the expansion, not the objects within it.  The galaxies, stars etc all have their own relative motions, some moving away some on a collision course.  I'm also not convinced that the expansion, while it has a positive changing rate  (albeit now reducing) and is casually referred to as an accelerating expansion - is  really an acceleration in the Newtonian sense ( ie unbalanced force acting on mass).  I'm not convinced it is (does space time have mass) but I'm equally open to being convinced that this is due to the limit of my understanding.

edited - just a thought are we (or I)  confusing the recessional velocity of galaxies (and hence acceleration and mass and forces and who feels what) with the expansion of space time (the universe)?

Jim

[Highburymark]

And yet space time can expand at (apparently) astonishing speed, while we still see superclusters of galaxies forming at immense scales.

[andrew s]

15 hours ago, Highburymark said:

And yet space time can expand at (apparently) astonishing speed, while we still see superclusters of galaxies forming at immense scales.

It's not quite like that. The expansion rate is proportional to distance so however far away a super-cluster of galaxies is from us (and hence moving rapidly away from us) their recession velocity with respect to each other is proportional to there "local" separation. Even on the scale of super-clusters this is relatively slow and overcome by there mutual gravitational attraction. 

Regards Andrew

[Highburymark]

Ok - that makes sense - thanks Andrew