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I was curious whether it is possible to make dark energy equivalent to dark matter.

Can this unification be done?

If it can, why do scientists prefer to separate dark energy from dark matter?

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At this point we don't know very much about either of these things (just a bit about dark matter and almost nothing about dark energy), but they solve different problem about the behavior of the universe. Come back in a few years and we may know a lot more about the dark matter. No promises on the dark energy. –  dmckee Aug 5 '12 at 13:08
    
arxiv.org/abs/1208.0449 –  John Rennie Aug 5 '12 at 13:57
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2 Answers 2

Despite the names, dark matter and dark energy are not related. Dark matter is similar to the baryonic matter that makes up galaxies, planets, you, etc. The key difference is that dark matter doesn't interact via the electromagnetic force, so you can't directly observe it. Dark matter is most likely a new sub-atomic particle, and it make be a WIMP (weakly-interacting massive particle). That is, it may interact through the weak force. One example of dark matter is hot dark matter, the neutrino. However, the type of dark matter relevant in cosmology is cold dark matter, which hangs around galaxies in very large clumps. We can indirectly observer dark matter through gravitational lensing, such as in the case of the Bullet Cluster.

Dark energy is quite different. Dark energy is the popular term for the cosmological constant, a negative pressure vacuum energy density. Usually, dark energy is modeled as a negative pressure fluid that has an equation of state of w = -1. This has the effect of accelerating the expansion of the universe.

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This is exactly what dark fluid hypothesis is about:

http://arxiv.org/abs/astro-ph/0601274

http://arxiv.org/abs/0804.1588

Not all the theories under the guise of the 'dark fluid' brand are the same though, that first paper postulates a scalar, while the other from Zhao assumes a U(1) electromagnetic-like vector field

However, the scientific bottleneck at the moment is not so much the lack of theories as the quality of the observational data, is still very hard to constrain many of these models by observations alone.

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This has a serious problem, as Mark M points out, that dark energy violates an energy condition, while dark matter can't. –  Ron Maimon Sep 8 '12 at 3:08
    
Interesting, no one mentions PRL paper which is the earliest on the subject I know: Purely Kinetic k Essence as Unified Dark Matter See also arxiv COPY –  Vitaliy Kaurov Jan 23 at 2:01
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