Dark matter passes another test to confirm its existence

By Robert Lea

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Robert Lea

Dark matter has become an object for scientists that is as fascinating as it is mysterious, and whilst many cosmologists and astrophysicists firmly accept its existence there are still groups of scientists that believe that effects attributed to dark matter could be explained by modified theories of gravity.

New research conducted by SISSA could answer some of the more recent questions regarding the presence of dark matter within galaxies — whilst disproving the empirical relations in support of alternative theories. The study — published in the Astrophysical Journal — also offers new insights into understanding the nature of dark matter and its relationship with ordinary matter.

From the expansion of the universe to the movement of stars in the galaxies, there are many phenomena that cannot be explained by the presence of baryonic matter alone — baryonic meaning composed of baryons like protons and neutrons, such as the matter we see around us every day.

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Acceleration as a function of radius in NGC 4455, one of the studied galaxies. (Di Paolo et al.)

The gravitation force generated by matter is insufficient to explain observable the attractive force which binds together the galaxies — leading to the theory of the existence of undetectable dark matter, and the idea that galaxies are embedded in spherical haloes of such matter.

Chiara Di Paolo, a doctoral student of astrophysics at SISSA, explains: “Three years ago, a few colleagues at Case Western Reserve University strongly questioned our understanding of the universe and the in-depth work of many researchers, casting doubt on the existence of dark matter in the galaxies.

Di Paolo and her team analysed the rotation curves of 153 galaxies, principally the ‘classical’ spiral kind, obtaining an empirical relationship between the total gravitational acceleration of the stars observed and the component which we would observe in the presence of only ordinary matter in the classical Newtonian theory.

Di Paolo continues: “This empirical relationship, which seemed valid in all the galaxies they analysed and at any galactic radius, motivated the explanation of gravitational acceleration without necessarily calling into question dark matter, but involving, for example, theories of modified gravity such as modified Newtonian dynamics (MOND).”

The researchers wanted to verify this relationship, analysing the rotation curves of galaxies other than the classical spiral kind — 72 galaxies with low surface brightness (LSB) and 34 dwarf disc galaxies. They uncovered a relationship, that aside from total gravitational acceleration and its ordinary component, also involved the galactic radius and the morphology of the galaxies.

Paolo Salucci, professor of astrophysics at SISSA and one of the research authors, says: “We have studied the relationship between total acceleration and its ordinary component in 106 galaxies, obtaining different results from those that had been previously observed.

“This not only demonstrates the inexactness of the empirical relationship previously described but removes doubts on the existence of dark matter in the galaxies. Furthermore, the new relationship found could provide crucial information on the understanding of the nature of this indefinite component.”

Original source: https://iopscience.iop.org/article/10.3847/1538-4357/aaffd6/meta