Light on dark matter
doi:10.1038/nindia.2021.50 Published online 8 April 2021
The identity of dark matter, which far outweighs visible matter in the universe, is still a mystery. Growing evidence shows that atoms or other known fundamental particles don’t make up dark matter — that large part of the universe which can not be seen since it neither absorbs nor emits light.
Dark matter doesn’t interact with any form of light or electromagnetic radiation, rendering it difficult to detect.
Physicists from Kolkata-based Saha Institute of Nuclear Physics (SINP) and St. Xavier’s College, have now moved a step closer to unveiling the secrets of dark matter. They have developed two models that provide new insights into the identity and mass of particles that possibly make dark matter1.
The models could help understand the particle nature of dark matter, throwing light on the unknown fundamental particles and the hitherto unfathomable laws of nature that such matter follows, SINP physicist Debasish Majumdar says.
Dark matter, he adds, is also crucial in explaining how large-scale structures such as galaxies and clusters of galaxies form and evolve in the universe.
In one of the models, the scientists show that dark matter has two components: one is the Feebly Interacting Massive Particle (FIMP) and the other is the Weakly Interacting Massive Particle (WIMP). The other model, they write, describes dark matter as consisting of Kaluza-Klein particles, which are much heavier than WIMP, and such particles can be explained only in extra dimensions; that is, beyond the four dimensions of space and time.
The gravity of massive celestial bodies at the centre of a galaxy or a dwarf galaxy could capture the WIMP dark matter particles, which, when accumulated in considerable numbers, collide with one another and destroy themselves completely, producing high-energy gamma-rays that are streams of photons, the researchers say. Such gamma rays, they say, travel great distances across space and point back to their source, providing indirect evidence of dark matter.
The energy of gamma rays ranges from thousands to hundreds of billions electron volts. Such an energy range also helps reveal the mass of dark matter particles. Combining the results of the model-based analysis and observational studies, the physicists calculated that the mass of WIMP as 50 billion electron volts, whereas the mass of Kaluza-Klein dark matter particles is around 900 billion electron volts.
Two-component or multi-component dark matter models are interesting because they can address different anomalies of dark matter searches, says astrophysicist Subinoy Das, who studies dark matter at the Indian Institute of Astrophysics.
Das, who is not involved in this research, says that the standard WIMP dark matter may explain excess gamma-rays. But, it sometimes fails to match galactic properties of dark matter such as the dark matter density profile and the abundance of satellite galaxies, he points out.
However, the models are useful, the researchers say. The models helped them estimate gamma-ray emission from dark matter annihilation from 45 dwarf satellite galaxies in the Milky Way that are thought to be very rich in dark matter.
The models also allowed the researchers to analyse gamma-ray signals from dark matter from extragalactic sources (from possible distribution of dark matter in galaxies and groups of galaxies or maybe point sources) and non-dark-matter sources such as pulsars, quasars and gamma-ray bursts.
The results of this study, Majumdar notes, indicate that the particle nature of dark matter can be probed by studying the gamma-rays that emanate from dwarf galaxies in the Milky Way and extragalactic sources.
1. Halder, A. et al. Addressing -ray emissions from dark matter annihilations in 45 Milky Way satellite galaxies and in extragalactic sources with particle dark matter models. Mon. Not. Roy. Astron. Soc. Lett. 500, 5589-5602 (2021)