3.1. The CMB is a cosmic frame of reference
Mansouri and Sexl 1977, among many others (e.g., Reinhardt, et al. 2007), has suggested that the cosmic microwave background (CMB) should be considered a cosmic frame:
The discovery of the cosmic back-ground radiation has shown that cosmologically a preferred system of reference does exist. This system is defined and singled out much more unambiguously to be a candidate for a possible “ether frame” than was the solar rest frame in Einstein’s days.
The CMB is the more accurate equivalent of the fixed stars as a cosmic frame—more accurate because it is changing less over time than the fixed stars. And, as discussed below, it exhibits some pronounced anisotropies, making its orientation detectable anywhere in the universe, as best we can tell. (A recent example of using the CMB as a preferred rest frame is found in Riess et al. 2016 (p. 15): “z is the redshift in the rest frame of the CMB corrected for coherent flows…”).
The existence of the CMB means that in practice there is always a common/preferred frame of reference for use in navigation and orientation more generally, no matter where we are in the universe. This alone doesn’t invalidate SR but it weighs against Einstein’s interpretations of the Lorentz transformations because SR postulates that the laws of physics are the same for all inertial frames (this is the “principle of relativity,” one of Einstein’s two postulates in his 1905 paper), thus there is no preferred frame. But in our actual universe there is a preferred frame formed by the CMB, or large-scale baryonic structures, or a combination of both, as discussed below.