Definitive test of the R_h = ct universe using redshift drift

The redshift drift of objects moving in the Hubble flow has been proposed as a powerful model-independent probe of the underlying cosmology. A measurement of the first- and second-order redshift derivatives appears to be well within the reach of upcoming surveys using as the Extremely Large Telescope high resolution spectrometer (ELT-HIRES) and the Square Kilometer Phase 2 Array (SKA). Here we show that an unambiguous prediction of the R_h = ct cosmology is zero drift at all redshifts, contrasting sharply with all other models in which the expansion rate is variable. For example, multiyear monitoring of sources at redshift z = 5 with the ELT-HIRES is expected to show a velocity shift Δv = -15 cm s^-1 yr^-1 due to the redshift drift in Planck ΛCDM, while Δv = 0 cm s^-1 yr^-1 in R_h = ct. With an anticipated ELT-HIRES measurement error of ±5 cm s^-1 yr^-1 after 5 yr, these upcoming redshift drift measurements might therefore be able to differentiate between R_h = ct and Planck ΛCDM at ~3σ, assuming that any possible source evolution is well understood. Such a result would provide the strongest evidence yet in favour of the R_h = ct cosmology. With a 20-yr baseline, these observations could favour one of these models over the other at better than 5σ.