Redshift Drift Baseline Challenges

The cosmic redshift drift — the secular change in the redshift of a distant source due to the changing expansion rate — is in principle a pristine cosmological signal, depending only on the global expansion history and requiring no standard candles. In practice, measuring drifts of order 10 cm/s per decade in the spectra of quasars at z ~ 2–4 is a formidable technical challenge. Instrumental stability over decadal timescales must be maintained at the sub-meter-per-second level, which requires laser frequency comb calibration, thermal and pressure stability of the spectrograph, and exquisite control of atmospheric effects. Current facilities have not yet achieved this, and even ESPRESSO requires multi-year baselines before a significant detection becomes feasible.

The cosmological tension emerges because ΛCDM's predicted drift signal at z ~ 2 is approximately −2 cm/s/yr — a tiny but definite value. If the expansion history differs from ΛCDM, the predicted drift changes sign or amplitude in characteristic ways. The baseline challenge means that for at least the next decade, this powerful discriminator is inaccessible, leaving the Hubble tension unresolvable by this method in the near term. Meanwhile, modified expansion histories proposed to address the Hubble tension — early dark energy, dynamical dark energy, or decaying dark matter — all predict drift signatures that differ from standard ΛCDM. The redshift drift test will eventually arbitrate these alternatives, but the long observing baselines required mean the problem will persist without resolution for many years under the current observational program.