In a study published in Monthly Notices of the Royal Astronomical Society, Dr. LI Rui and Prof. WANG Jiancheng from Yunnan Observatories of the Chinese Academy of Sciences reported their new finding. By analyzing a modified gravity (Rastall gravity), they observationally constrained the parameters in this gravity with galaxy-scale gravitational lensing.

Rastall gravity is a kind of modified General Relativity that assumes that the energy-momentum is not conserved in curved space-time. The energy-momentum conservation law is one of the basic elements in General Relativity, people have only observationally checked it in flat space-times, but it is maybe not the case in curved space-time. In order to release the energy-momentum conservation law, Rastall gravity was proposed.

Rastall gravity introduces a new interaction between the geometry and the matter field. It has many applications in cosmology and black-hole space-time recently. But in galaxies, there are no applications and observational constraints to the parameters in Rastall gravity.

For elliptical galaxies, their mass density is approximately a power-law form, which has not been explanted by General Relativity. Dr. LI and Prof. WANG found that this power-law form is a natural result of the Rastall gravity, and the power-law index is determined by two parameters in Rastall gravity.

Researchers found that how to obtain the power-law index for the mass density of elliptical galaxies could be inferred from strong gravitational lensing.

It is known that the light can be blended by massive objects. If a source galaxy is located behind a foreground galaxy, its light would be blended by the gravity of the foreground galaxy. Therefore, around the foreground galaxy, several images of the source galaxy could be found. This phenomenon is known as galaxy-galaxy strong gravitational lensing. The shapes and locations of the images of the background galaxy are determined by the mass and mass distribution of the foreground lens.

With gravitational lensing, combined with galactic dynamics, the power-law index for the mass density could be obtained. Then it could be used to constrain the parameters in Rastall gravity and judge the rationality of this modified gravity.

"This study provides a new way to prove the gravity theory,” said Prof. WANG.