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dkgolfer16
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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.dkgolfer16 said: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?
Naty1 said: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...
That's what most people used to think a decade or so ago, but now it looks like the expansion is accelerating.dkgolfer16 said: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.dkgolfer16 said:Thanks i'll take a look. I just remembered reading somewhere that eventually the universe will stop expanding and recollapse.
MeJennifer said:That actually depends on the density of the universe. If it is not dense enough it will never collapse.
granpa said: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.granpa said:if its dense enoigh it will collapse (due to gravity)? if its gravitationally wanting to collapse then why would it be accelerating?
MeJennifer said:Accelerating expansion is due to the energy of the vacuum, e.g. a positive cosmological constant.
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.dkgolfer16 said:didn't einstein call his cosmological constant his greatest mistake?
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.dkgolfer16 said:Just because an object is being pulled on by gravity doesn't mean it can't accelerate.
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.dkgolfer16 said:Basically it would have to reach the density of a massive star right before it collapses into a black hole?
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.
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.
(items in parenthesis () are added by me.)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...
..As the universe expands matter and radiation lose energy to gravity while an inflation field gains energy from gravity.
Naty1 said:It is space itself expanding, not objects in space, and for basically unknown reasons.
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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?
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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.
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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?.
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Thanks
Austin0 said:You should probably learn to use the "quote" tags. Use the quote button to see what they look like.Naty1 said:It is space itself expanding, not objects in space, and for basically unknown reasons.
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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
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).
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.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.
The current understanding is that the universe is expanding at an accelerating rate. This means that the space between galaxies is growing larger, and the distance between objects in the universe is increasing over time.
The main force driving the expansion of the universe is dark energy. This mysterious force is thought to make up about 68% of the total energy in the universe. Another force that plays a role in the expansion is dark matter, which makes up about 27% of the universe's energy.
Dark energy is a repulsive force that counteracts the effects of gravity, pushing galaxies and other objects further apart. As the universe expands, the amount of dark energy remains constant, causing the expansion to accelerate over time.
While dark energy is currently the dominant force driving the expansion of the universe, gravity can have a slowing effect on the expansion. As objects with mass move further apart, gravity will eventually cause them to slow down and potentially even reverse the expansion.
Scientists use a variety of methods to measure the rate of expansion, including observing the brightness of supernovae, studying the cosmic microwave background radiation, and measuring the distribution of galaxies. These methods provide different but complementary data to help determine the rate of expansion and any changes over time.