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Beyond general relativity: probing gravity with gravitational redshifts

Eleni Tsaprazi, Giorgio F. Lesci, Federico Marulli, Alan F. Heavens, Piero Rosati, Sofia Contarini, Enrico A. Maraboli, Pratika Dayal, Ofer Lahav, Lauro Moscardini

Abstract

Despite the success of general relativity (GR), the unexplained nature of dark energy on cosmological scales leaves open the question of whether GR provides a complete description of gravity. This quest is further motivated by growing tensions among cosmological observations when interpreted within $Λ$CDM. Gravitational redshifts of cluster member galaxies probe cluster potentials on megaparsec scales directly, complementing conventional large-scale structure tests. Here, we investigate how redshift precision and survey design propagate into constraints on modified gravity using an end-to-end pipeline run on mock catalogues, focusing on mis-centring and spectroscopic completeness. We find that competitive measurements require wide-field spectroscopic cluster surveys explicitly designed to maximise the number of spectroscopically confirmed members per cluster, to enable high-purity stacking, and to control systematic effects.

Beyond general relativity: probing gravity with gravitational redshifts

Abstract

Despite the success of general relativity (GR), the unexplained nature of dark energy on cosmological scales leaves open the question of whether GR provides a complete description of gravity. This quest is further motivated by growing tensions among cosmological observations when interpreted within CDM. Gravitational redshifts of cluster member galaxies probe cluster potentials on megaparsec scales directly, complementing conventional large-scale structure tests. Here, we investigate how redshift precision and survey design propagate into constraints on modified gravity using an end-to-end pipeline run on mock catalogues, focusing on mis-centring and spectroscopic completeness. We find that competitive measurements require wide-field spectroscopic cluster surveys explicitly designed to maximise the number of spectroscopically confirmed members per cluster, to enable high-purity stacking, and to control systematic effects.

Paper Structure

This paper contains 3 sections, 1 figure.

Table of Contents

  1. Introduction
  2. Observational status
  3. Technology requirements

Figures (1)

  • Figure 1: (a) Mock gravitational redshift signal (black) as a function of projected distance from the cluster centre normalised to $r_\mathrm{500}$, compared to the binned theoretical prediction for GR (blue), sDGP (self-accelerating Dvali-Gabadadze-Porrati) gravity (purple) and f(R) gravity (red) models described in 6. In the top plot, we show a simulated data set assuming a redshift uncertainty of $\sigma_\mathrm{z}$ = $10^{-4}$, whereas in the bottom plot $\sigma_\mathrm{z}$ = $10^{-2}$ is assumed. (b) Corresponding posteriors on the modified gravity parameter, $\alpha$. The self-consistency of the pipeline has been validated to the high-S/N limit. Any fluctuations are due to the random velocity dispersion and redshift-uncertainty realisations. Our analysis assumes 560 000 clusters out to z = 2 and 1 400 000 spectroscopically identified members, using a mass- and redshift-dependent halo completeness. (c) Gravitational redshift error bars as a function of projected distance from the cluster centre normalised to $r_\mathrm{500}$, for different completeness scenarios. A perfect completeness scenario is shown in blue, a galaxy-like completeness scenario in purple, and a cluster-like completeness scenario in red.