And, either way, I think it is as JP stated. No 'free energy' available from a vacuum to drive a flashlight. Assume that planets and suns moving uniformly, frictionlessly, all used up 'usable' energy while moving. Furthermore assume that the unusable final product of this energy is heat. Then assume that our universe has been doing this for 13.7 billion years about. Do Space have a temperature?

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Eh, couldn't help myself there, just exchange space for matter (proper mass), and see if the temperature has raised. If everything is energy, and we split it in usable, to become unusable (heat) then the temperature of mass should raise, shouldn't it? As for the Casimir force I don't know how to think of it, assuming it is a property of space I wonder if one couldn't refer to it as a probability becoming a certainty, instead of waves? Or it can be property of matter maybe?

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'Usable energy to unusable', in this case, would be the transformations between a Higgs field and proper mass, or Higgs bosons. Maybe one could argue that this is 'dark tension (energy)' though, as a idea :) This because assuming bosons, or field(s) with bosons as their 'concentrations', interacting with mass must involve transformations, and transformations cost 'energy'. So, if it won't be heat, then, what is it?

(As soon as we discuss particles, of whatever kind, there should be a cost interacting. A pure 'field' (Higgs field) on the other hand? I don't know what a 'field' is. Neither have I seen it defined what the cost for a field interacting would be? And furthermore, how do you get to observer dependencies from a field?)