Are all reference frames equally valid? Are accelerated reference frames as valid / real as inertial frames, or are accelerated frames a convenience of integrating across a continuum of inertial frames?
For instance, it makes sense to consider a passing relativistic observer for some experiment done on earth, if only to imagine what they would observe on earth due to relativistic effects. It then makes sense to consider any and all possible relative velocities / momenta, meaning that any and all inertial frames must be considered in that they all should find a way to agree on the events taking place on earth.
Does the same property hold for non-inertial frames? Eg, does it make sense to, or is there a need to consider every accelerating frame of reference such that they would agree on the outcome of an observed experiment on earth?
If so what are the consequences of acceleration horizons giving rise to a set of accelerated frames that, due to distance and acceleration, are unable to observe an experiment on earth until they stop accelerating, while another set of accelerating frames can?
 A: Einstein's equivalence principal states that an accelerated reference point is indecipherable from a reference frame in a gravitational field, so an accelerated reference frame will act in the same way that a gravitational field with the same acceleration would act.
As for if all reference points are equally valid, the answer is generally "yes" with some dependence on what your definition of "valid" means. While most reference frames will agree on an outcome (I encourage you to look up relativity's "train tunnel paradox" if you don't know what that is), there theoretical are examples of perspectives having quite different outcomes such as an idea in string theory in which falling into a black hole feels harmless for the person as they pass the Schwarzschild radius (and shortly after feels a lot less harmless as tidal forces increase to infinity) while the person would appear to be killed near the Schwarzschild by an outside observer. These views can be considered equally valid, but disagree entirely; due to the nature of a black hole there is no way for the person on the inside to tell the outside observer that they survived, so as long as from a specific perspective no laws of physics are broken then it can be considered valid which is always the case.
