The Sandage–Loeb redshift-drift signal is an extremely small, slowly accumulating change in the redshift of distant sources, so ESPRESSO and future ELT-class forecasts require multi-decade baselines, ultra-stable spectrographs, and precise control of systematics to reach the centimetre-per-second sensitivity needed to distinguish ΛCDM acceleration from alternative expansion histories (Loeb 1998; Liske et al. 2008). In ΛCDM this test is touted as a clean, model-independent probe of the background expansion, yet realistic forecasts show that inhomogeneities, peculiar accelerations, instrumental drifts, and limited sample sizes can bias or even mimic the expected signal, turning what looks like a straightforward prediction into a long-term observational and modeling challenge (Quercellini et al. 2010; Martins et al. 2024).