Cosmic dynamics in the era of Extremely Large Telescopes

TL;DR

This paper explores how next-generation Extremely Large Telescopes could directly measure the universe's expansion rate over time by detecting redshift drift in high-redshift quasars, providing a model-independent test of dark energy.

Contribution

It assesses the feasibility of detecting cosmological redshift drift with future telescopes, specifically quantifying the observational requirements and potential for independent dark energy evidence.

Findings

A 42-m telescope can detect redshift drift over ~20 years with 4000 hours of observation.

Redshift drift measurement is feasible in the 2<z<5 range using Ly alpha forest data.

The method offers a direct, model-independent probe of the universe's expansion and dark energy.

Abstract

The redshifts of all cosmologically distant sources are expected to experience a small, systematic drift as a function of time due to the evolution of the Universe's expansion rate. A measurement of this effect would represent a direct and entirely model-independent determination of the expansion history of the Universe over a redshift range that is inaccessible to other methods. Here we investigate the impact of the next generation of Extremely Large Telescopes on the feasibility of detecting and characterising the cosmological redshift drift. We consider the Lyman alpha forest in the redshift range 2 < z < 5 and other absorption lines in the spectra of high redshift QSOs as the most suitable targets for a redshift drift experiment. Assuming photon-noise limited observations and using extensive Monte Carlo simulations we determine the accuracy to which the redshift drift can be measured from the Ly alpha forest as a function of signal-to-noise and redshift. Based on this relation and using the brightness and redshift distributions of known QSOs we find that a 42-m telescope is capable of unambiguously detecting the redshift drift over a period of ~20 yr using 4000 h of observing time. Such an experiment would provide independent evidence for the existence of dark energy without assuming spatial flatness, using any other cosmological constraints or making any other astrophysical assumption.