Measuring the Sandage–Loeb redshift-drift signal demands decades-long baselines, centimetre-per-second radial-velocity precision, and exquisite instrument stability, because the expected ΛCDM signal is only a few centimetres per second per decade even at high redshift, easily swamped by calibration drifts and astrophysical noise (Loeb 1998; Liske et al. 2008). In practice, peculiar accelerations of sources, inhomogeneities in large-scale structure, our own local acceleration, and the need to combine data from different facilities and epochs all introduce additional, hard-to-model contributions at the same order as the cosmological drift, so turning the elegant theoretical prediction into a robust observational test of ΛCDM’s expansion history is far more difficult than the simple FRW calculation suggests (Quercellini et al. 2010; Martins et al. 2024).