What is Dark Energy

[Time_Travel]

What exactly is the nature of Dark Energy(DE).

Is it the derivative of the 4 known fundamental forces namely Strong Nuclear,Weak Nuclear,Gravity,Electromagnetism.

Or is it a totally Different Energy?

[Jay-qu]

I would guess that it is totally different.

 

Dark energy can be described by entering a cosmological constant like term into the Einstein field equations, but this does not mean dark energy is a gravitational phenomena - it is just at home in the equations since they can describe the large scale structure and evolution of the universe.

[sanctus]

We don't really know what dark energy is, there are different models describing it, I think the main model is the one Jay-qu said, i.e. the cosmological constant (called [math]\Lambda CDM[/math] model). But then even there, there are modifications, for instance models with varying (both spatially and in time) cosmological constants (these models are called quintessence...)

[Time_Travel]

We don't really know what dark energy is,............/QUOTE]

 

If we dont know what it is then is it part of the fundamental forces or totally different and separate from these forces.

[sanctus]

Lol, I guess it depends on the model you use when trying to fit the data. For example there are also so called big-void models, where we are in an underdensity (not forcefully in the center) and hence the acceleration which in standard [math]\Lambda CDM[/math] is due to the cosmological constant, is now just due to gravitational attraction of the the higher mass concentration at the border of the underdensity and is hence purely gravitational...

[Rade]

See this paper:

 

A classical treatment of the problems of dark energy, dark matter, and accelerating expansion

[HydrogenBond]

Dark energy is the energy output connected to the lowering of gravitational potential. Just as the EM force gives off photons when it lowers potential, the lowering of gravity potential gives off what we call dark energy. Just as the photon, given off by the lowering of EM potential, can increase the EM potential elsewhere (for example excite an electron) the dark energy output of gravity will cause what appears to be an apparent reversal of gravity such as universe expansion.

 

Science has been looking for the graviton, which are the theoretical particles of gravity, but it has not been easy to find. Even without this smoking gun particle, we model gravity in terms of observed effects. We also have not directly seen gravity give off dark energy, but we can also infer this by the effect.

 

For example, if we look at the universe, the densest matter seems to bunch up into what we call galaxies. If gravity was given off dark energy as it lowers potential, we would expect the universe to expand relative to the galaxies. Could the existing model predict this without just correlating to observation?

 

If the lowering of gravity potential is given off dark energy, and the universe is accelerating expanding, this would imply the accumulative impact of gravity within galaxies, stars, etc, has been accelerating. Is this consistent with observation? Sometimes simplicity is closer to truth, but fancy sells.

[Jay-qu]

Dark energy is the energy output connected to the lowering of gravitational potential. Just as the EM force gives off photons when it lowers potential, the lowering of gravity potential gives off what we call dark energy. Just as the photon, given off by the lowering of EM potential, can increase the EM potential elsewhere (for example excite an electron) the dark energy output of gravity will cause what appears to be an apparent reversal of gravity such as universe expansion.

 

This is incorrect HB, lowering gravitation potential using only gravitational interacting is thought to give off gravitational waves, not dark energy.

 

General relativity predicts gravitational waves. It was observed that as two pulsars orbit eachother the orbit decays - and it decays at the precise rate that GR predicts (see graph below). This does not leave room to say that there is dark energy radiated.

The blue is the prediction by GR and the red points is the data taken from the binary pulsar system PSR B1913+16

(source: PSR B1913+16 - Wikipedia, the free encyclopedia)

[Time_Travel]

Did Dark Energy existed always.

If yes did it cause the Big Bang.

If no then it implies that it must have born after Big Bang and what might have created it

[Jay-qu]

The amount of dark energy in the universe is closely tied to the size of the universe. Essentially as the universe expands there is more dark energy and back when the universe was young there was a comparatively small amount of dark energy.

[Time_Travel]

The amount of dark energy in the universe is closely tied to the size of the universe. Essentially as the universe expands there is more dark energy and back when the universe was young there was a comparatively small amount of dark energy.

 

 

If more dark energy is at present than it was at beginning, what is creating that dark energy?

Weakening Gravity is creating it or something else creating it?

[Jay-qu]

No one knows for sure, but it seems to be some sort of vacuum energy, so that as the universe grows there is more and more of it.

[Moontanman]

Possibly there is no dark energy?

 

Three cosmic enigmas, one audacious answer - physics-math - 09 March 2006 - New Scientist

[Jay-qu]

I find this hypothesis hard to believe. It predicts that all dark matter will be made up of these dark energy stars and that they would be around 10^3 solar masses (1000 times the mass of the sun) for comparison the largest known star Canis Majoris is only ~25 solar masses. While that does not mean that these things are large or should be observable it does imply that there would be about 300 million of these things floating around our galaxy. While this is still possible all of these objects would provide a very large chance of observing micro-lensing events (gravitation lensing of background stars and galaxies).

[sanctus]

But Jay, wouldn't it be the same lensing as from black holes?

 

I think this is not so hard to believe, when you think of it, black holes are much harder to believe, we are just used to them ;-) (kind of).

[Jay-qu]

But Jay, wouldn't it be the same lensing as from black holes?

 

I think this is not so hard to believe, when you think of it, black holes are much harder to believe, we are just used to them ;-) (kind of).

But black holes are not proposed to make up all the dark matter of the galaxy and so there are much less of them..

[sanctus]

Should be no more ;-) proposed to make up etc...but that is a good point.