Professor explores mystery of dark matter
Natalie Weber | Wednesday, March 1, 2017
In the most recent installment of the “Our Universe Revealed” lecture series, associate professor of physics Antonio Delgado discussed dark matter.
“Normal matter — which is you, me, Jupiter, Mars, whatever — is only 4 percent of what we see in the universe,” Delgado said. “Only 4 percent of what we say [is] the energy budget of the universe is what you and me are made of.
“The other 96 percent is something we have never been able to detect. … Out of the other 96 percent, 21 percent is dark matter.”
Delgado said the rotation of the galaxies has helped scientists prove the existence of dark matter. The further galaxies were away from each other, the slower they were predicted to rotate around each other. However, Delgado said scientists found that the speed of rotation did not decrease with distance.
“The farther away they are, the slower they have to rotate,” Delgado said. “This is what you expect. This is also why your dryer works, or your washing machine. If you’re on a merry-go-round, the closer you are to the rotating axis, the faster you go.
“We saw that the rotation of the limits of the galaxies were more or less constant. It didn’t go down as expected.”
Delgado said this unexpected discovery could be explained in two plausible ways: Either our understanding of gravity is incorrect or we don’t see all the matter that is affecting the orbital speeds. The invisible matter potentially affecting these speeds is known as dark matter, he said.
Delgado said one gravitational explanation for the constant rate of orbital speeds is Modified Newtonian Dynamics (MOND). However, he said, MOND cannot explain certain phenomena in the bullet cluster — which consists of a collision of two galaxies.
In particular, Delgado said, MOND cannot explain why the gas and the center of gravity of the collision are not in the same spot. Dark matter, however, could explain this discrepancy, Delgado said.
“The center of gravity — where all the mass of this collision is — is not where you see the gas,” Delgado said. “So there has to be something else there to make sure that the mass is where you have seen it. You will never be able to explain ever that you’re modifying gravity. … There has to be more matter there.”
Delgado said the process of lensing occurs when matter between a source of light and an observer bends the light traveling towards the observer. According to Delgado, the lensing of light between other galaxies and the earth could be caused by dark matter.
“This is how we know that between the galaxy and the earth, the light is parted and you can get images where you see, from the earth, two different points,” Delgado said. “That’s because the light is parted by something between that you don’t see.”
According to Delgado, dark matter is able to explain a variety of different occurrences throughout the universe, but at the moment, methods for detecting dark matter is limited.
“Dark matter is a very possible explanation of different phenomena,” Delgado said. “We can explain different phenomena … but we have only detected [dark matter] right now through gravity. We haven’t been able to see dark matter in any other way.”