Spectrographs and Spectroscopists for the Sandage Test

TL;DR

The paper discusses the potential of measuring the redshift drift using advanced spectrographs like ESPRESSO and ANDES on the ELT, which can directly probe the Universe's expansion and test cosmological models.

Contribution

It highlights recent technological advancements enabling the measurement of redshift drift, a novel cosmological probe independent of gravity, geometry, or clustering assumptions.

Findings

Spectrograph improvements make redshift drift measurement feasible.

Redshift drift provides a direct, model-independent test of cosmic expansion.

Synergies with other probes enhance understanding of dark energy.

Abstract

The redshift drift is a small, dynamic change in the redshift of objects following the Hubble flow. Its measurement provides a direct, real-time, model-independent mapping of the expansion rate of the Universe. It is fundamentally different from other cosmological probes: instead of mapping our (present-day) past light-cone, it directly compares different past light-cones. Being independent of any assumptions on gravity, geometry or clustering, it directly tests the pillars of the Lambda-CDM paradigm. Recent theoretical studies have uncovered unique synergies with other cosmological probes, including the characterization of the physical properties of dark energy. At the time of the original proposal by Sandage (1962) the expected change in the redshift of objects at cosmological distances appeared to be exceedingly small for reasonable observing times and beyond technological capabilities. In the last decades progress in the spectrographs (e.g. ESPRESSO), in the collecting area of telescopes and in the samples of cosmic beacons, enabled by new datasets and new machine-learning-based selections, have drastically changed the situation, bringing the Redshift Drift Grail within reach. As a consequence, this measurement is a flagship objective of the Extremely Large Telescope (ELT), specifically of its high-resolution spectrograph, ANDES.