Dark Matter: Exotic phenomenon or convenient hypothesis?

Review

Asish Philip Monai

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  For decades, astronomers, physicists, and cosmologists have hypothesized that the universe is made up of an exotic material known as "dark matter,” which explains the strange gravitational behaviour of galaxies and galaxy clusters. According to mathematical models, dark matter accounts for three-quarters of all matter in the universe. However, it has never been seen or fully explained. While dark matter has become the dominant theory to account for the unexplained motion of stars within galaxies, some scientists have looked for alternative explanations for why galaxies hold together these stars.

  Now, an international team of scientists claims to have discovered new evidence that dark matter may not exist at all. The scientists report tiny discrepancies in the speeds of distant stars as they orbit around their galaxy. The study was published in the Astrophysical Journal in November of last year. They believe this shows that a faint gravitational effect, known as the Strong Equivalence Principle (SEP), breaks down. This principle states that in any freely falling local inertial frame (experiencing only gravitational forces), all physics is unaffected by the external field, including internal dynamics under self-gravity. In simple words, SEP tells us that if we measure the acceleration of the Solar System relative to the Milky Way, the external gravitational fields from other galaxies should not influence the measurement.

  This is noteworthy because it further validates the idea known as modified Newtonian dynamics (MOND) or “modified gravity” as a feasible solution for a cosmological quandary: galaxies appear to defy the long-accepted rules of gravity traced back to Sir Isaac Newton. Newton based his theory on the fact that objects attract each other with a force that varies with their mass, something inferred based on observations of planetary orbits. It has remained astonishingly accurate with refinements from the theories of the German-born physicist Albert Einstein in the twentieth century.

Credits: Pexels   However, observations of the Coma cluster of galaxies by Swiss astronomer Fritz Zwicky in the 1930s revealed that it was subject to greater-than-expected gravitational forces. When American astronomers Vera Rubin and Kent Ford discovered anomalies in star orbits in galaxies in the 1970s, many scientists hypothesized that they were caused by masses of invisible “dark matter” within and around galaxies, and the theory has dominated astrophysics ever since.

  According to some estimates, dark matter accounts for approximately 85% of all matter in the universe. It is said to interact with light and visible matter only through gravity, and it explains anomalies observed in distant galaxies. However, it has never been observed, and no one has fully explained what it could be. Possible dark matter candidates include some weakly interacting massive particles, the oldest of black holes or even neutrinos.

Credits: Pexels   The MOND theory, proposed by physicist Mordehai Milgrom in the early 1980s, states that this excess gravitational pull exists because the laws of gravity are slightly altered for such large structures. Rather than attributing the excess gravitational pull to an unseen, undetectable dark matter, MOND proposes that gravity at low accelerations is stronger than a purely Newtonian understanding would predict.

  Furthermore, MOND made a bold prediction: the motions of an object in the universe should be determined not only by the mass of the object but also by the gravitational pull of all other masses in the universe, a phenomenon known as "the external field effect" (EFE). Simply put, the spin of the Earth around the Sun is affected by the gravitational pull of every other body in the universe. The group claimed to have discovered the EFE by noticing that galaxies in strong external fields slowed (or displayed declining rotation curves) more frequently than galaxies in weaker external fields, as predicted by MOND alone.

  MOND has so far withstood several scientific tests. Only time will tell if it will truly replace the undetected exotic phenomenon known as dark matter.

References

Chae, K.-H., Lelli, F., Desmond, H., McGaugh, S. S., Li, P., and Schombert, J. M., “Testing the Strong Equivalence Principle: Detection of the External Field Effect in Rotationally Supported Galaxies”, The Astrophysical Journal, vol. 904, no. 1, 2020. doi:10.3847/1538-4357/abbb96.

Asish Philip is an M.Sc graduate in Astrophysics from St. Xaviers College, Mumbai. His research interest lies in observational astronomy, particularly the study of binary stars and exoplanets.

Note: This article was submitted by Asish Philip as an assignment during the workshop Scicomm for Scientists 2021, organised by Cogito137, IISER Kolkata, funded by the Department of Science and Technology, Govt. of India. The assignment was selected for publication and has undergone due editorial process. Team Cogito137 thanks Spoorthy Raman for the initial editorial review of this article.