Gravitational redshift of galaxies in clusters as predicted by general relativity

Abstract

The theoretical framework of cosmology is mainly defined by gravity, of which general relativity is the current model. Recent tests of general relativity within the Lambda Cold Dark Matter (ΛCDM) model have found a concordance between predictions and the observations of the growth rate and clustering of the cosmic web. General relativity has not hitherto been tested on cosmological scales independently of the assumptions of the ΛCDM model. Here we report an observation of the gravitational redshift of light coming from galaxies in clusters at the 99 per cent confidence level, based on archival data. Our measurement agrees with the predictions of general relativity and its modification created to explain cosmic acceleration without the need for dark energy (the f(R) theory), but is inconsistent with alternative models designed to avoid the presence of dark matter.

Gravitational redshift and asymmetric redshift-space distortions for stacked clusters

“…It can, in principle, be detected by stacking a large sample of clusters. This has been predicted by Nottale (1990), Cappi (1995) and Kim & Croft (2004), and the first few tentative measurements from stacked clusters from SDSS data sets have been reported (Wojtak, Hansen & Hjorth 2011;Jimeno et al 2015;Sadeh, Feng & Lahav 2015).…”

“…In a situation where the density of galaxies is constant in real space, this will introduce, at leading order, a linear ramp in the density. However, in analyses of GRedshift such as those of Wojtak et al (2011) and Jimeno et al (2015), this gets removed because they fit for the local large-scale gradient using the density of galaxies well separated in velocity from the cluster. Similar effects arise from the fact that the cluster will be at finite distance, so a beam through the cluster in which the distribution of galaxies is measured will be broadening, and also because of variation of the selection function.…”

“…The first measurement of the GRedshift signal from stacked clusters has been conducted by Wojtak et al (2011) using the SDSS MaxBCG group catalogue from (Hao et al 2010). The idea is that the BCG is likely to live close to the bottom of the potential well of the host halo, while other galaxies (satellites and field galaxies) further away from the centre of the halo tend to occupy locations where the gravitational potential is shallower.…”

“…This requires an accurate prediction of the observed redshift. Wojtak et al (2011), for example, have modelled the effect by assuming a power-law mass function for clusters that are individually spherically symmetric and have an NFW (Navarro, Frenk & White 1996) profile, and that the observed redshifts are given as the sum of the first-order Doppler shift and the GRedshift with respect to the cluster centre. It was subsequently realized that several additional physical processes, such as the transverse Doppler redshift, the past light cone effect and relativistic beaming, would cause additional contributions that are generally of the same order of magnitude as the GRedshift signal (Kaiser 2013;Zhao, Peacock & Li 2013) and which complicate the analysis.…”

maggie: Models and Algorithms for Galaxy Groups, Interlopers and Environment

“…Groups and clusters are also an important tool to probe cosmological parameters, such as the dark energy parameter (Wang & Steinhardt 1998). Moreover, clusters have been recently used to test a major prediction of general relativity, with the recent discovery of weak but significant signs of gravitational redshifts (Wojtak et al 2011).…”