How does Dark energy fit into the standard model?

[jnorman]

At this point, I am rather confused about the concept of DE, and even why it is called DE since it does not seem to fit into what the concept of energy is. but my question is where/how does DE fit into the standard model? thanks.

 

[Coin]

Hi, let me see if I can get this right.

"Dark Energy" is maybe something of a misnomer. When we say "dark energy" we really mean "whatever unexplained, undiscovered mechanism is responsible for the surplus expansion of the universe".

It is maybe worthwhile to compare "Dark Matter". Dark Matter got named when they discovered an effect that as if there was an extra undiscovered matter scattered through the universe. Although it was not proven that dark matter was a kind of matter-- the dark matter effect could have been due to a flaw in our understanding of gravitational theory-- they called it "dark matter" anyway. Based on recent astronomical observations we by now can conclude with very good confidence that "dark matter" is a kind of matter, that there is a specific particle or family of particles that the dark matter is made of. This particle is not part of the standard model (that is: we don't know what the particle is, but we know its properties, and its properties are unlike anything in the standard model) and the standard model will have to be expanded when the particle is found.

"Dark Energy", on the other hand, was named when they discovered an effect that was as if there were an unexplained energy density of unknown source permeating the universe. Calling this "dark energy" may be a little misleading, maybe? But they were probably trying to come up with a name which draws an analogy to "dark matter". Once a naming gimmick catches on people will follow it even where it doesn't apply well (for example, see "dark flow"). If you look at the wikipedia article for dark energy, it explains:

Two proposed forms for dark energy are the cosmological constant, a constant energy density filling space homogeneously,[2] and scalar fields such as quintessence or moduli, dynamic quantities whose energy density can vary in time and space. Contributions from scalar fields that are constant in space are usually also included in the cosmological constant.

So: We don't know what dark energy is. But we have two ideas:

Maybe it's the "cosmological constant" in the Einstein field equations. If so, it really is like a "dark energy"-- the equation that relates the stress energy tensor to the curvature of spacetime just happens to have built in a "bonus" energy that doesn't seem to come from anywhere. If this is the case, the "energy" is not part of the standard model and it wouldn't make sense to put it in the standard model, since the standard model is a collection of particles/fields.

On the other hand, maybe the 'dark energy' is a field, a particle-- maybe there's something permeating space that is interacting with all the matter to push it apart. In this case it would not be part of the standard model (unless the field turns out, somehow, to be the Higgs) but it would be totally appropriate to add it to the standard model.

So, in short: Either dark energy is actually a "dark" energy density, or it is possible to fit it into the standard model, but not both?

 

[jnorman]

v good - danke.