Interpreting Negative Dark Energy Density in the Expanding Universe

[zeebo17]

Could someone please explain how to interpret a negative dark energy density $$-\Omega_{\Lambda}$$? How is a density allowed to be negative?

 

[Chalnoth]

Could someone please explain how to interpret a negative dark energy density $$-\Omega_{\Lambda}$$? How is a density allowed to be negative?

Well, this can be explained in a variety of ways. One is that the cosmological constant isn't actually an energy density, but is instead a parameter on the other side of Einstein's equations that can be understood as part of the behavior of space-time, and not part of the matter stress/energy tensor on the right hand side of Einstein's equations.

Another way to understand it is that we see negative energy density all the time, in the form of potential energy. Consider, for instance, if we start with a situation where you have two masses far apart and stationary. As time moves forward, their mutual gravitational attraction will cause the two to pick up speed toward one another and eventually collide. But where did this positive kinetic energy come from? It had to come from potential energy, which became more negative.

Finally, a third way is to mention that the zero for energy is a bit arbitrary, and we might interpret finding a negative energy as just a demonstration that we haven't taken the right zero point for estimating energy density.

 

[edgepflow]

Consider that the energy density of the dark energy is constant. So as the universe expands, work is done to the dark energy stuff. So this allows a constant energy density as the volume of the universe increases.

The cosmological constant for the dark energy has a negative effective pressure. So work is required to expand it. This is different than a piston expanding from positive internal pressure and doing work => positive internal pressure, expansion does work; negative internal pressure, expansion requires work.