An international group of scientists has come closer to the final proof of the existence of dark matter and the refutation of alternative theories of gravity. The astronomers published their conclusions based on the observation of several hundred thousand galaxies in the journal Astronomy & Astrophysics. Future research should put an end to what actually generates the effect of hidden mass that affects the movement of stars in galaxies.
It is known that the outer regions of galaxies rotate faster than can be expected from the Newtonian dynamics of the visible (baryonic) matter that makes up stars, planets, and gas-dust clouds. This mysterious phenomenon is explained by the existence of an unknown source of gravity, called dark matter. The presence of a hidden mass in galaxies is confirmed by weak gravitational lensing, baryon acoustic oscillations, and the cosmic microwave background. However, the non-detection of dark matter particles is one of the main unsolved problems of cosmology, which leaves room for alternative theories of gravity that exclude the hidden mass from cosmological models. Thus, the existence of dark matter particles is not required in the modified Newtonian dynamics (MOND) and Erik Verlinde’s theory of entropic (or emergent) gravity.
A consequence of any of the theories of alternative gravity is that the contribution of the assumed dark matter should be very closely correlated with the contribution of baryonic matter. The radial acceleration ratio (RAR) is known, which describes a strict relationship between the observed gravitational acceleration of the galaxy — g(obs) — and what is expected from the observed baryon mass — g(bar). However, RAR can also be explained within the framework of the standard cosmological model ΛCDM.
In the new work, the scientists have expanded the observations of RAR to extremely low radial accelerations, which are currently impossible to detect using galactic rotation curves or any other kinematic measurements. For this purpose, the researchers used weak gravitational lensing from the curvature of the path of light from the galaxy due to the gravitational mass of another galaxy. They determined the apparent distribution of the density of matter in about a million galaxies to a radius of three mega parsecs, which is a hundred times larger than the radius of the visible disk of the galaxy (about 30 kilo parsecs). This allows us to understand which better describes the results-ΛCDM or alternative theories of gravity.
The scientists compared the obtained RAR, measured by the gravitational lensing of 259,383 galaxies, with the predictions of ΛCDM, MOND, and the theory of emergent gravity. It turned out that the ratio between g(obs) and g(bar) agrees well with the predictions of alternative theories of gravity. However, the RARs of early and late galaxies differ significantly, which is expected only in one of the models of cold dark matter ΛCDM, in which the RAR depends on the age of the galaxies. The theory of Verlinde and MOND predicts that the ratio is always fixed.
Nevertheless, there is a loophole for alternative theories of gravity: the fact is that the differences in RAR can be explained by the presence of a diffuse cloud of hot gas in old galaxies, which has remained unnoticed in the current study. Future cosmological reviews will have to eliminate this uncertainty.