Have you ever wondered what everything in our Universe is made of? The answer may surprise you. Dark matter, a mysterious and elusive substance, makes up about 27% of the universe, while the matter we can see and interact with makes up only 5%. But what exactly is dark matter, and how do we study it?
In 1932 Dutch astronomer Jan Oort does research on the dynamics of stars in the Milky Way galaxy’s disk. By analysing the observed motion of stars in the Milky Way’s disk, Oort came to the conclusion that the mass of all visible matter could not explain the gravitational force experienced by these stars. This discrepancy led Oort to conclude that there should be “hidden mass” in our Milky Way which is not visible to telescopes [1]. Around the same time, Swiss astronomer Fritz Zwicky studied the motions of galaxies in the Coma Cluster. Zwicky also found that the motion of the stars in this galaxy could not be explained by the gravitational pull generated from all visible matter and he suggested the existence of ”dunkle Materie” to make up for the rest of the observed gravitational pull [2]. Oort’s and Zwicky’s suggestions for invisible matter were met with scepticism at the time but gained a wider acceptance as the evidence continued to pile up over the decades after.
One of the most compelling pieces of evidence for the existence of dark matter came from a study of the Bullet Cluster in 2006 [17]. In this study, astronomers observed the collision of two galaxy clusters and found that the majority of the mass in the clusters was not in the form of visible matter but rather in the form of dark matter. This observation provided strong evidence for the existence of dark matter and helped to cement its place in modern cosmology.
Nowadays there is a scientific consensus that visible matter only makes up a small amount of all the energy in the Universe. A 2015 study by the Planck Collaboration used a variety of research estimates that our Universe is made up of 4.9% baryonic matter (the ordinary matter that makes up planets, stars, etc.), 26.8% Dark Matter and 68.3% dark energy [3].
The exact nature of Dark Matter is still up for debate. Generally, Dark Matter candidates are referred to as Weakly Interacting Massive Particles (WIMPs) [4]. The most distinct characteristic of Dark Matter, and where it gets its name from, is that it does not emit, absorb or reflect electromagnetic radiation, causing the matter to be ”dark” and unable to be detected by conventional telescopes [5]. Dark Matter can however interact with baryonic matter through its gravitational influence. This has been observed through phenomena like gravitational lensing and galaxy rotation curves [6]. Dark matter is also said to be predominantly collisionless, meaning that they do not interact with baryonic matter through the strong or electromagnetic forces [7]. Dark Matter may however have weak self-interactions [8]. One of the leading theories suggests that Dark Matter consists of slow-moving, non-relativistic particles which are known as Cold Dark Matter (CDM) [9]. Although there are contesting models, such as Hot Dark Matter, that suggest otherwise [10].
Direct detection of dark matter researches the interactions between dark matter particles and ordinary matter in a laboratory setting. This approach relies on the assumptions that:
- Dark matter is composed of particles that have weak interactions with baryonic matter,
predominantly via the weak nuclear force. - Our galaxy, including our solar system, is embedded within a dark matter halo, leading to a constant flux of dark matter particles passing through Earth.
The expected interaction rate between dark matter particles and baryonic matter is extremely low, therefore the equipment is highly sensitive and is usually located underground to shield the detectors from other sources of background radiation [11]. Indirect detection of dark matter refers to the search for signals produced as a result of dark matter particles interacting with, annihilating, or decaying into other particles, such as standard model particles like photons, neutrinos, or charged particles. The underlying assumption in indirect detection is that dark matter particles may have self-interactions or interactions with other particles via weak-scale forces, which could produce detectable signals in various forms, such as an excess of gamma rays, cosmic rays or neutrinos and can therefore be observed by gamma-ray. observatories, neutrino telescopes and cosmic-ray detectors [12].
Collider detection of dark matter refers to the search for dark matter particles or their signatures in high-energy particle collisions, typically produced at particle accelerators like the Large Hadron Collider (LHC) at CERN. Collider detection aims to directly produce dark matter particles or other new particles that could be related to dark matter, by colliding known particles (such as protons) at high energies and subsequently analysing the resulting particle decay products and interaction signatures. The underlying principle of collider detection is that if dark matter particles interact with standard model particles through weak-scale forces or if they are related to other new particles predicted by beyond-the-standard-model theories, then they could be produced in high-energy particle collisions [13].
In conclusion, dark matter remains one of the most fascinating and mysterious substances in the universe. Although we have made significant progress in understanding its properties and its role in the universe, we are still far from a complete understanding of this elusive substance. The ongoing research in direct, indirect, and collider detection techniques is crucial to unravel the mysteries of dark matter and to provide new insights into the fundamental nature of the universe. As our understanding of dark matter continues to evolve, we can only hope that we will one day unravel this mystery and gain a deeper understanding of the fundamental nature of the cosmos.
[1] J. Oort, “The force exerted by the stellar system in the plane of the galactic equator,” Bulletin of the Astronomical Institutes of the Netherlands, vol. 6, pp. 249–287, 1932.
[2] F. Zwicky, “On the masses of nebulae and of clusters of nebulae,” Astrophysical Journal, vol. 86, p. 217, 1937.
[3] P. Collaboration, P. A. R. Ade, N. Aghanim, et al., “Planck 2015 results. xiii. Cosmological parameters,” Astronomy & Astrophysics, vol. 594, A13, 2016.
[4] K. Freese, “Dark matter: A primer,” Annalen der Physik, vol. 529, no. 3, p. 1 600 218, 2017.
[5] J. R. Bond and G. Efstathiou, “Cosmic background radiation anisotropies in universes dominated by nonbaryonic dark matter,” The Astrophysical Journal Letters, vol. 285, pp. L45–L48, 1984.
[6] V. C. Rubin, J. Ford W. K., and N. Thonnard, “Rotational properties of 21 sc galaxies with a large range of luminosities and radii, from ngc 4605 (r=4kpc) to ugc 2885 (r=122kpc),” The Astrophysical Journal, vol. 238, p. 471, 1980.
[7] S. W. Randall, M. Markevitch, D. Clowe, A. H. Gonzalez, and M. Bradac, “Constraints on the self-interaction cross-section,” Astrophysical Journal Letters, vol. 679, no. 2, pp. L117–L120, 2008.
[8] D. N. Spergel and P. J. Steinhardt, “Observational evidence for self-interacting cold dark matter,” Physical Review Letters, vol. 84, no. 17, pp. 3760–3763, 2000. doi: 10 . 1103 / PhysRevLett.84.3760. [Online]. Available: https://journals.aps.org/prl/abstract/ 10.1103/PhysRevLett.84.3760.
[9] P. J. E. Peebles and J. T. Yu, “Primeval adiabatic perturbation in an expanding universe,” The Astrophysical Journal, vol. 162, pp. 815–836, 1970.
[10] S. D. M. White, C. S. Frenk, and M. Davis, “Clustering in a neutrino-dominated universe,” The Astrophysical Journal Letters, vol. 274, pp. L1–L5, 1983. doi: 10.1086/184141.
[11] R. J. Gaitskell, “Direct detection of dark matter,” Annual Review of Nuclear and Particle Science, vol. 54, no. 1, pp. 315–359, 2004.
[12] L. E. Strigari, S. M. Koushiappas, J. S. Bullock, et al., “Indirect dark matter detection from dwarf satellites: Joint expectations from astrophysics and supersymmetry,” The Astrophysical Journal, vol. 678, no. 2, pp. 614–620, May 2008. doi: 10.1086/529428. [Online]. Available: https://doi.org/10.1086%2F529428.
[13] G. Bertone, D. Hooper, and J. Silk, “Dark matter candidates from particle physics and methods of detection,” Physics Reports, vol. 405, no. 5-6, pp. 279–390, 2005.
[14] A. Einstein, “Lens-like action of a star by the deviation of light in the gravitational field,” Science, vol. 84, no. 2188, pp. 506–507, 1936. doi: 10.1126/science.84.2188.506.
[15] G. Bertone and D. Hooper, “History of dark matter,” Reviews of Modern Physics, vol. 90, no. 4, p. 045 002, Oct. 2018. doi: 10.1103/RevModPhys.90.045002.
[16] J. A. Tyson, F. Valdes, and R. A. Wenk, “Detection of systematic gravitational lens galaxy image alignments – mapping dark matter in galaxy clusters,” The Astrophysical Journal Letters, vol. 349, pp. L1–L4, 1990. doi: 10.1086/185588.
[17] D. Clowe, M. Bradac, A. H. Gonzalez, et al., “A direct empirical proof of the existence of dark matter,” The Astrophysical Journal Letters, vol. 648, no. 2, pp. L109–L113, 2006.
[18] T. M. C. Abbott, F. B. Abdalla, A. Alarcon, and D. Collaboration, “Dark energy survey year 1 results: Cosmological constraints from galaxy clustering and weak lensing,” Physical Review D, vol. 98, no. 4, p. 043 526, 2018.
