Universe expanding

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What allows the universe to expand faster than the speed of light? What force will slow it down, causing it to start contracting on itself?
 

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  • #2
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What allows the universe to expand faster than the speed of light? What force will slow it down, causing it to start contracting on itself?
For all these questions: the distribution of mass, energy and pressure in the universe.
 
  • #3
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It is space itself expanding, not objects in space, and for basically unknown reasons.

So over vast distances the expansion (additional volume) of space causes distant objects to move apart at velocities greater than "c" as space pushes them. The latest thinking has this acceleration possibly increasing for several billion years, but the cause is unknown. Some think it's dark energy powering the expansion. Many currently believe, without experimental proof, that the universe will continue to expand and cool down and eventually everything may approach absolute zero and an "empty" dead universe.
Hubble's measurements show the expansion is accelerating, originally much to everyone's surprise, including Hubble, but the mechanisms are still not understood.
 
  • #4
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Brian Greene's FABRIC OF THE COSMOS has a nice, non mathematical explanation of expansion of the universe.....whats been observed, some theories and implications, what's known and what isn't. I'd guess about 50 pages or so...
 
  • #5
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Brian Greene's FABRIC OF THE COSMOS has a nice, non mathematical explanation of expansion of the universe.....whats been observed, some theories and implications, what's known and what isn't. I'd guess about 50 pages or so...
Thanks i'll take a look. I just remembered reading somewhere that eventually the universe will stop expanding and recollapse. I'm not sure how a prediction like this may be made without knowing what is causing an accelerated expansion as of now and what force will hault this acceleration.
 
  • #6
DrGreg
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I just remembered reading somewhere that eventually the universe will stop expanding and recollapse.
That's what most people used to think a decade or so ago, but now it looks like the expansion is accelerating.
 
  • #7
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Thanks i'll take a look. I just remembered reading somewhere that eventually the universe will stop expanding and recollapse.
That actually depends on the density of the universe. If it is not dense enough it will never collapse.
 
  • #8
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if its dense enoigh it will collapse (due to gravity)? if its gravitationally wanting to collapse then why would it be accelerating?
 
  • #9
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That actually depends on the density of the universe. If it is not dense enough it will never collapse.
Basically it would have to reach the density of a massive star right before it collapses into a black hole?
 
  • #10
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if its gravitationally wanting to collapse then why would it be accelerating?
Just because an object is being pulled on by gravity doesn't mean it can't accelerate.
 
  • #11
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if its dense enoigh it will collapse (due to gravity)? if its gravitationally wanting to collapse then why would it be accelerating?
Accelerating expansion is due to the energy of the vacuum, e.g. a positive cosmological constant.
 
  • #12
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Accelerating expansion is due to the energy of the vacuum, e.g. a positive cosmological constant.
didn't einstein call his cosmological constant his greatest mistake?
 
  • #13
Fredrik
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didn't einstein call his cosmological constant his greatest mistake?
Yes, when he realized that he had introduced it for a really bad reason. But that doesn't mean that there were no good reasons.
 
  • #14
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Just because an object is being pulled on by gravity doesn't mean it can't accelerate.
Just for the good order, objects undergo no proper acceleration in an accelerating FRW universe. Distances are dynamic under general relativity, they can increase and decrease in time due to the curvature of spacetime.
 
  • #15
Fredrik
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Basically it would have to reach the density of a massive star right before it collapses into a black hole?
No, in the "closed" FLRW solution, the density keeps decreasing until the moment when the rate of expansion has slowed to zero. That universe ends in a big crunch.

By the way, if you just consider a massive enough cloud of gas under the influence of gravity, it will eventually collapse to a black hole no matter how low the density is right now. So you don't need high densities to create black holes.
 
  • #16
One view would be looking at dark matter/energy as the fuel of the universe's expansion. As if the universe itself is a rocket ship, running on this dark fuel, and gravitational forces are all pulling it back to its original point. At the moment we can't be sure if the universe will ever run out of gas(or if the gravitational forces pulling the universe back will ever be greater than the forces at work expanding it), since the expansion is in fact accelerating. Since we arn't able to pinpoint the cause of this acceleration, whether it be the creation of more dark fuel as the expansion continues or some vacuum effect happening, we arn't able to determine whether its plausable that the universe could ever collapse.
 
  • #17
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if its dense enoigh it will collapse (due to gravity)? if its gravitationally wanting to collapse then why would it be accelerating?
Yes, and this is not intended to modify any of the above posts....

If the sum total of all the attractive gravitational force exceeds the total of the expansionary force, the universe would eventually stop expanding and begin contracting.

As Mejennifer posted:

Accelerating expansion is due to the energy of the vacuum, e.g. a positive cosmological constant.
But nobody knows if this acceleration is uniform and nobody yet knows why the force would be strong enough to be expansionary. Maybe it's a 'fundamental' part of all universes, maybe not. As I posted, it appears to have increased in the last several billion years (I'm unsure how "proven" that is.) so maybe it could decrease in the future...but I have not read that as a likely scenario anywhere...so MAYBE expansion could slow and then reverse...that's not current thinking from what I have read.

In any case if anyone has read anything linking today's cosmological constant/vacuum energy/zero point energy with the origin of the universe, I'd be really interested. "Unification" appears to have combined three of our four well known forces (fields) but much remains unknown....mass,energy,space,time
 
  • #18
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Wikipedia says:
http://en.wikipedia.org/wiki/Cosmological_constant


It is now thought that adding the cosmological constant to Einstein's equations does not lead to a static universe at equilibrium because the equilibrium is unstable: if the universe expands slightly, then the expansion releases vacuum energy, which causes yet more expansion. Likewise, a universe which contracts slightly will continue contracting.
I haven't seen quite so assertive an explanation about expansion elsewhere. If this is correct, there's no turning back!!! I think they are implying the the constant energy density of the vacuum effectively increases as more vacuum (space) appears....if that's the case the acceleration would appear to increase..... without bound??

Brian Greene says:
Pressure is also a source of gravity...for ordinary matter pressure is positive (attractive)...but a cosmological constant exerts an overall repulsive gravitational force...(resulting from a negative pressure)...At the origin of the universe the non zero (and temporarily very high energy) Higgs field generated a gigantic gravitational repulsion ....usually called the inflation field to distinguish it from (today's) electroweak Higgs field giving mass to familiar particles...
(items in parenthesis () are added by me.)

..As the universe expands matter and radiation lose energy to gravity while an inflation field gains energy from gravity.
Here again it appears acceleration of the universe will be increasing...
 
  • #19
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It is space itself expanding, not objects in space, and for basically unknown reasons.
______________________________________________________________________
I remain unclear whether the redshift that is observed is attibuted to the relative velocity resulting from space expanding [doppler] or if it is considered to result from the metric of space itself expanding, stretching photons with it ,thus longer wavelengths.
I am not sure whether I am unclear on the concept or whether there is just a prevailing ambiguity as I have read conflicting accounts. Have you gotten a more definite idea????
==========

So over vast distances the expansion (additional volume) of space causes distant objects to move apart at velocities greater than "c" as space pushes them.
______________________________________________________________________-
Hi Does this mean that observed redshifts exceed the redshift of v=c as calculated in SR
thus indicating speed>c

or is it based on the apparent size of the present universe relative to its projected age???.
=========

Thanks
 
  • #20
Fredrik
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It is space itself expanding, not objects in space, and for basically unknown reasons.
______________________________________________________________________
I remain unclear whether the redshift that is observed is attibuted to the relative velocity resulting from space expanding [doppler] or if it is considered to result from the metric of space itself expanding, stretching photons with it ,thus longer wavelengths.
I am not sure whether I am unclear on the concept or whether there is just a prevailing ambiguity as I have read conflicting accounts. Have you gotten a more definite idea????
==========

So over vast distances the expansion (additional volume) of space causes distant objects to move apart at velocities greater than "c" as space pushes them.
______________________________________________________________________-
Hi Does this mean that observed redshifts exceed the redshift of v=c as calculated in SR
thus indicating speed>c

or is it based on the apparent size of the present universe relative to its projected age???.
=========

Thanks
You should probably learn to use the "quote" tags. Use the quote button to see what they look like.

The observed redshift of galaxies is due to the expansion of space. The wavelength expands with the cosmological expansion as the light approaches us. The SR formula says that the redshift goes to infinity (infinite wavelength) as v goes to c, and the observed redshifts are certainly not more than infinite. If you'd like to know how redshifted the most distant galaxies and the background radiation are, you could try finding a post by Marcus where he explained it. (It was probably in the cosmology forum).
 
  • #21
Ich
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The SR formula says that the redshift goes to infinity (infinite wavelength) as v goes to c, and the observed redshifts are certainly not more than infinite.
That depends on how you define "v". Old Smuggler https://www.physicsforums.com/showpost.php?p=1366649&postcount=24" to me. So, contrary to the claims made in the Davis/Lineweaver paper, one may interpret redshift as due to recession velocity.
 
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