Position drift with Gaia

The proper motion (also known as position drift) field of extragalactic sources at cosmological distances across our sky can be used to measure the acceleration of the Solar System through the aberration effect. If measured very precisely, the signal would also hold cosmological information, for instance about bulk flows of distant sources or the presence of tensor modes. In the $Λ$ cold dark matter ($Λ$CDM) model, the acceleration of the Solar System is by far the dominant contributor to the position drift signal for sources at cosmological distances, and the measurement is therefore expected to yield a constant spheroidal dipole across redshifts as long as convergence to the cosmic restframe has been reached. The aim of this paper is to test this hypothesis. We analyze data from the cosmic reference frame dataset of Gaia data release 3 focusing on constraining the dipole and quadrupole in the position drift signal, with an emphasis on redshift dependence of the signal as a consistency test of the $Λ$CDM model. The spheroidal dipole that we find is in mild tension, at the level of $2-3σ$, with the constant-in-redshift signature expected from the local acceleration of the Solar System. We also find significant quadrupole components, that however do not have any significant evolution with redshift. The most straightforward interpretation of these findings is (unknown) systematic errors related to the Gaia instrumentation, but a cosmological origin is a possibility. Our analysis remains inconclusive on the cause of the redshift dependence of the dipole and warrants further investigations with upcoming data releases. We discuss possible implications of our results and highlight the importance of proper motion measurements for rest frame determinations in cosmology. In our discussion, we highlight interesting avenues for doing cosmology with Gaia data.