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what is a dark energy??


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Not dark matter - that's something else they (we) don't know about. Dark energy was cooked up to explain the observation that the expansion of the universe appears to have sped up, instead of being slowed down by the pull of gravity of matter in the universe. So they've come up with something that acts as a repulsive force and called it dark energy. You have to admit its a cool sounding name.

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cosmology and physics are not my strong point. the mighty kong has already explained dark energy as much as I understand it. I think dark matter is a kind of matter which exerts a gravitational pull but they have no way of seeing it hence "dark" As far as I understand they have been able to calculate the mass of objects by totting up star emissions and calculating. The mass that they can find comes nowhere close to explaining the motions of stars and things so they have inferred there must be more mass than can be seen this mass comes from dark matter.

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The term "Dark" is used to denote that it is unknown,

So it is not known what "Dark Energy" is, nor is it known what "Dark Matter" is.

If you have the correct and provable answer you have at least one Nobel prize heading your way and your name in the history books for the next few thousand years, and you would possibly have the right to have it named after you - like the Higgs Boson. Note that the conditions are "correct" and "provable". Correct and provable does not include "How about this idea?"

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and you would possibly have the right to have it named after you - like the Higgs Boson.

What if your name is ..

" Wolfeschlegelsteinhausenbergerdorffvoralternwarengewissenhaftschaferswessenschafewaren­ wohlgepflegeundsorgfaltigkeitbeschutzenvonangreifendurchihrraubgierigfeindewelchevoralternzwolftausend­ jahresvorandieerscheinenwanderersteerdemenschderraumschiffgebrauchlichtalsseinursprungvonkraftgestart­ seinlangefahrthinzwischensternartigraumaufdersuchenachdiesternwelchegehabtbewohnbarplanetenkreise­ drehensichundwohinderneurassevonverstandigmenschlichkeitkonntefortplanzenundsicherfreuenanlebens­ langlichfreudeundruhemitnichteinfurchtvorangreifenvonandererintelligentgeschopfsvonhinzwischen­ sternartigraum" ;) ?

http://en.wikipedia.org/wiki/Longest_name#Personal_names

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well, dark energy and dark matter, I believe, are linked.

The universe itself is infinite correct? So in that infinite space, you would think that area would be full, wouldn't you? But, from what we can see, it isn't! there are dark spaces and huge gaps between stars, planets and outer objects, yet as I said the universe is infinite so that space should be filled with something, thus we say that this made up of dark matter, thus if spaces change (like a change in orbit), there is an influx of negative energy of that space compacting, or that space growing (positive energy), we have dark energy!

Nobel prize please! :grin:

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well, dark energy and dark matter, I believe, are linked.

The universe itself is infinite correct? So in that infinite space, you would think that area would be full, wouldn't you? But, from what we can see, it isn't! there are dark spaces and huge gaps between stars, planets and outer objects, yet as I said the universe is infinite so that space should be filled with something, thus we say that this made up of dark matter, thus if spaces change (like a change in orbit), there is an influx of negative energy of that space compacting, or that space growing (positive energy), we have dark energy!

Nobel prize please! :grin:

I'm a bit in the dark here (pun most definately intended) :). Do we know absolutely whether the universe is finite or infinite and if so, how? If the universe is expanding (and this much we know) into what is it expanding? and if it's expanding into something then surely it must be finite.can it be expanding into nothing (no space, no time)? must lie down, head hurting.

Scott

Edited by auspom
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Do we know absolutely whether the universe is finite or infinite and if so, how?

Nope, nothing in science in certain. If a theory starts to pass certain tests it becomes more probable, but never certain.

If the universe is expanding (and this much we know) into what is it expanding?

Do we know though (see above)? The evidence in regards to a Doppler Shift (Hubble's Law) certainly suggests so, but it's by no means proven. As to what it's expanding into I could say Strawberry jelly and it'd be as good a guess as any (prove it's not!)

Edited by Spec-Chum
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I think... I'm fairly sure this is right:

Much of the mass of the (observable) Universe is "missing". The mass inside galaxies does not explain how they are the way they are, thus it is thought that there is mass or matter there that we can't see, this is Dark Matter.

The mass of the Universe, assuming there is nothing stopping it, be contracting due to gravity, however, it is expanding at an accelerating rate, and this is Dark Energy.

HTH

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Im not religiouse in anyway but just a thought maybe dark energy is evidence of God or the remains off. From what i remember from school s/he's supposed to be everywhere!

Schools should not be imposing 'beliefs' onto children like that :( .. school ort to be about education.

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I think... I'm fairly sure this is right:

Much of the mass of the (observable) Universe is "missing". The mass inside galaxies does not explain how they are the way they are, thus it is thought that there is mass or matter there that we can't see, this is Dark Matter.

The mass of the Universe, assuming there is nothing stopping it, be contracting due to gravity, however, it is expanding at an accelerating rate, and this is Dark Energy.

HTH

This is also my understanding, it is interesting that these two components, which are still 'hyperthetical' would need to form a high proportion of the universe. Dark energy, the force thought to be accelerating the expansion, it is felt would need to account for something like 73% of the total mass energy of the universe (assuming E really does = mc2), and dark matter would need to constitute 84% of the total mass. What we can see is only a tiny proportion.

Phil

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What if your name is ..

" Wolfeschlegelsteinhausenbergerdorffvoralternwarengewissenhaftschaferswessenschafewaren­ wohlgepflegeundsorgfaltigkeitbeschutzenvonangreifendurchihrraubgierigfeindewelchevoralternzwolftausend­ jahresvorandieerscheinenwanderersteerdemenschderraumschiffgebrauchlichtalsseinursprungvonkraftgestart­ seinlangefahrthinzwischensternartigraumaufdersuchenachdiesternwelchegehabtbewohnbarplanetenkreise­ drehensichundwohinderneurassevonverstandigmenschlichkeitkonntefortplanzenundsicherfreuenanlebens­ langlichfreudeundruhemitnichteinfurchtvorangreifenvonandererintelligentgeschopfsvonhinzwischen­ sternartigraum" ;) ?

http://en.wikipedia.org/wiki/Longest_name#Personal_names

You'd be brushed under the carpet and all the credit would be given to someone called Bob!

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My understanding is this. A vast protion of the observable universe has been mapped and the presence of a dark matter helps explain why much of the matter is clumped together in strings or fillaments. It also helps explains the way galaxies rotate and also why galaxies rotating in such a way dont tear themselves apart under Newtons first law of motion (this point could be very wrong but I believe there is not enough gravity within galaxies to explain the curved paths of matter within them). We have no idea what dark matter is or how it interacts, it is currently totally undetectable. Then we observe the fact that our universe appears to be expanding and that that expansion is increasing in speed. This does not conform with any of the proposed theories on the creation of the universe. Therefore a new source of energy has been put forward to explain this observation "dark energy". Again we have no idea what generates it and can not currently detect it in anyway. We also have no idea if dark energy and dark matter are connected. It is of course also possible that both could be some effect of processes we are already aware of but have no idea it also exerts these other strange phenomenon.

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There are a lot of reasons for Dark Matter, and a few for Dark Energy. A lot of it comes from the CMB data. Plotting the power spectrum constrains a lot of parameters. If there were different ratios the curves would look different to what are seen.

The amount of stuff is usually work out in ratios. There is a critical density of the universe which is a fine line between the universe having negative and positive curvature. From the CMB data, this is very very close to 1 - so the universe is very very close to flat.

So the total energy density of the universe as a fraction of the critical density is 1.

From large scale structure, supernova data, and being consistent with CMB data is the total matter density is 0.27 (27%).

From big bang nucleosynthesis and verified from CMB data, the total baryon density (normal matter we see) is 0.05 (5%).

Thus there is 22% of matter that is non baryonic (so called Dark Matter). There is also missing matter from rotation curves, gravitational lensing and cluster dynamics which fits very closely with the CMB data.

The remainder is dark energy 73%. This also fits with the accelerating expansion.

So - there are several lines of evidence that converge for each of these ratios.

Hence the belief in dark matter and dark energy - as this fits with the CMB data.

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