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Author Topic: Why is dark energy considered to be a push rather than a pull?  (Read 2955 times)

Gerhard Viljoen

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Gerhard Viljoen  asked the Naked Scientists:
   
Dear Chris
 
Our 8 year old son is very very interested in dark energy and the origins of the universe.  We have read bits about it (including what we could find on the Hubble site) but are thoroughly ignorant of dark energy.  Our son has a question that we can't answer and we thought that possibly you might be able to help.
 
Why is dark energy necessarily an unexplained push?  Why can't it be a pull explained by the fact that we / Earth is inside a kind of unit like our galaxy or solar system or something that is pulled towards something outside our known universe with greater mass than us that pulls our universe towards it/apart?
 
Best wishes
 
Jeanne-Marie

What do you think?
« Last Edit: 22/07/2010 17:30:02 by _system »


 

Offline Madidus_Scientia

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Well hopefully someone will correct me if i'm wrong but I don't think it's a push or a pull, but an expansion of space, which has the effect of increasing the distance between any two points, which makes us think of it in terms of a push or pull. But nothing is actually being moved.
 

Offline JP

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I know two possible answers to this, but in checking for more details, I found this site from NASA, which seems very good:
http://science.nasa.gov/astrophysics/focus-areas/what-is-dark-energy/

Basically, if you plopped a bunch of normal energy down into space, you would expect it to cause the universe to contract, or at least slow down in its expansion, according to general relativity.

But the universe is accelerating in its expansion, which could be attributed to a few things:
If general relativity is correct,
1) space itself might contain energy, so that when more space is created, more energy appears--this has the effect of accelerating the expansion,
2) there might be some new substance, called "quintessence" that has the opposite effect of energy (it causes the universe to expand) [this theory is new to me],

Or 3) general relativity could be wrong at the scale of the entire universe.  In this case, the accelerating expansion might be perfectly well explained in terms of some new theory of gravity without the need for one of the above two explanations.

There isn't enough evidence to really favor any of these options, as far as I know, and there may be other plausible explanations.
 

Offline LeeE

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No one actually knows what Dark Energy is, or is sure that it exists.  The term 'Dark Energy' is really just a definition of what is needed to explain an observed anomaly, rather than a definition of the thing itself that is causing the anomaly; it is more a description of the problem than it is a description of the answer.
 

Offline Pmb

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No one actually knows what Dark Energy is, or is sure that it exists.  The term 'Dark Energy' is really just a definition of what is needed to explain an observed anomaly, rather than a definition of the thing itself that is causing the anomaly; it is more a description of the problem than it is a description of the answer.
The dark energy is the quantity that must be added to the mass density/c^2 to give the overall active gravitational mass density. I.e. when

active gravitational mass density = dark energy + the mass density/c^2 > 0

the gravitational force gives a push. When its negative it gives a pull. Right now measurenents give a pull.
 

Offline yor_on

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For me it seems like Gravitation is a 'pull' though? And thinking that way I would expect the universe to contract, sooner or later.

But as we have an accelerating expansion, as measured by those very old nebula moving away from us we do need something explaining it. A weird property of space is its inherent energy, meaning that if we have an expansion we also will have more 'space' as a consequence, every point of those points 'expanding' into more points, growing as water rings when you throw a stone into a pond.

If that is true we also must get more energy into this universe as it grows, as space itself contains energy. So what would that`'new' energy do to our universe, expand it or contract it. Assume that we have a 'hidden balance' that must be fulfilled for a universe to exist. As more space comes to be more energy will manifest, that energy will need an outlet and so will 'push' on space around it, creating new space that, as a direct consequence, will do just the same. And what started this would have to be the Big Bang and possibly that inflationary period when space seems to have 'expanded' faster than the speed of light. I'm not sure we need 'dark energy' for it, seems to me that this 'virtual energy' will do as well?
« Last Edit: 26/07/2010 05:23:32 by yor_on »
 

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