Redshift Drift Null Prediction

In ΛCDM with the Planck 2018 best-fit parameters, the redshift drift at z ≈ 2 is precisely predicted to be negative, with a value of approximately −2 cm/s per year of observation. This is a null-like prediction in the sense that it is a specific, non-zero, quantitatively calculable number — not merely a qualitative expectation. The importance of this prediction is that it tests the model in a region of the expansion history — the matter-dominated era transitioning to dark-energy domination — that is not directly accessible through distance measurements alone. Future ELT/ANDES observations over a 20-year baseline would accumulate enough signal to test this prediction to a few percent precision.

The tension is prospective but structurally important: if the real Hubble constant is closer to 73 km/s/Mpc than to 67, or if the dark energy equation of state evolves with redshift (as DESI data hints), then the predicted drift value at z ~ 2 can differ from the Planck prediction by 10–20%. This is a detectable difference with ELT. The null prediction problem for ΛCDM is therefore a ticking clock: if and when ANDES comes online, ΛCDM's expansion history will be tested at a level of precision that cannot be mimicked by any astrophysical systematic. Any result inconsistent with the Planck prediction would be a direct, assumption-free falsification of the ΛCDM expansion history — a qualitatively different kind of evidence from all current tensions.