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.