Scientists say they have removed the recent doubts cast on the presence of the elusive dark matter within galaxies, disproving the empirical relations in support of alternative theories. Dark matter is one of the greatest enigmas of astrophysics and cosmology, and is thought to account for 90 per cent of the matter in the universe.
However, its existence has been demonstrated only indirectly, and has recently been called into question. The study, published in the Astrophysical Journal, 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. The attractive force generated by matter is insufficient to explain observable gravitational effects. This had led to the theory of the existence of undetectable dark matter, and the idea that galaxies are embedded in its spherical halo.
“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,” said Chiara Di Paolo, a doctoral student at International School of Advanced Studies (SISSA) in Italy.
“Analysing the rotation curves of 153 galaxies, principally the ‘classical’ spiral kind, they obtained an empirical relationship between 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 said.
“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),” she said.
Di Paolo and her collaborators 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 produced more extended results, finding a relationship, which, besides total gravitational acceleration and its ordinary component, also involves the galactic radius and the morphology of the galaxies.
“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,” said Paolo Salucci, professor of astrophysics at SISSA and one of the research authors.
“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,” Salucci said.