Stability and null energy condition

My collaborator, UO postdoc Roman Buniy, is off to sunny Miami to attend a particle theory conference. His talk is on stability of quantum field theories and the null energy condition.

Recently, cosmologists have discovered that 80% of the energy in the universe is in a very unusual form called "dark energy" (not to be confused with "dark matter," which also exists but is clumped around galaxies, rather than diffuse, and doesn't seem as weird). Dark energy causes the expansion of the universe to accelerate, due to an equation of state with negative pressure. How negative can the pressure be? There is some theoretical prejudice that it cannot be more negative than minus its energy density (in natural units where Planck's constant and the speed of light are unity), even though the data allow for and perhaps even suggest this possibility.

What do I mean by "theoretical prejudice"? Well, I mean many theorists would be shocked if things turned out otherwise. If the pressure is too negative something called the "null energy condition" (NEC) used in general relativity is violated. The NEC says that the contraction of any null, or light-like, four vectors with the stress energy tensor must be non-negative. Assuming the NEC, one can prove a number of pleasing properties of solutions to the Einstein equations. It is believed to be satisfied by any reasonable types of matter.

In our paper we show that, in a broad class of models including any constructed out of interacting scalar and gauge fields, or any model describing a perfect fluid, if the NEC is violated by some configuration of the fields, then that configuration is unstable (i.e., will fall apart). This makes it very unlikely that the dark energy violates the NEC, since it seems to have been stable over billions of years.