Does dark energy have mass?

Question

For personal writing purposes, I’ve been researching dark energy, amongst other similar subjects. I’ve found it hard to discern whether or not we have any conclusive answers or generally accepted theories for this question, and would appreciate any insight.

To my understanding, energy is not typically regarded as having mass, but rather a process or force and its potentials, though mass itself is a form of energy. Based on what I've seen about dark energy and the way it interacts, it seems to be implied that it has mass, or another method of interaction that seems to parallel having mass in some way. It is often referred to in a way that reads more as a substance, rather than a force or process. Is dark energy used interchangeably to refer to the conceptual force itself, and a mass derived from that energy? Do we suspect it could have massless particles, similar to photons? Any clarification would be appreciated.

Answer 1

In general relativity mass and energy are treated as equivalent using Einstein's famous equation $E=mc^2$. When we are calculating the curvature of spacetime we use the total energy density i.e. the energy per cubic metre plus the mass times $c^2$ per cubic metre.

This applies to more familiar forms of energy like light, as well as to the mysterious dark energy, so in this sense dark energy is no different from any other form of energy. It does not need to have a mass to curve spacetime any more than light does.

Where dark energy is unique is that that it has, in effect, a negative pressure. More precisely it has an equation of state that differs from light and matter, but negative pressure is a decent layman interpretation. The spacetime curvature depends on pressure as well as energy density, though somewhat counterintuitively a positive pressure tends to cause a contraction while a negative pressure tends to cause an expansion.

We don't know what dark energy is, or indeed if it really exists, so we have no idea exactly how it interacts. It's possible it is a fundamental property of spacetime (a cosmological constant) and not a field like the electromagnetic field.