Dark Energy: Observer Dependent or Absolute Rest Frame?

[Khashishi]

If yes, does dark energy define a frame of absolute rest?

In the Einstein field equations, the cosmological constant is just a scalar, so it is independent of frame. But, sometimes, the cosmological constant is described as being a kind of vacuum energy density. Wouldn't a vacuum energy density be frame dependent? Or is it assumed that the vacuum energy density is always combined with a negative vacuum pressure density which cancels out the frame dependence?

 

[PeterDonis]

No. Dark energy is Lorentz invariant, whether you view it as a cosmological constant or as a vacuum energy density, since both appear the same way in the GR equations.

is it assumed that the vacuum energy density is always combined with a negative vacuum pressure density which cancels out the frame dependence?

It's not "assumed"; it's required by the equations. Dark energy (whether it's a cosmological constant or a vacuum energy density) appears in the equations as a constant that multiplies the metric; that means that in any local inertial frame, it looks like a positive energy density and an isotropic negative pressure that both have the same magnitude.