What Is Phantom Energy? Normal matter/energy in cosmology is classified according to the equation of state:
where p is the pressure and ρ the energy density of the matter/energy. For radiation w = 1/3, for matter/dust w = 0 and for the cosmological constant w = -1. What's interesting to note, looking at the image at the top, is the larger (more positive) w is, the faster it dilutes in the universe as the universe expends. The cosmological constant is right at the point where it's density remains constant throughout the expansion of the universe.
Phantom energy is energy that has w less than -1. If this form of energy existed, it would actually increase in density as the universe expanded!
Furthermore, as the above plot shows, certain phantom energy models do fit current data, such as the supernova observations shown in this plot, and so we have to be willing to probe these models especially if they further go on to make experimental predictions. But do they?
Possible Experimental Prediction: A Sudden Big Rip. First, as stated above, phantom energy models by definition predict w to be less than -1. This itself can be measured and therefore tested. Second, if we can somehow demonstrate that the energy density of the universe is is increasing with expansion, that would be a tale tale sign.
But lastly I want to discuss the big rip. It turns out, that if dark energy is driven by phantom energy, the universe gets ripped apart in finite time. More technically, the scale factor "a" controlling the expansion of the universe becomes infinite in finite time. This means the distance between you and everything else in the universe goes to infinity without having to wait an infinite amount of time for this to happen.
In fact, a model discussed in this paper predicts the universe will experience this big rip in only 8.7 million years from now!
No wonder the authors call this model the "sudden future singularity" model. If the universe becomes singular, the scale factor "a" becoming infinite in only 8.7 million years, compared to the age of the current universe that will be one sudden singularity!
Conclusion. I do want to remind people that the standard cosmology scenario where dark energy is a cosmological constant has worked so well for so many years that we have no reason to abandon it. That said, dark energy being driven by phantom energy is technically a possibility and so we are justified looking into it, especially since it's existence may be experimentally verified/falsified.
Basically if we ascertain w is less than -1, see the energy density of dark energy increase with expansion, or find our universe suddenly being ripped apart we will know the dark energy is actually a form of phantom energy.
Let's hope that last one doesn't happen any time soon. :)
Mariusz P. Dabrowski, & Tomasz Denkiewicz (2009). Exotic-singularity-driven dark energy AIP Conference Proceedings, 1241 arXiv: 0910.0023v1