Test of parametrized post-Newtonian gravity with galaxy-scale strong lensing systems

TL;DR

This study tests the post-Newtonian parameter gamma using galaxy-scale strong lensing systems, finding results consistent with general relativity, and highlights the potential of future surveys to improve these constraints.

Contribution

It provides the first constraints on gamma at galaxy scales using strong lensing, incorporating systematic uncertainties and exploring degeneracies with cosmic curvature.

Findings

Gamma constrained to 0.995^{+0.037}_{-0.047} with current data

Future LSST data could constrain gamma to within ~0.0023

Strong lensing can serve as a probe for testing general relativity

Abstract

Based on a mass-selected sample of galaxy-scale strong gravitational lenses from the SLACS, BELLS, LSD and SL2S surveys and using a well-motivated fiducial set of lens-galaxy parameters we tested the weak-field metric on kiloparsec scales and found a constraint on the post-Newtonian parameter $\gamma = 0.995^{+0.037}_{-0.047}$ under the assumption of a flat $\Lambda$CDM universe with parameters taken from \textit{Planck} observations. General relativity (GR) predicts exactly $\gamma=1$. Uncertainties concerning the total mass density profile, anisotropy of the velocity dispersion and the shape of the light-profile combine to systematic uncertainties of $\sim 25\%$. By applying a cosmological model independent method to the simulated future LSST data, we found a significant degeneracy between the PPN $\gamma$ parameter and spatial curvature of the Universe. Setting a prior on the cosmic curvature parameter $-0.007< \Omega_k <0.006$, we obtained the following constraint on the PPN parameter: $\gamma=1.000^{+0.0023}_{-0.0025}$. We conclude that strong-lensing systems with measured stellar velocity dispersions may serve as another important probe to investigate validity of the GR, if the mass-dynamical structure of the lensing galaxies is accurately constrained in the future lens surveys.