Improvements in cosmological constraints from breaking growth degeneracy

¹ Department of Physics and Astronomy, University of the Western Cape, Cape Town 7535, South Africa
e-mail: perenon.louis@yahoo.fr
² Université PSL, Observatoire de Paris, Sorbonne Université, CNRS, LERMA, 75014 Paris, France
³ CEICO, Institute of Physics of the Czech Academy of Sciences, Na Slovance 2, Praha 8, Czech Republic
⁴ IRAP, Université de Toulouse, CNRS, CNES, UPS, Toulouse, France
⁵ Institute of Cosmology & Gravitation, University of Portsmouth, Portsmouth PO1 3FX, UK
⁶ Cosmology & Gravity Group, Department of Mathematics & Applied Mathematics, University of Cape Town, Rondebosch 7701, Cape Town, South Africa

Received: 13 May 2020
Accepted: 22 July 2020

Abstract

Context. The key probes of the growth of a large-scale structure are its rate f and amplitude σ₈. Redshift space distortions in the galaxy power spectrum allow us to measure only the combination fσ₈, which can be used to constrain the standard cosmological model or alternatives. By using measurements of the galaxy-galaxy lensing cross-correlation spectrum or of the galaxy bispectrum, it is possible to break the fσ₈ degeneracy and obtain separate estimates of f and σ₈ from the same galaxy sample. Currently there are very few such separate measurements, but even this allows for improved constraints on cosmological models.

Aims. We explore how having a larger and more precise sample of such measurements in the future could constrain further cosmological models.

Methods. We considered what can be achieved by a future nominal sample that delivers an ∼1% constraint on f and σ₈ separately, compared to the case with a similar precision on the combination fσ₈.

Results. For the six cosmological parameters of ΛCDM, we find improvements of ∼5–50% on their constraints. For modified gravity models in the Horndeski class, the improvements on these standard parameters are ∼0–15%. However, the precision on the sum of neutrino masses improves by 65% and there is a significant increase in the precision on the background and perturbation Horndeski parameters.