When it was learned that the expansion of the universe is speeding up, I assumed that meant the universe would never contract in the "big crunch" and create another big bang.

But this dark matter has me wondering.

Have we been able to determine if the rate of acceleration is changing or constant? If acceleration of expansion is dropping off, like a car topping out on the current gear, maybe gravity has a chance.

One thing I as a layman can offer is that gravity obviously has no chance. Four forces once did not exist and at some point will either cease to exist or be overwhelmed by an emergent force. Neither you nor I can visualize the universal future given cosmological time scales, so why even try?

Well, I mean you can if you wanted to. It requires not the greatest amount of imagination...

Experiments seem to indicate that the rate of expansion is accelerating.

I think he's talking about the 3rd derivitive.


PairTheBoard

I believe current theories state that as the universe gets more 'spread out' from the expansion, the contribution from dark energy will become a larger influence on the evolution of the galaxy. Since it's dark energy that causes an acceleration of the universe (and dark matter along with regular matter) slow the expansion, the "force" producing the expansion of the universe should increase, and hence the acceleration should increase.

Is that a conclusion of the theory or are you drawing the implication? The reason I ask is because it's not clear that the rate of universe expansion works analogously to the velocity of a mass, where the acceleration is proportional to the force applied to the constant mass. It might be more like the rate of expansion of a balloon where you get no coasting for the rate and the rate is proportional to the increase in force. Or something along that line for the expansion of the universe. I don't know. Just asking.


PairTheBoard

I mean, the universe is expanding faster now than it was expanding before.

Yeah, but the above (as worded) only says that the expansion is accelerating, not that the acceleration is accelerating. Is this what you mean to say?

Two caveats; I was thinking about the cosmological constant formulation of dark energy, and two, it's been a while since I've heard anyone talk about this stuff. So my memory might be a little hazy. I looked at the wiki article on dark matter to bring myself somewhat up to speed....

The cosmological constant explanation of dark energy is that a volume of space has an associated energy. That energy exerts a force (pressure) causing the expansion of the universe. The wiki article stopped there, but I think this follows: If the cosmological constant is actually constant, then the force causing expansion is constant. However, as the universe expands, the force slowing expansion (from regular and dark matter) decreases. The net force causing expansion then increases, so the acceleration of the universe should increase.

Now, that's probably a simplistic (and hopefully not incorrect!) explanation of what is going wrong. I think to get a complete model, one would need to use the spacetime metric tensor, and I have bad memories of it!

The rate of acceleration is almost surely changing. It's dependent on dark energy but also the density of the universe. In the past, the expansion was likely slowing down since the gravitational attraction between bodies closer together is larger and the universe was smaller back then. Now, matter is spread out enough to where the dark energy term overcomes the gravitational one in terms of how the rate of expansion of the universe is changing.

In terms of what will happen in the future.....I don't think anything has been totally ruled out. I think expansion forever is what most people think, but a big crunch is also possible. Cyclical universes have problems not violating thermodynamics but are not totally ruled out as well afaik.

This sounds to me like the rate of expansion should max out to what the rate would theoretically be if the space had no matter of any kind and thus no gravity within it. If that's the case then expansion would continue to accelerate but at a declining rate of acceleartion as the rate of expansion continues to increase asymptotically to its max value. While the acceleration would decline toward zero it would never reach zero and never turn negative.


PairTheBoard

That sounds right--but as I said before I could be missing something.

Also, this is assuming the cosmological constant is actually constant and not dependent on something that changes as the universe expands.

To my knowledge, the following is observationally true, as of today:

- The rate of expansion is currently accelerating. This is known to very high confidence (with assorted caveats)
- Of order 5-10 Gyr ago, the rate of expansion was probably not accelerating. That is, there is evidence that the Universe went from decelerating to accelerating expansion around 5-10Gyr ago.
- The rate of change of the acceleration of the rate of expansion is undetermined.

As for what is causing the acceleration... there's no consensus. The mere fact that it's (probably) happening is pretty mind blowing. The cosmological constant is one possibility, but its among a class of solutions that involve constructing an equation of state for a `thing' that could cause the observed effects. In some senses it's not helpful to think about it as an energy density of space, since the key fact of accelerating expansion is that it happens on comoving length scales. The phrase 'dark energy' should be taken to mean "this weird phenomena that doesnt seem to have anything to do with light, and that seems to make something happen".

I can envision a cessation of life but cannot conceive of the conditions through which such a cessation will be realized, or the conditions which would exist in the time immediately preceding this cessation. No human imagination can envision this, so about your requirements....

an immortal Peter Parker will in fact deliver us to the clearly-next-most-hospitable universe in our infinite (?) multiverse, yes

Could you explain a little what this means and why it's such a "key fact"?

Thanks,

PairTheBoard

It's the observation that (accelerating) expansion happens on very large length scales, of order 100Mpc or greater (such length scales are sometimes called comoving for largely historical reasons). Which is, coincidentally or otherwise, also the scale at which homogeneity seems to be established. We don't observe accelerating expansion on scales much smaller than this. Our cluster of galaxies sticks together, as do (probably) superclusters of galaxies. And so do individual galaxies, obviously.

This (to me at least) is key as it gives a nice starting point for working out why we observe this to happen. Do we not see accelerating expansion on smaller length scales because it doesn't exist on those scales, or because something (like gravity) stops it? In other words - if I had an assembly of massless, chargeless particles grouped closely together (say, within a few metres) then, as time goes by, would they stay together with their original separations, or would they move apart at an accelerating rate? It would be a fascinating experiment to do.

I'm like pre-K compared to most ITT, so know that, but...if dark matter and/or energy is currently helping to "push" mass outward as opposed to "pull" it, does that push cause the acceleration?

Also, could we construct a craft that utilizes this push for space travel?

The expansion is of space itself, propelled by dark energy (aka wtf), not dark matter. Think dots on a balloon being blown up. The dots do not have velocity relative to each other, but the space between them expands. The rest of your questions are above my pay grade.

For all interested to remain current on standard reasonably recent results and potential explanations with variable degrees of confidence.

http://en.wikipedia.org/wiki/Lambda-CDM_model

http://pdg.lbl.gov/2012/reviews/rpp2...-cosmology.pdf

http://pdg.lbl.gov/2012/reviews/rpp2...parameters.pdf

http://en.wikipedia.org/wiki/Dark_energy

Read it and then recognize why it is really not reasonable to speculate about such things yet while we are missing fundamental understanding of the mechanisms involved and just before physics will experience a remarkable framework altering breakthrough that can reframe many calculations.

The best one can do at this point is simply to study the observations and the most respected results and interpretations and remain conservative about speculations regarding the very early universe or the very distant future.

No reason to feel even remotely confident with current theories and measurements that this universe will recollapse. It doesnt look that way and frankly it never did look that way (moreover the theoretical possibility when we started measuring cosmological parameters as one of options) but pending deeper understanding we may yet have to go through a phase transition we do not understand that could change even that. The most problematic however in all these is that many of the features we observe can easily turn out to be explained by theories that are completely different than current ones and make things like dark matter or dark energy only effective concepts not fundamental ones or if real understand their nature in ways that opens new considerations and possibilities essentially rewriting equations of expansion.

Some very detailed book for anyone interested to start easy and get to more interesting topics eventually.

http://www.amazon.com/Introduction-M...2nd+edition%29

At the very least go over the particle data group pdf files (answers about where we stand in terms of what kind of universe cosmological parameters suggest exist in their figures) because only then you will appreciate how it all comes together and why it becomes very speculative after a point.

Sorry but it cant be avoided, its just sci fi and story telling often and fancy but not on solid ground documentaries otherwise.


Here are some lectures too

That sounds like the accelerated expansion is not happening uniformly in all regions of space. It's evidently happening in the vast empty regions but not observed to be happening close to where there's a lot of matter and gravity.

So I wonder what is happening close to a black hole. Could space actually be contracting close to a black hole? If so, has it been observed - maybe indirectly?


PairTheBoard

Noway near a black hole or anything like that, the dark energy in terms of the contribution to the field equations and the resulting metric solutions eventually is tiny at local scales almost unimportant. Its basically Schwarzschild solution with a cosmological constant thats all.

The expansion is a very long distance effect. Of course all is expanding but i mean locally you do not notice stars in the same galaxy expanding away in any prominent sense to be able to substantially modify local general relativity solutions. So imagine what happens in the scale of a few astronomical units that is the neighborhood of a black hole.

http://en.wikipedia.org/wiki/De_Sitt...zschild_metric


Read also the section on cosmological constant on dark energy entry;

http://en.wikipedia.org/wiki/Dark_energy

to see how small of a density term it is to understand why locally its a triviality of no material interest. Its like a 10^-29 gr/cm^3 type of density term in the field equations. Its a negative pressure term with pressure proportional to -density.

Locally where you have matter/energy they dominate the field ie near a star or a galaxy etc. Those mass terms i mean are much larger vs the cosmological constant term in the resulting differential equations that determine the metric.

Originally Posted by PairTheBoard
"That sounds like the accelerated expansion is not happening uniformly in all regions of space. It's evidently happening in the vast empty regions but not observed to be happening close to where there's a lot of matter and gravity.

So I wonder what is happening close to a black hole. Could space actually be contracting close to a black hole? If so, has it been observed - maybe indirectly?"





I'm not sure you understood my question. I wasn't asking if accelerated expansion was still happening close to a black hole. From the post I was responding to I understood that accelerated expansion was not occuring close to galaxies or even near clusters of galaxies. So that makes me wonder, if the presence of relatively small gravitational forces can cancel the accelerating pressure that happens far from gravity, is it possible that greater gravitational forces might decelerate nearby expansion. And in the extreme case of space near a black hole, with a little time might the extreme gravity actually reverse the expansion causing nearby space to contract?


PairTheBoard

This is why i gave you an example of a solution of spherically symmetric gravitational source (bh or star or even any spherically symmetric mass) in the presence of a cosmological constant.

All you have to do is look at the metric to see what you asked;

1-2MG/r-b*r^2 (c=1 units) is the important term (would be 1 for flat empty space) where M is the mass and b i think if i recall correct is 1/3*L (With L the cosmological constant))

Near the neighborhood of the hole or the object say a few times 2*MG/c^2 the L*r^2 terms is tiny and doesnt have even an experimentally measurable effect. As you go far from the hole however it becomes the dominant term reflecting of course an expanding spacetime.


Now solutions that describe what happens to an entire galaxy with a cosmological constant dont exist but the behavior will be similarly unimportant.

Would it be satisfactory to you to calculate for example how much 2 objects separated by 1 au expand in 1 year???

Using Hubble's law the constant being 70 km/s per Mparsec one obtains 11m per year or 7% of au per billion years. Of course local gravity is so much more dominant that this practically cannot be observed and can only be seen by looking at distant galaxies.

What else other than that would you mean???

The dark energy if it is real and not an effective description of something else covers all spacetime but mass is only a rare localized coincidence in the vastly empty at large universe (consider what fraction of a cluster of galaxies is space between galaxies to see what i mean plus you also know local density is like order 1 solar mass per 50 say cubic light years = already tiny with the vast majority of the mass inside less than 1% of au radius). Of course you do not expect cosmological constant to play any role close to gravitational sources and small distances/timeframes. By all that i mean that the avg density (source of gravity) of the universe is many orders of magnitude smaller than that of a galaxy and the latter is also many orders of magnitude smaller than that of a solar system etc.

However when you study the effect at global scales eg in the Robertson Walker metric which attempts to describe the entire universe and which clearly has to use some effective avg density of the universe as the source term in the stress tensor and which is itself super small (tiny density ) the Cosmological constant term dominates the metric at present era and dictates expansion and acceleration of it.


Ps: If the black hole is not dynamical the spacetime around it is not contracting.

Also global metric solutions like FLRW http://en.wikipedia.org/wiki/Friedma...3Walker_metric assume homogeneity isotropy and are obviously idealizations, locally you have other solutions and you cannot exactly carry the observations ie the expansion ala Hubble law the same way locally that the system is no longer obeying those symmetries and the solutions will be different. Basically to fully satisfactory answer all this you need a solution that is using both local sources and distant global sources (eg a Scharzschild metric embedded in rotating galaxy metric which is itself embedded in a galaxy cluster etc which then belongs to a FLRW type metric expanding universe with cosmological constant term and then using that solution you can see what is the behavior locally and how it differs from the accelerated expansion of the large scale solution. Do not expect however anything spectacular (plus no so complex known solutions exist really), the solution locally is dominated so vastly by the local gravity distributions/motions and the effects of other stars and the rotation of the galaxy that any cosmological constant effect is trivially tiny in comparison. This is why i gave you the de Sitter Schwarschild solution as an example of a solvable problem which fully answers your question actually if you ignore the effect of the rest of the universe.