why the universe is expanding faster and faster

Garth

The Cosmological Constant ([itex]\Lambda[/itex]) is a possible component of Einstein's field equation. It serves the function of an integration constant in that its presence does not violate the conservation properties of the Einsteinian tensor with respect to covariant differentiation. It need have no counterpart in quantum physics, it is simply part of how gravity on its own might behave.

[itex]\Lambda[/itex] represents a repulsive force that becomes significant only at large ranges, whereas the normal Newtonian force becomes increasingly significant at shorter ranges. If weak enough [itex]\Lambda[/itex] would be undetectable in the solar system yet dominant at cosmological ranges.

Cosmic acceleration may be evidence of [itex]\Lambda[/itex] having a non-null value.

The zero point energy field, detected only as the weakest of forces, the Casimir force, is something different; although it should behave gravitationally identically to [itex]\Lambda[/itex] and is often confused with it.

Theoretically ZPE is of huge energy density and if it affected gravitational fields it would totally dominate over all else, being OOM 10¹²⁰ times larger than [itex]\Lambda[/itex].

As you indicate the resolution of this enigma awaits a full quantum gravity theory.

Garth

notknowing

I've been reading about quintessence which has the property that it has a spatial and temporal dependence. As you explained before, the cosmological constant is a possible component of Einstein's field equation but it is really a constant, independent of space and time. What I don't understand however is how this can be reconciled with quintessence. Why is there suddenly a new component which is not a real constant and yet compatible with Einstein's field equation? How does this mathematically makes sence?

Garth

A good question!

One problem with [itex]\Lambda[/itex] is that its energy density is roughly equal to that of matter in the present epoch. Why should this be so as [itex]\Lambda[/itex] is a constant, as you say, and matter-energy density has decreased by a huge factor ~~10⁸⁰ since the Inflation era?

If we look at the Einstein Field Equation:

[tex]G_{\mu \nu} = 8\pi G T_{\mu \nu}[/tex]

[itex]\Lambda[/itex] enters in on the LHS of the equation as part of the [itex]G_{\mu \nu}[/itex] description of the nature of space-time curvature, whereas quintessence, ZPE, etc. etc. would enter in on the RHS of the equation, if they exist.

The differences between these various speculative possibilities is their equation of state, that is, how their energy relates to their pressure.

Cosmic acceleration demands that overall p < -[itex]\frac{1}{3} \rho[/itex]
and if DE is given the equation of state p = - 1[itex]\rho[/itex], then that fits the data pretty well.

If you move [itex]\Lambda[/itex] across to the RHS of the equation then it also has this equation of state, which also happens to be the equation of state of the false vacuum, ZPE.

Hence either [itex]\Lambda[/itex] or false vacuum is the leading candidate, but that suggestion then has to explain why its density is so small.

As an alternative you can make up whatever equation of state you want for a hypothetical 'quintessence' to make it fit the data, but until you have identified it in the laboratory its just 'pixie dust' IMHO!

Garth

aman malik

has ether has to do something with dark matter or dark energy??

EL

Probably not with dark matter. Depending on what you mean by "aether" it could have something to do with dark energy. Please define what you mean by aether first.

aman malik

actually it was belived or rather it is believed that the universe is filled of a material known as ether. a space ship moving in a ether would see a light ray travelling from behind moving at a faster pace.
for further reading read stephen hawkings universe in a nut shell page 3-5
the spelling is ether not aether

jquark

i refer ether to be an anesthetic and solvent, and aether as a medium for light waves. my preference.

Chronos

The case for 'aether' was refuted by the michelson-morley experiment. See
http://galileoandeinstein.physics.vi...michelson.html

keepitmoving

hypothetical thought about the dark energy - if the extra dimensions do exist and were compactified during the early high density gravitation of the early universe, could the extra dimensions be decompactifiying into our visible dimensions in the relatively low density gravity universe of today, and that is the extra space that is occurring between galaxies (dark energy)?

keepitmoving

hypothetical thought - if the extra dimensions are found and they were compactified in the early high density universe and are now decompactifiying into the visible dimensions of our relatively low density universe, could that be the source of the extra space that is occurring (dark energy)?

Hal King

Actually, whether the universe expansion is accelerating or decelerating -- now or in the past -- is completely model dependent.

The current popular model has been modified to incorporate 'dark energy' -- something made up to make the model better fit the data because it couldn't account for the departure -- i.e. EITHER the data compared to the model made it look like acceleration OR the model didn't work. So the normal cosmology solution was applied. Make up something 'dark' that can't be seen to account for it.

Same process was used for exotic dark matter. The models weren't working so some matter was added to fix it.

hippy dippy cosmologist says: ITS DAAARK ....

Wallace

Yeah, those cosmologists are idiots. Imagine changing the theory in light of new evidence! That Galileo bloke has a lot to answer for....

Naty1

Post 27 and 28 above reflect some possibilities and uncertantities in the big bang model.

A recent related thread, http://www.physicsforums.com/showthread.php?t=318568
might be of interest to the original poster, and also my post #7 there which refers to another model, a cyclic model (instead of a one time big bang model) which has incorporated many findings of the last 20 years and eliminates the required fine tuning in the big bang model.

Current expansion and dark energy required further fine tuning of the big bang model....another way to say it is that the original big bang expansion hypothesis has no explanation for the current dark energy....so it had to be "glued on"....doesn't make it wrong, but suggests there are fine points, perhaps major ones, we have not yet understood.

Let's face it, cosmological models (and our understanding) are in their infancy: Not so long ago, about the 1920's?? maybe, it was believed our puny galaxy WAS the universe!!!!!!!!!!!!!!!! ....then we "discover" black holes, which Einstein thought impossible, then dark matter and dark energy, the latter two accounting for perhaps 96% or so of all matter and energy!!!!!; next maybe we'll find supermassive black holes by the billions have gobbled up another 5% 10% 50%?? or whatever of original matter and energy....
and just recently Hawking decides information is not really lost forever in black holes when multiverses are considered...."consensus science" on big issues has never been initially right; no reason to expect it would suddenly change now.

Hal King

All normal for human nature. And cosmologists have to recognize that they are human too.

Its good that people -- as individuals or groups -- support and work on their models of choice.
But one must always remember that 'consensus' is NOT the goal -- the science is.

If full consensus were ever reach, scientific investigation would come to a halt.

It's not going to ever happen -- for there will NEVER be a 'theory of everything'. (see Godel)

I guess my problem is the 'labels' people put on the problems in a model. Such as 'dark matter' or 'dark energy'. Such labels carry mental baggage implying that they are a known commodity of existence. In fact, in both cases they are used to describe collections of problems that have been lumped together. Sometimes the only reason for their groupage is that the problems were enountered in a similar timeframe.

This results in any solution not only having to address one problem -- but many others that may be completely unrelated -- even the 'unrelation' has to be 'proved'.

This system of thinking is slowing progress and diverting efforts.

Wallace

What are you referring to??

I love this debating tactic (politicians and advertisers use it all the time). State something that everyone agrees on/does in order to (falsely) imply that some other group does. E.g.

"We on our side of politics support freedom and democracy"
"Our clothes are Lead free"
etc etc

The statements above are in the same vein.

Dark energy and dark matter are different and probably unrelated things. You need to be much more specific in your rantings if you want to make sense. For instance, detail where and why they were introduced. What were they problems that were being addressed by their introduction?

Again, you need to be specific. It is not at all clear what you are talking about here???

Really? Because progress in cosmology has never happened more rapidly than in the last decade.

What is your alternative??? I don't mean what is your alternative theory, but an alternative system for doing cosmology? At present there is a lively and intelligent debate within cosmology about all kinds of interesting ideas. It is a very exciting time to be doing cosmology, precisely because things are changing so fast. The idea that there is a rusted on sets of ideas that are being held irrationally due to the resistance to change is just fanciful.

Hal King

Afraid it's not.

And I wasn't saying dark matter and dark energy were the same -- if fact just the opposite. Even going so far as saying that the problems people are using dark energy to solve may not be the same in all cases ... LIKEWISE for dark matter.

In fact, I believe your response 'proves' my point.

mark2929

Dark matter is not understood and like God is used as a fixit for whas unknown What we do know is the universe is expanding faster and the likliest cause is new matter is entering so
1) The big bang is ongoing and creating new matter well time hasnt stopped so it must be.
2) Matter is coming through from another dimension maybe thats what dark matter is? The result of another dimensions black hole coming out this side?
3) We are being pulled towards an outer edge and as we get nearer so its gravity pulls us faster.
4) The equipment for measuring time is faulty or something missed.