Re: Can a vacuum have a speed?
Agreed, as long as some folks don't drop deliberate slurs targeting me.. no problem. So back on target again.
I agree that Space can't have a Speed relative to itself, no more than I can have a Speed relative to Myself.
I had posited the question if Expansion can be included in the the actual observations that remote locations in space do, in fact, have their own Unique Velocities. The Doppler evidence of remote Galaxies, that are intrinsically bound by their local Space-Time neighborhoods, do prove that Space can have differential velocities over large enough distances. That's based on pure observation and can't be disputed.
What might be disputed is if a good reason, such as Expansion, can be used to disqualify our observations.
If a rocket can push a Spaceship away from me and that qualifies as a solid reason for a Velocity differential between Me and said Spaceship.. and given that Science currently explains Expansion is the result of Dark Energy Pressure, then perhaps we shouldn't be too quick to disqualify Expansion as a good reason for our observations that Space is being observed as having unique Private Differential Velocities or independent Speeds.. as the case may be.
I'm not convinced that supplying an explanation (Dark Energy or Hubble Constant) for our Real Observations is any different than supplying an explanation for why a rocket accelerates by burning fuel.
Basically: Can an Explanation disqualify an Observation?
The OP is <NOT> asking "why" a Volume of Vacuum can have its own unique speed but rather have we ever observed such to be true. If we base our answer on observations of the Universe only.. then the answer must be "Yes".. a Volume of Vacuum can have its own unique Speed relative to other Volumes of Vacuum. The further the faster and visa-versa.
Just because those Speed Differentials follow some specific Constant relative to Distance doesn't disqualify the Cosmic Observation that we do witness Unique Speed Differentials (regarding Space-Time or vacuum) in the Observable Universe.
Perhaps.. if we could prove that Remote Galaxies are in fact moving at near the Speed of Light independent of their local Space-Time neighborhoods.. then perhaps a case can be made to answer the OP with a resounding "NO".
But honestly, this latter idea is not the side I would want to support.