What is Dark Matter?

Dr. Karen Masters, Associate Professor of Physics and Astronomy (left), Dr. Daniel Grin, Assistant Professor of Physics and Astronomy (right).

Dark matter is mysterious to many, and exploring this phenomenon usually leads to more questions than answers. Professors Dr. Karen Masters, associate professor of Physics and Astronomy, and Dr. Daniel Grin, assistant professor of Physics and Astronomy, provide key answers and explanations. 

This NASA Hubble Space Telescope image shows the distribution of dark matter in the center of the giant galaxy cluster Abell 1689, containing about 1,000 galaxies and trillions of stars. Dark matter is an invisible form of matter that accounts for most of the universe’s mass. Hubble cannot see the dark matter directly. Astronomers inferred its location by analyzing the effect of gravitational lensing, where light from galaxies behind Abell 1689 is distorted by intervening matter within the cluster. Image and caption via NASA.

Dark Energy vs. Dark Matter

The first thing that both Professor Masters and Professor Grin want to make clear is that dark energy and dark matter are two very different things, even though they are usually lumped together.

Dark matter’s name is rather misleading … It is transparent rather than dark, and it doesn’t absorb, produce, or reflect light in any way. 

Professor Masters says that dark matter is the name we give material that we see in galaxies. Professor Grin also mentions the gravitational effect of dark matter, stating that “dark matter is matter that fundamentally interacts with other matter through gravity.” We know that dark matter exists because we have evidence of matter pulling on other matter with gravity, yet it doesn’t give off light. Dark matter’s name is rather misleading, Professor Masters points out. It is transparent rather than dark, and it doesn’t absorb, produce, or reflect light in any way.  Because of how difficult dark matter is to find, the identity of dark matter is still unknown. It really is a mystery! 

Dark energy is very different. We know that the universe is expanding; however, this shouldn’t be happening given what we know about gravity. Gravity should be slowing this expansion down, but instead, the universe’s expansion is accelerating. This is due to dark energy, which Professor Grin says has a “negative pressure.” Professor Masters also posits that dark energy could be a modification of gravity. What we can conclude, however, is that dark matter and dark energy are both still very mysterious to this day. 

Professor Dan Grin’s Research at Haverford

Professor Masters and Professor Grin are both completing research related to dark matter. Professor Grin explores cosmic microwave background radiation (CMB), which is electromagnetic radiation left over from the early universe. This shows a snapshot of the universe at 380,000 years old. The universe’s current age is around 400 billion years, so this is one of the only ways we can see what the early universe truly looked like.

Examining sound waves in the CMB can reveal how clumpy matter is based on how hot or cold the waves are. The more dark matter there is, the clumpier normal observable matter seems. When looking at a map of the CMB, dark energy determines how wide and large these spots of sound waves are, which can be used to determine how far light has travelled. The angular size of these spots gives insight into how quickly the universe is expanding and, thus, how much dark energy is hidden in this map.

Planck Maps the Microwave Background. Image from European Space Agency, Planck Collaboration

Professor Grin compared CMB to a baby photo because it takes a snapshot of the infancy of the universe. He explained that if you look at this baby photo from a distance, you can figure out how far away it is by knowing the size of a baby’s features and by using angles and geometry.

Professor Grin always loved how basic things, such as sound waves, harmonic oscillations, and the wave-particle duality can be used to explore abstract ideas. This is the case for CMB and exploring the concept of dark matter. Professor Grin is excited about the future of his research, and hopes to go from a 2D map of CMB and sound waves to a 3D map.

Professor Grin notes that his research is key in drawing a line between experimentalists and theorists. It helps push technology along, such as in precision measurements, which benefits both observers and theorists.

Professor Karen Masters’ Research at Haverford

Professor Masters studies galaxies and their dynamics (how they move). She cites a useful analogy often used in the field: Imagine you take a photo of a crowd. How does the crowd change over its lifetime? Notice individuals’ hair colors and heights. Professor Masters essentially takes measurements of people in the crowd and studies how they change over time.

Professor Masters also studies galaxies’ rotating disk of stars. This rotation width is much too large for the stars in the galaxy. Therefore there must be something else in the galaxy — dark matter.

… at least 90% of the mass in galaxies, and the universe, is invisible and unknown.

Professor Masters highlighted that galaxies with more stars relative to its dark matter halo act differently to those with fewer stars. She also pointed out that some galaxies have spiral arms, and some don’t. That some galaxies have a galactic bar and some don’t. A galactic bar is a line across the disk of a galaxy. Dark matter affects whether a galaxy will have this bar or not. A galaxy is more likely to have this bar if there are lots of stars compared to the dark matter halo in a galaxy.

Professor Masters has always had an interest in physics and cosmology. However, it was Vera Rubin who helped Professor Masters realise her interest in galaxies and dark matter. Vera Rubin and her calculations showed that galaxies contain about ten times as much dark matter than can be accounted for by the stars that we can see. Therefore, at least 90% of the mass in galaxies, and the universe, is invisible and unknown. Professor Masters is planning on continuing her research and is very excited about the data being taken with the Greenbank telescope. 

Both Professor Masters’ and Professor Grin’s research not only helps people in their field, but also the wider scientific community. Professor Masters also explains that Astronomy is part of what makes us human, and that human curiosity is what drives people to want to explore the sky above. If it wasn’t for astronomy research, we wouldn’t have WiFi, GPS, or camera phones. Without knowledge of general relativity and its effects, GPS systems would be universally incorrect by about 20 meters.

Further Reading

Now that dark matter and its links to research at Haverford College have been explored, you may be asking yourself where you can learn more. People with no science background at all  can learn and understand dark matter. Professor Masters suggested looking at Phil Plait’s crash course in astronomy as a good place to start. It is used in 101 astronomy courses at Haverford and is rich in scientific knowledge. Professor Masters also suggested reading “Dark Matter and The Dinosaurs” by Lisa Randall (available in the Tri-Co Libraries). These are two accessible resources for people looking to expand their understanding of dark matter.

Professor Grin also recommended some informative TED talks by Risa Wechsler and physicist Patricia Burchat. If you would like to learn more about his research exploring the identity of dark matter as axions, he suggests watching “Are Axions Dark Matter?” from PBS. You might also explore Chanda Prescod-Weinstein’s book, The Disordered Cosmos: A Journey into Matter, Spacetime, and Dreams Deferred, which discusses dark matter and marginalized groups.

The world really is at your fingertips. So go out, explore your interests, and expand your mind! There is something out there for everyone.

This article was edited by Rebecca Stevens and Lydia Guertin.