Why does "dark energy" push things apart?

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Ronald W. Heiby

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Why does "dark energy" push things apart?
« on: 02/02/2010 11:30:02 »
Ronald W. Heiby  asked the Naked Scientists:
   
I have a point of confusion related to the recent talk of "dark energy" tending to push the universe towards increasing expansion.

I have the idea that gravity is a property of mass. Einstein showed the equivalence of mass and energy. So, how is it that mass in the non-energy state wants to draw things together, while mass in its energy state wants to drive things further apart?

I understand how heating (for example) a gas causes its molecules to bounce around more and want to use more volume. But that does not seem to be what is being described when dark energy is discussed.

Thanks for the interesting programmes!

Ron Heiby
Mahomet, Illinois, US

What do you think?
« Last Edit: 02/02/2010 11:30:02 by _system »

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Offline graham.d

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« Reply #1 on: 02/02/2010 12:16:54 »
There are two aspects to the properties of matter that contribute to the gravitational attraction; one is mass density and the other is "pressure". This results from Stress-Energy tensor component in Einstein's Field Equations. The concept, in this case, is that dark energy has a negative pressure so that as the universe expands the pressure gets greater (making it expand more).

« Last Edit: 02/02/2010 12:19:56 by graham.d »

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Offline acsinuk

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« Reply #2 on: 02/02/2010 17:25:19 »
Dark energy I think is electric as the stars all have positive surface potentials and repel each other.  See alternative electric universe for more info.
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Offline graham.d

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« Reply #3 on: 02/02/2010 17:45:13 »
This is not a very conventional view is it? Are you saying that there is net positive charge in the universe too?

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Offline PhysBang

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« Reply #4 on: 02/02/2010 19:02:06 »
The electric universe idea is a fairly old and greatly discredited idea.

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Offline LeeE

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« Reply #5 on: 02/02/2010 23:16:39 »
Quote
Why does "dark energy" push things apart?

Because the definition of Dark Energy is pretty much 'That which pushes things apart'

No one knows what Dark Energy is, which doesn't stop plenty of people from hypothesising about it, but the bottom line is that the only evidence we have for it is that the universe doesn't seem to be working as we would expect it to do so.

Dark Energy then, hasn't been directly discovered, but is more a requirement to explain what we think we see.
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Offline PhysBang

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« Reply #6 on: 02/02/2010 23:19:20 »
Dark energy does more than just push things apart, it also contributes to the overall mass-energy density of the universe and this determines the overall geometry of the universe. So we can measure dark energy in at least two different ways.

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Offline yor_on

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« Reply #7 on: 03/02/2010 20:02:38 »
There are two aspects to the properties of matter that contribute to the gravitational attraction; one is mass density and the other is "pressure". This results from Stress-Energy tensor component in Einstein's Field Equations. The concept, in this case, is that dark energy has a negative pressure so that as the universe expands the pressure gets greater (making it expand more).

What you're saying here, if I get you right, is that you can imagine space as something in where we have a lot of invisible boxes containing a negative pressure, restricting and constraining those boxes in a inverse direction, and as space around them grows bigger due to the 'expansion' the inverse pressure automatically become larger? The problem here is that you will need a 'box' it seems to me, to explain how this can happen?

As I'm not really sure how you otherwise expect that 'negative pressure' to become 'larger'?
And just how does it help an expansion?
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Or are you using 'negative' as a 'positive expanding force' here?

And then dark matter would be something 'welling forward' somehow, expanding space at the same time as holding it at a constant rate of approximately 74 percent to explain the apparent mass in our universe.

Doesn't make sense, why would our universe be closed, flat (Analysis of data from WMAP implies that the universe is spatially flat with only a 2% margin of error), and constantly balanced at the same apparent mass-mix, when it also is seen as expanding. If this is happening and our universe are 'growing' and our mass-mix is a constant? Then our universe as a whole will mass more if we trust in the idea of distance. And if we don't trust in that idea, we shouldn't speak of 'expansion' at all as it has to be something different, well, as I see it?
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Awhh, I had enough problems trying to understand a photon.
Will I have to understand this too?

Where will it end???
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And how can it be closed if it is growing (expanding) yet keeping a constant percentage of mass, mixed as matter, dark energy, energy, dark mass, and 'vacuum energy'? It doesn't matter if it its percentage is getting 'thinned out' or 'staying the same' or  'begetting more'. Either one of them should indicate a non closed universe it seems to me?

So what am I missing here?
« Last Edit: 04/02/2010 04:02:10 by yor_on »
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Offline Ron Hughes

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« Reply #8 on: 04/02/2010 01:15:54 »
Shouldn't this discussion be in new theories Benv?
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Offline Geezer

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« Reply #9 on: 04/02/2010 07:35:21 »
Ben is hopefully asleep.

As far as I can tell the original question did not propose a new theory, so it's probably in the right place.

However, it would be good if those who post new theories to try to answer the question make that clear.

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Offline acsinuk

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« Reply #10 on: 04/02/2010 11:19:38 »
No Graham
I think that overall the universe is in exact electric balance.  But having said that it is because of the continual movement of magnoflux energy that the systems within in go out of balance and this cause another movemet of electromagnetic energy and so on for ever.
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Offline graham.d

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« Reply #11 on: 04/02/2010 13:24:04 »
Yor-on, I think you have it right. The negative pressure concept means that as the universe's volume (V) increases, the vacuum energy increases (dW = -PdV) because P is itself negative. So the vacuum energy density stays the same. However the Pressure term in Einstein's field equations will dominate, and being a negative term, results in a net increasing gravitational repulsion.

However, I don't think this explains anything but rather just says what the equations reveal. I can't think of any analogy with everyday familiar behaviour that can help. I have tried to find a clear explanation on the web but have failed to do so.

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Offline yor_on

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« Reply #12 on: 04/02/2010 15:16:27 »
Thanks Graham. I've tried to avoid dark energy, as I said I can't even understand a photon :) But I got to admit that there seems to be a lot of sense in the concept. And reading what I wrote lastly I think I got it wrong. It does matter if we see the mix of 'mass' stay the same. If it somehow would be 'thinned out' the idea of a closed but 'expanding' universe would make my sort of sense :)

But as it is not you have two alternatives it seems to me? Either we have a universe that's constantly are begetting 'mass' of some remarkable kind to keep the balance correct, and there I frankly don't care from where it would get this 'energy/mass'. It won't be a closed universe as long as you think the concept of entropy is correct. Or what we call 'expansion' isn't the right definition of what's happening. The later makes more sense to me as it fits right in with my 'pet theory's' about 'distances'.

Although I don't understand that either :::)))

Anyway Graham, you're a real decent guy and it's always nice reading you. Just want to point out that this site have a very nice balance between 'crackpots' like me and the more serious guys. And it's a pleasure participating.
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Offline Good Elf

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« Reply #13 on: 06/02/2010 03:06:59 »
Oops.. deleted!
« Last Edit: 06/02/2010 03:10:53 by Good Elf »
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