By: Saatvik Sanjay
(NASA Hubble Space Telescope via Unsplash.com)
Introduction
Dark matter is an invisible form of matter that behaves like ordinary matter nonetheless – it takes up space and holds mass. However, it doesn’t interact with electromagnetic radiation making it difficult to observe. Today, scientists are able to infer the existence of such matter due to its gravitational effect on other objects.
Findings
In 1933, Swiss astronomer Fritz Zwicky, while observing the Coma cluster of galaxies, Zwicky noted an inconsistency with the theory and observation of the galaxies. The average speed of galaxies in a cluster is determined by the total mass of the cluster. Thus, Fritz calculated the total mass using the observed speeds of the galaxies. He then proceeded to calculate the mass of the galaxies again using their total light output. Surprisingly, he observed that the mass measured via speed was 10 times greater than the mass measured through light. As a result, he came to the conclusion that there was a tremendous amount of unseen matter or “dark” matter which was keeping the galaxies from flying apart.
Further research by American astronomer Vera Rubin in 1963 indicated that the outer and more sparsely populated edges of galaxies moved at the same velocity as the ones near the core, seemingly violating Newtonian physics. Only large amounts of invisible mass could satisfy both the recorded observations and the laws of physics, thus providing definitive evidence that dark matter does exist.
Based on current observations, dark matter resides in a “halo” surrounding the normal matter in a galaxy. In particular, dwarf galaxies are less bright and contain a higher concentration of dark matter. The galaxy cluster known as the Bullet Cluster is a prime example for proof of dark matter. This cluster is made up of two clusters that collided in the past leading to a shock wave similar to a bullet. Using gravitational lensing, scientists found that most of the mass is collected around the galaxies and not in the center with the gasses. With the help of fluctuations in the cosmic microwave background, dark matter is determined to account for roughly 27% of all matter in the universe, while tangible normal matter makes up 5%. The remaining 68% is occupied by dark energy.
Dark Matter vs Dark Energy
Unlike dark matter, dark energy is repulsive and pushes everything apart. Dark energy is believed to be the leading factor for the accelerated expansion of the universe. First observed by astronomers such as Edwin Hubble in the 1920s, galaxies seemed to be moving away from us and the farther they were, the faster they receded. While it was known that the universe was expanding, it wasn’t until the late 1990s when two independent groups of researchers measured it to a higher degree of precision that we discovered it was also accelerating which led to the hypothesis of dark energy being the reason. Scientists are still in the process of understanding dark energy. It is known, however, that dark energy is evenly distributed throughout the universe, not just in space but also in time. Thus, its effect doesn’t waver with the expansion of the universe. Moreover this distribution means that it doesn’t have any local gravitational effects as is the case with dark matter, but rather a global effect on the universe as a whole.
Conclusion
There are many theories circulating about the true nature of dark matter. One idea is that dark matter is made up of “supersymmetric particles”, which are hypothesized particles that are partners to those already known in the Standard Model. Experiments at CERN’s Large Hadron Collider (LHC) may provide more insight into dark matter. For instance, it is theorized that dark matter particles are light enough to be produced in the LHC and would escape the detector unnoticed. However, energy and momentum would be dissipated in this process leading scientists to infer their existence with the help of the missing energy and momentum after the collision. Other theories that suggest physics beyond the Standard Model including supersymmetry and extra dimensions also incorporate dark matter elements. One such theory is the existence of a “Hidden Valley” or a parallel world made entirely of dark matter. In the end, a breakthrough in any one of these theories would prove vital to our understanding of the universe.
Works Cited
Building Blocks - NASA Science. (n.d.). https://science.nasa.gov/universe/overview/building-blocks/#dark-matter
Dark matter. (2024, August 30). CERN. https://home.cern/science/physics/dark-matter
Dark Energy and Dark Matter | Center for Astrophysics | Harvard & Smithsonian. (2024, July 18). https://www.cfa.harvard.edu/research/topic/dark-energy-and-dark-matter
Vera Cooper Rubin: Uncovering Dark Matter, a Missing Chunk of the Universe - CTAO. (2024, April 25). CTAO. https://www.ctao.org/news/building-from-diversity-article-vera-rubin/
Fritz Zwicky: Underrecognized Astronomer | AMNH. (n.d.). American Museum of Natural History. https://www.amnh.org/learn-teach/curriculum-collections/cosmic-horizons-book/fritz-zwicky
Vera Rubin on Dark Matter: A Factor of Ten | AMNH. (n.d.). American Museum of Natural History. https://www.amnh.org/learn-teach/curriculum-collections/cosmic-horizons-book/vera-rubin-dark-matter