Acceleration towards c without a reference frame and changes

[Chris Miller]

A universe devoid of everything but one spaceship would = that spaceship. Hard to make any sort of physical predictions about such a hypothetical, small and different "universe." In this universe, possibly expanded to some future point where nothing is observable (as most of it already isn't) you'd still have the CMB to measure your relative speed and direction against.

 

[PeterDonis]

A universe devoid of everything but one spaceship would = that spaceship

Not if you are using SR/GR as your theory. In SR/GR, if we assume the mass of the spaceship is negligible, the universe would be Minkowski spacetime. If we assume the mass of the spaceship is non-negligible, the universe would be some curved spacetime (if we idealize the spaceship as spherically symmetric, it would be a vacuum region described by the Schwarzschild geometry surrounding a non-vacuum region occupied by the spaceship).