Young Astronomers Blog, Volume 28, Number 15.
In a previous article, I explored the standard model of particle physics and discussed what ordinary matter is made of. All the matter that we can see and touch is composed of baryons (particles containing three quarks) and, as such, is referred to as baryonic matter. However, the universe is turning out to be much more mysterious (and darker).
In the early 17th century, Johannes Kepler put forth his three laws of planetary motion. One of which states that the closer a planet is to the Sun, the faster it moves. This and much, if not all, of the motion of stars and galaxies is controlled by gravity. Isaac Newton published the first complete thesis on gravity in 1687 with his Philosophiae Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy). In it he established the mathematical foundations for gravity and explained why Kepler’s three laws work. Albert Einstein refined how we view gravity with his General Theory of Relativity in 1915. Einstein viewed gravity as a something that curves space/time rather than a force that attracts objects with mass.
It was generally assumed that gravity associated with ordinary matter would account for the motion of the cosmos based on Newton’s and Einstein’s theories.
The first indication that something might be missing from the universe came in 1932 from Jan Oort, who suggested that there must be more matter in the Milky Way that what is observed. Oort was followed by Fritz Zwicky, who studied the motion of the Coma galactic cluster and noticed that there was not enough mass to account for the movement of the cluster. He theorized that there might be hidden matter that, in 1933, he termed dunkle Materie (dark matter), although some say Oort was first to coin the term.
In 1970, Vera Rubin and Kent Ford measured the rotational velocity of the Andromeda galaxy. They expected the galaxy to follow Kepler’s laws and that matter should move slower about the galactic center the farther it is from the center. This did not turn out to be the case. They found that galactic material moved at the same rate regardless of the distance from the center. Over the next few years, this was confirmed with observations of other galaxies.
In the 1970s, James Peebles and Jeremey Ostriker began to investigate the theoretical underpinnings of what Oort, Zwicky, Rubin and Ford had found. Their simulations indicated that galaxies acted as if they were enclosed within a halo of hidden matter. In 1982, Peebles published his theory of a “cold dark matter (CDM)” universe, which is the foundation for our modern view of the universe. Peebles recently was awarded the 2019 Nobel prize for this pioneering work in this field.
Further evidence for dark matter comes through gravitational lensing. Light from distant galaxies is bent as it passes by nearby galaxies due to the interaction with the gravity of dark matter surrounding the galaxies.
Other theories have been proposed to explain the evidence leading to the theory of dark matter. In 1983, Mordehai Milgrom offered MOND (Modified Newtonian Dynamics) suggesting that Newton’s 2nd law of motion should be modified for large distances. MOND is still being discussed, although it hasn’t been successful in replacing the dark matter hypothesis.
In any event, dark matter remains elusive. it does interact with gravity, but not with light. Astronomers do think it accounts for around 85% of the matter in the Universe. Three types of particles have been proposed for dark matter.
- Massive Compact Halo Objects (MACHOs) are normal matter that do not emit (much) light including black holes.
- Weakly Interacting Massive Particles (WIMPs) are large more exotic particles that interact with gravity, but not the other forces of nature including electromagnetism.
- Axions are extremely light particles with no electrical charge. They are a hypothetical particle associated with the field that preserves something called CP-invariance for the strong nuclear force.
Currently, WIMPs and axions appear to be the more likely candidates for dark matter.
Many experiments have attempted (and will attempt) to detect dark matter. They include:
- The planned LUX-ZEPLIN (LZ) detector in South Dakota.
- The PICO Experiment and SuperCDMS detector at the Sudbury Neutrino Observatory Laboratory (SNOLAB) in Canada.
- The XENON1T and planned XENONnT experiments near L’Aquila, Italy.
- The Axion Dark Matter eXperiment (ADMX) located at the University of Washington.
So far none of these experiments have found definitive evidence for a dark matter particle, although they have put limitation on what such a particle could be like. However, the XENON1T experiment recently announced that it may have detected axions, not as dark matter, but from the sun. Although there are other possible explanations for their results.
If you explore this topic some more, you will read that ordinary matter is 15% and 5% of everything we see. How can both be correct? In the first case, only matter is considered, which gives us 15% ordinary matter and 85% dark matter. In the second case, dark energy is also considered giving us 5% ordinary matter, 27% dark matter, and 68% dark energy. We will explore dark energy in another article.
Selected Sources and Further Reading
“Dark matter facts for kids.” Kiddle Encyclopedia. May 22, 2020. https://kids.kiddle.co/Dark_matter
“What is Dark Matter?” NASA. (accessed June 20, 2020). https://www.nasa.gov/audience/forstudents/9-12/features/what-is-dark-matter.html
Paul Sutter. “What is Dark Matter?” LiveScience/YouTube. February 16, 2020. https://www.youtube.com/watch?v=pd_3YHMaffk
Phil Plait. “Dark Matter: Crash Course Astronomy #41.” CrashCourse/YouTube. December 3, 2015. https://www.youtube.com/watch?v=9W3RsaWuCuE
Stephanie M. Bucklin. “A history of dark matter.” Ars Technical. February 3, 2017. https://arstechnica.com/science/2017/02/a-history-of-dark-matter/
William Harris & Craig Freudenrick, PH.D. “How Dark Matter Works.” HowStuffWorks.com. September 4, 2007. https://science.howstuffworks.com/dictionary/astronomy-terms/dark-matter.htm
“Dark Matter 101 – Learn more about one of the most sought-after substances in the universe.” Symmetry magazine. (accessed June 20, 2020). https://www.symmetrymagazine.org/collection/dark-matter-101
Kurt Winkler. “Fritz Zwicky and the Search for Dark Matter.” Swiss American Historical Society Review, 50(2), (2014) 23-41. https://scholarsarchive.byu.edu/cgi/viewcontent.cgi?article=3017&context=facpub
Sarah Scoles. “How Vera Rubin confirmed dark matter.” Astronomy.com. October 4, 2016 / Astronomy Magazine. June 2016. http://www.astronomy.com/news/2016/10/vera-rubin
“Big Bang theory.” Nature Physics 15, 1103. November 4, 2019. https://www.nature.com/articles/s41567-019-0720-4
Laura Dattaro. “The other dark matter candidate.” Symmetry magazine. January 21, 2020. https://www.symmetrymagazine.org/article/the-other-dark-matter-candidate
Ethan Siegel. “Is It Dark Matter? Mystery Signal Goes ‘Bump’ in World’s Most Sensitive Detector.” Forbes. June 17, 2020. https://www.forbes.com/sites/startswithabang/2020/06/17/is-it-dark-matter-mystery-signal-goes-bump-in-worlds-most-sensitive-detector/#632140547179
Richard Panek. The 4 Percent Universe. Houghton Mifflin Harcourt. Boston, New York, 2011. https://www.hmhbooks.com/shop/books/The-4-Percent-Universe/9780547577579
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