[pests]

There is no underlying reference frame. All motion is relative. Everyone, no matter how fast they are already going, will measure the speed of light as c. Accelerate to .99c and shine a flashlight in front of you. That light is moving ahead of you at the speed of c. Because to you, you are not moving.

[eru]

That's true for the laws of physics, yes, but our universe does have a 'natural' frame of reference.

> A comoving observer is the only observer who will perceive the universe, including the cosmic microwave background radiation, to be isotropic. Non-comoving observers will see regions of the sky systematically blue-shifted or red-shifted. Thus isotropy, particularly isotropy of the cosmic microwave background radiation, defines a special local frame of reference called the comoving frame. The velocity of an observer relative to the local comoving frame is called the peculiar velocity of the observer.

From https://en.wikipedia.org/wiki/Comoving_and_proper_distances#...