During the 60s and the 70s, the field of astronomy was blooming with lots of new discoveries that could not be understood with the scientific knowledge of the time. In the search for a satisfactory explanation to the newly observed phenomena, two main hypothesis arose: the dark matter model and the modified gravity model. Given that our current standard cosmological model is called the “Lambda-cold dark matter (ΛCDM) model”, it is not too hard to guess which theory ended up being the favourite of the mainstream scientific community. But, what led to this preference of dark matter over modified gravity? On the 16th of May of 2024, Dr. Antonis Antoniou addressed this question in our cosmology seminar from a historical and philosophical point of view.
Already in 1933, Swiss astronomer Fritz Zwicky pointed out that the galaxies in the Coma galaxy cluster were moving too fast for what is expected from the cluster’s observed mass. His claims were dismissed, as they were interpreted to be caused by observational errors. However, several other researchers confirmed this anomaly, also in galaxies, in the next few decades. Similarly, astronomers in the 70s and in the 80s found that the rotational velocities in galaxies did not decrease at large galactocentric radii – as it is expected in Newtonian gravity – but they remained constant instead. In the same decade, other research groups found in their galaxy simulations that the stellar disks of spiral galaxies are not stable in Newtonian dynamics. Additionally, cosmologists noticed that, in order to produce the observed structure of the Universe, a non-interactive component should dominate the matter budget of the Universe. The signature of such non-interacting component in the CMB anisotropies was also consistent with observations at the time.
For these four problems, Dr. Antoniu proposed four possible reasons that could have led to the establishment of dark matter as the preferred solution during the 80s and onwards. These are: problem-solving potential, compatibility with established theories, feasibility of incorporation, and independent testability. In short, the incorporation of a dark matter particle to address these problems is a lot simpler and easier than modifying the well-established theory of general relativity, and the implications that dark matter might have in galactic dynamics and cosmology are more straight-forward to infer than in a non-linear gravity model (e.g. MOND). That is, it was clear how dark matter would solve the four aforementioned problems, but it was not known how modified gravity could address the structure formation problem and the CMB patterns. Also, the dark matter hypothesis was expected to be easier to test, as candidate particles could be searched for in particle accelerators (among other tests).
After the talk, we had a brief discussion on how the scientific opinion on the two alternative hypotheses might have changed with the more recent developments (e.g the dark matter particle still has not been found after all these years, Skordis and Zlosnik presented in 2021 a relativistic MOND theory that fitted the CMB, etc.). Dr. Antoniou considers that, while the number of supporters of modified gravity has clearly grown since the 80s, many scientists are still reluctant to change their mind about dark matter, in part, due to the lensing observations of the Bullet Cluster (which show a non-interactive component of the cluster contributing to the gravitational bending of the space around it).
Only time (and research!) will be able to tell whether new developments in modified gravity theories can provide an alternative interpretation for the Bullet cluster observations, or if they can reveal underlying issues in the dark matter lensing modelling. But, in the meantime, we would like to thank Dr. Antoni Antonious for his fantastic talk at our seminar. We would also like to thank our audience, both online and in person, for attending and making these seminars possible. Until next time!
Dr. Antonis Antoniou is a Humboldt Postdoctoral Fellow at the Lichtenberg Group for History and Philosophy of Physics of the University of Bonn, where he works on a project on the Philosophy of Dark Matter and Modified Gravity. His research interests primarily concern philosophical questions about the methodology of high-energy physics and cosmology, especially the debate between dark matter and modified gravity theories.
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