The seemingly preferred cosmic frame

The Universe appears to have a 'preferred' frame of reference, within which the cosmic microwave background is completely isotropic. Earth's motion through this relic radiation field is measurable, and some have viewed this type of observation as a possible contradiction of Einstein's principle of relativity. Several experiments have been proposed (and some have been carried out) to test for violations of Lorentz invariance when transforming out of this background frame, e.g., based on expected velocity differences in a Michelson-Morley type of probe. But what exactly is this cosmic frame? Is it truly an absolute frame attached to the Universe, or is it an observer-dependent construction? Such possibilities represent very different Physics, of course. In this paper, we work through these issues carefully, without the introduction of unsubstantiated assumptions. We show that the preferred frames originate through general (not special) relativity, and are indeed observer-dependent. Their existence does not at all conflict with Einstein's fundamental hypothesis of special relativity.