dark energy

[Spin]

What is dark energy?

[RyanJ]

What is dark energy?

 

In cosmology' date=' dark matter refers to hypothetical matter particles, of unknown composition, that do not emit or reflect enough electromagnetic radiation to be detected directly, but whose presence can be inferred from gravitational effects on visible matter such as stars and galaxies. The dark matter hypothesis aims to explain several anomalous astronomical observations, such as anomalies in the rotational speed of galaxies (the galaxy rotation problem). Estimates of the amount of matter present in galaxies, based on gravitational effects, consistently suggest that there is far more matter than is directly observable. The existence of dark matter would also resolve a number of inconsistencies in the Big Bang theory, and is crucial for structure formation.

 

If dark matter does exist, it vastly outmasses the "visible" part of the universe [1']. Only about 4% of the total mass in the universe (as inferred from gravitational effects) can be seen directly. About 23% is thought to be composed of dark matter. The remaining 73% is thought to consist of dark energy, an even stranger component, distributed diffusely in space, that probably cannot be thought of as ordinary particles. Determining the nature of this missing mass is one of the most important problems in modern cosmology.

 

Have a look here for more informaiton:

 

http://en.wikipedia.org/wiki/Dark_matter

 

Cheers,

 

Ryan Jones

[Sisyphus]

The short answer is that nobody really has any idea. It's just something that probably has to be there, somehow, for a lack of a better explanation. The universe is apparently expanding at an accelerating rate, nobody knows why, and so they postulate the existence of some gravitationally repulsive force that, they calculate, has to make up more of the universe than everything else put together. It's really just kind of a "placeholder" until we can figure out what's really going on.

[RyanJ]

The short answer is that nobody really has any idea. It's just something that probably has to be there, somehow, for a lack of a better explanation. The universe is apparently expanding at an accelerating rate, nobody knows why, and so they postulate the existence of some gravitationally repulsive force that, they calculate, has to make up more of the universe than everything else put together. It's really just kind of a "placeholder" until we can figure out what's really going on.

 

Good point, no-one is actually shure if it exists, right nw it is thought that it must exist to account for apparent observations

 

Cheers,

 

Ryan Jones

[Severian]

I think it is most likely to be just a cosmological constant.

[tholan]

RyanJ, the text from wikipedia you are quoting refers to dark matter but not to dark energy.

 

In cosmology the content of the universe is modelled as a perfect fluid with an equation of state p = w d relating pressure p and density d. Dark energy is supposed to be something that permeates space homogeneously and exerts a negative pressure acting against gravitation of matter (or energy densities). This means that w < 0. One can see that this may lead to an accelerated expansion of space when inserted in the corresponding Friedmann equation.

 

The WMAP first year data suggest that -1.3 < w < -0.7. If w < -1 then the dark energy is called phantom energy and it violates some energy conditions. If w = -1 and constant in time it is usually refered to it as the cosmological constant.

[luc]

RyanJ' date=' the text from wikipedia you are quoting refers to dark matter but not to dark energy.

 

In cosmology the content of the universe is modelled as a perfect fluid with an equation of state p = w d relating pressure p and density d. Dark energy is supposed to be something that permeates space homogeneously and exerts a negative pressure acting against gravitation of matter (or energy densities). This means that w < 0. One can see that this may lead to an accelerated expansion of space when inserted in the corresponding Friedmann equation.

 

The WMAP first year data suggest that -1.3 < w < -0.7. If w < -1 then the dark energy is called phantom energy and it violates some energy conditions. If w = -1 and constant in time it is usually refered to it as the cosmological constant.

 

That's right. Indeed recent observations seem to reinforce the idea that Dark energy is the Cosmological constant. In that case Einstein equations should add a term involving the Cosmological constant (that term was rejected by Einstein, after having introduced it himself)

 

I have a question: I know the form of the Einstein equations if dark energy turns out to be the Cosmological constant, but what's the form of the equations if it turns out to be, say, quintessence?

[Daecon]

What does quintessence mean?

[luc]

What does quintessence mean?

 

quintessence is the other major candidate for dark energy. It's possible that the energy density of dark energy is not constant, but shows time dependence. Then the cosmological constant wouldn't be appropriate, because its energy density is constant. Quintessence is an hypothetical scalar field proposed by Steinhardt, and has an energy density that is not constant.

 

There's an article in wikipedia about quintessence

http://en.wikipedia.org/wiki/Quintessence_%28physics%29