The ESPRESSO Redshift Drift Experiment^⋆

I. High-resolution spectra of the Lyman-α forest of QSO J052915.80-435152.0

¹ INAF – Osservatorio Astronomico di Trieste, Via G.B. Tiepolo, 11, I-34143 Trieste, Italy
² Dipartimento di Fisica dell’Università di Trieste, Sezione di Astronomia, Via G.B. Tiepolo, 11, I-34143 Trieste, Italy
³ INFN – National Institute for Nuclear Physics, via Valerio 2, I-34127 Trieste, Italy
⁴ Centro de Astrofísica da Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal
⁵ Instituto de Astrof’ısica e Ciências do Espaço, Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal
⁶ Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 4150-007 Porto, Portugal
⁷ IFPU – Institute for Fundamental Physics of the Universe, via Beirut 2, I-34151 Trieste, Italy
⁸ Hamburger Sternwarte, Universitaet Hamburg, Gojenbergsweg 112, D-21029 Hamburg, Germany
⁹ European Southern Observatory (ESO), Karl-Schwarzschild-Str. 2, 85748 Garching bei Munchen, Germany
¹⁰ INAF – Arcetri Astrophysical Observatory, Largo E. Fermi 5, I-50125 Florence, Italy
¹¹ Instituto de Astrofísica de Canarias (IAC), Calle Vía Láctea s/n, E-38205 La Laguna, Tenerife, Spain
¹² Departamento de Astrofísica, Universidad de La Laguna (ULL), E-38206 La Laguna, Tenerife, Spain
¹³ Centre for Astrophysics and Supercomputing, Swinburne University of Technology, Hawthorn, Victoria 3122, Australia
¹⁴ Instituto de Astrofísica e Ciências do Espaço, Faculdade de Ciências da Universidade de Lisboa, Campo Grande, PT1749-016 Lisboa, Portugal
¹⁵ Departamento de Física da Faculdade de Ciências da Universidade de Lisboa, Edifício C8, 1749-016 Lisboa, Portugal
¹⁶ Observatoire Astronomique de l’Université de Genève, Chemin Pegasi 51, CH-1290 Versoix, Switzerland
¹⁷ Center for Space and Habitability, University of Bern, Gesellschaftsstrasse 6, Switzerland
¹⁸ Cerro Tololo Inter-American Observatory/NSF NOIRLab, Casilla 603, La Serena, Chile
¹⁹ INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, I-35122 Padova, Italy
²⁰ INAF – Osservatorio Astrofisico di Torino, Via Osservatorio 20, I-10025 Pino Torinese, Italy
²¹ Centro de Astrobiología, CSIC-INTA, Camino Bajo del Castillo s/n, 28602 Villanueva de la Cañada, Madrid, Spain

^⋆⋆ Corresponding author: andrea.trost@inaf.it

Received: 12 March 2025
Accepted: 27 May 2025

Abstract

Context. The measurement of the tiny temporal evolution in the redshift of distant objects, the redshift drift, is a powerful probe of universal expansion and cosmology.

Aims. We performed the first steps towards the measurement of such an effect using the Lyman-α forest in the spectra of bright quasars as a tracer of cosmological expansion. Our immediate goal is to determine to which precision a velocity shift measurement can be carried out with the signal-to-noise (S/N) level currently available and whether this precision aligns with previous theoretical expectations. A precise assessment of the achievable measurement precision is fundamental for estimating the time required to carry out the whole project. We also aim to study possible systematic effects of an astrophysical or instrumental nature arising in the measurement.

Methods. We acquired 12 hours of ESPRESSO observations distributed over 0.875 years of the brightest quasar known, J052915.80-435152.0 (z_em = 3.962), to obtain high-resolution spectra of the Lyman-α forest, with a median S/N of ∼86 per 1 km s⁻¹ pixel at the continuum. We divided the observations into two distinct epochs and analysed them using both a pixel-by-pixel method and a model-based approach. This comparison allows us to estimate the velocity shift between the epochs, as well as the velocity precision that can be achieved at this S/N. The model-based method is calibrated using high-resolution simulations of the intergalactic medium from the Sherwood Simulation Suite, and it provides greater accuracy compared to the pixel-by-pixel approach.

Results. We measure a velocity drift of the Lyman-α forest consistent with zero: Δv = −1.25_−4.46^{+ 4.44} m s⁻¹, equivalent to a cosmological drift of v˙ = −1.43_−5.10^+5.08 m s⁻¹ or ż = −2.19_−7.78^+7.75 × 10⁻⁸ yr⁻¹. The measurement uncertainties are on par with the expected precision. We estimate that reaching a 99% detection of the cosmic drift requires a monitoring campaign of 5400 hours of integration time over 54 years with an ELT and an ANDES-like high-resolution spectrograph.