Strong lensing constraints on modified gravity models

Abstract

We impose the first strong-lensing constraints on a wide class of modified gravity models where an extra field that modifies gravity also couples to photons (either directly or indirectly through a coupling with baryons) and thus modifies lensing. We use the nonsingular isothermal ellipsoid (NIE) profile as an effective potential which produces flat galactic rotation curves. If a concrete modified gravity model gives a flat rotation curve, then the parameter Γ that characterizes the lensing effect must take some definite value. We find that Γ = 1.24 ± 0.65 at 1σ, consistent with general relativity (Γ = 1). This constrains the parameter space in some recently proposed models. PACS numbers:Astrophysical observations on scales greater than the solar system do not match predictions from standard gravity sourced by ordinary matter. On the scales of galaxies and clusters of galaxies the problem might be resolved if we postulate the existence of particle dark matter. However, the persistent null results from direct and indirect searches for particle dark matter strengthen the case to seriously explore alternative explanations. Those include alternative dark matter candidates such as primordial black holes and macros, but also the possibility that General Relativity must be altered.The principal alternative gravity framework, MOND, has been around for decades as a phenomenological fit [1]. More recently concrete dynamical models with well defined relativistic Lagrangians have emerged[2]. By construction, such models reproduce flat galactic rotation curves, however, as hinted in [3], so far it has not been considered whether these models can pass the strong lensing test on galactic scales.General relativity has been tested to high precision on the solar system scale. Recently such tests have been also extended to galactic scales [4][5][6][7]. These tests mainly focus on testing the post Newtonian parameters, in particular γ, which is the leading order term. The geometric metric in the post-post-Newtonian form can be written aswhere we have included only the leading-order terms.

Symmetron scalar fields: Modified gravity, dark matter, or both?

“…First constraints on the strength of the coupling between an additional modified-gravity scalar, which is responsible for galactic dynamics in the absence of particle dark matter, and photons were derived in Ref. [49] from observations of the lens system SDSS J2141-0001. It was found that the effect of the modified-gravity scalar on the lensing of photons must be of the same order as its effect on the motion of matter within the galaxy.…”

Astro- and Quantum Physical Tests of Screened Scalar Fields

“…Ref. [122] provides first constraints on the strength of the coupling between photons and an additional scalar field from modified gravity theories, which is responsible for galactic dynamics in the absence of particle DM.…”