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What theories propose to close the gap (as suggested by http://arXiv.org/abs/1210.6008) between 'phenomenological behavior of dark energy' and 'physical-level understanding of dark energy' and to what extent do observations support such theories?

That 2012 paper states, "Dark energy is not understood at all at a physical level but phenomenologically behaves like a smoothly distributed fluid with equation of state close to p = −ρ, as for a cosmological constant Λ."

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    $\begingroup$ Well, the zero-point energy of quantum fields could be a physical-level explanation of dark energy, except that we have no idea how to get the observed value. Some popular calculated values are infinity, zero, or finite but 120 orders of magnitude too large. Not good! $\endgroup$ – G. Smith May 25 at 0:41
  • $\begingroup$ If you think of the Einstein-Hilbert action as an expansion in powers of the curvature, the cosmological constant is essentially the coefficient of the 0th order term and $G$ essentially the coefficient of the 1st order term. Do we need to explain term 0 but not term 1? In this approach, the cosmological constant is not really energy at all but just part of gravity’s dynamics. $\endgroup$ – G. Smith May 25 at 0:45
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Just having a cosmological constant is not looking "natural" therefore theorist try to find modification of the Einstein equation which would be more "natural" than a cosmological constant. There is basically to way to do so. First by modifying gravity (let say gravitational theory with have not the same vacuum that general relativity) or by including an additional source in the stress-energy tensor this additional source is called dark energy.

But current state of observation give $\Lambda$CDM as the best fit, so we are still in the "who knows". Missions (like Euclid/LSST/...) have as one of their main purpose to get a better constraint on the equation of state of dark energy and will maybe start to change the picture, but current as said, $\Lambda$CDM is the best fit.

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