ESPRESSO Redshift Drift Forecast

The Sandage-Loeb test proposes to directly detect cosmic acceleration by measuring the drift in the spectroscopic redshifts of distant objects over a timescale of decades. If the universe is accelerating, Lyman-alpha forest absorption lines in quasar spectra should shift at a rate dz/dt that depends on the expansion history H(z). The ESPRESSO spectrograph on the VLT, and its future successor ACES on the ELT, have been designed specifically for this test, with forecasts suggesting that a 20-year baseline of monitoring bright quasars at z ~ 2–4 will yield a detection of the expected drift at several sigma significance under ΛCDM. The forecast assumes a constant Λ cosmology; if the true expansion history deviates from ΛCDM, the measured drift will deviate from the ΛCDM forecast value and provide a direct, model-independent measurement of H(z) at the quasar redshifts.

Successive Collision Theory predicts that the ESPRESSO/ACES redshift drift measurement will deviate from the ΛCDM forecast in a specific and detectable way. Because Λ_eff evolves over cosmic time as the tensor mesh dissipates, the expansion history H(z) in SCT is not the constant-Λ function assumed in the ΛCDM drift forecast. The rate of tensor mesh dissipation translates into a slowly varying dΛ_eff/dt that modifies H(z) at a level of a few percent relative to ΛCDM across the redshift range 2–4 probed by the drift test. The resulting drift signal will be systematically offset from the ΛCDM prediction, with the sign and magnitude of the offset determined by the rate of orbital decay in the parent frame hierarchy — a physically constrained quantity tied to the same mechanism that explains the late-time acceleration and Hubble tension.

Additionally, the SCT prediction for the redshift drift includes an anisotropic component arising from the spatial variation of Λ_eff. Quasar lines of sight in different sky directions traverse different densities of the surrounding large-scale structure, experiencing different effective expansion histories. The ESPRESSO forecast assumes an isotropic drift signal, but SCT predicts the drift to vary across the sky at the percent level correlated with the local density field along each line of sight. Mapping this anisotropy with the full ELT quasar monitoring sample would provide a uniquely powerful three-dimensional measurement of the evolving Λ_eff field — far more diagnostic than the isotropic drift alone — and would constitute a decisive test of the SCT tensor mesh dissipation mechanism.