How come we can tell what motion we have with respect to the CMB? Doesn’t this mean there’s an absolute frame of reference?
The theory of special relativity is based on the principle that there are no preferred reference frames. In other words, the whole of Einstein’s theory rests on the assumption that physics works the same irrespective of what speed and direction you have. So the fact that there is a frame of reference in which there is no motion through the CMB would appear to violate special relativity!
However, the crucial assumption of Einstein’s theory is not that there are no special frames, but that there are no special frames where the laws of physics are different. There clearly is a frame where the CMB is at rest, and so this is, in some sense, the rest frame of the Universe.
But for doing any physics experiment, any other frame is as good as this one. So the only difference is that in the CMB rest frame you measure no velocity with respect to the CMB photons, but that does not imply any fundamental difference in the laws of physics.
“The cosmic microwave background (CMB, CMBR) is electromagnetic radiation as a remnant from an early stage of the universe in Big Bang cosmology. In older literature, the CMB is also variously known as cosmic microwave background radiation (CMBR) or “relic radiation”.
The CMB is a faint cosmic background radiation filling all space that is an important source of data on the early universe because it is the oldest electromagnetic radiation in the universe, dating to the epoch of recombination. ...”
https://en.wikipedia.org/wiki/Cosmic_microwave_background
We are in fact travelling about 600 kilometers per second with respect to the cosmic microwave background, which means we are moving about 600 kps relative to the Big Bang. The Big Bang itself represents the universe’s reference frame. However, it is just as valid to say we are stationary while the universe is moving as to say we are moving while the universe is stationary.