Timelike vs null deceleration parameter in tilted Friedmann universes

Information exchanged between observers in the universe typically travels along the null rays of the associated light signals. One may therefore decompose the luminosity distance of a given radiation source along these null geodesics, instead of the timelike worldlines of the emitting/receiving observers. In so doing, one obtains (among others) the null counterparts of the familiar Hubble and deceleration parameters. Although the timelike and the null sets of these parameters coincide in an exact Friedmann universe, they generally differ in perturbed cosmological models. The situation becomes more involved in tilted cosmologies, namely in those equipped with two families of relatively moving observers. There, the two observer groups generally disagree on the values of these parameters, simply because of their relative motion. Assuming a tilted, perturbed Friedmann universe with dust, we find that the relative-motion effects alone can locally change the sign of the null deceleration parameter from positive to negative, while at the same time the host universe is globally decelerating. An exactly analogous effect has also been reported with regard to the timelike deceleration parameter. The difference is that, in the null case, the impact of the relative motion is twice as strong, thus making it easier for the unsuspecting observers to misinterpret a local change in the sign of the deceleration parameter as recent global acceleration.