ESPRESSO Redshift Drift Forecast

One of the most direct probes of the expansion history is the Sandage-Loeb test: measuring the slow drift in the redshift of distant objects over a period of years to decades as the Hubble flow carries them. Unlike distance indicators, this method measures the rate of change of the expansion — dH(z)/dt — in real time without any standard candle. The ESPRESSO spectrograph on the VLT, and the planned ELT/ANDES instrument, are designed to detect this drift in the Lyman-alpha forest of distant quasars at z ~ 2–5 over a ~20-year baseline. ΛCDM makes a precise prediction for the magnitude and sign of this drift at each redshift: it should be negative at z < 2 (slowing due to past deceleration) and positive at higher redshifts. Early ESPRESSO calibration work has validated the instrument's extraordinary radial velocity precision at the centimeter-per-second level.

The tension at present is a forecasting tension: ΛCDM's prediction for the drift signal is sufficiently precise that if ESPRESSO or ELT/ANDES measures a drift rate inconsistent with the Planck best-fit cosmology — even after 20 years of monitoring — it would constitute a direct, model-independent falsification of the standard expansion history. The current worry is that the Hubble tension, if real, implies an expansion history that deviates from ΛCDM in the late universe, which would produce a measurably different redshift drift profile compared to the Planck-concordance prediction. Any future detection of the drift inconsistent with ΛCDM's forecast cannot be attributed to distance ladder systematics; it would be written directly into the fabric of spacetime kinematics.