Galactic expansion can be understood in terms of bread dough, guest lecturer says

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An art piece titled 'Hard Work' rests upon Umberger Hall. Jo Dunkley, professor of physics and astrophysical sciences at Princeton University, gave a nontechnical talk titled "Our Window on the Universe," speaking on the fascination humans have about the night sky. (Archive photo by Evert Nelson | Collegian Media Group)

A dough of bread is left to rise. As the yeast works, it expands. Now, imagine if we lived inside the dough, said Jo Dunkley, professor of physics and astrophysical sciences at Princeton University.

“There are raisins in the dough,” Dunkley said. “All the raisins would move away as the dough grows. The raisins wouldn’t move but the dough gets bigger. The raisins on the edge would appear to be moving away faster.”

There’s no point of growth — it expands everywhere. There is no middle point of growth either, which is depicted in the universe, Dunkley said at Tuesday’s 2018 Chester Peterson Jr. Public Lecture in Physics in Umberger Hall.

“Our universe is continually growing,” Dunkley said. “We are the result of 14 billion years of its evolution.”

Scientific advances are constant and go back in time. A woman named Henrietta Swan Leavitt observed that the rate of pulsation of stars depends on the star’s brightness in the late 19th century.

“How’s that useful?” Dunkley asked. “Because if you can see stars in these distant galaxies, you can figure out where the galaxy is.”

Another woman named Vera Rubin was an astronomer in the 1940s. She worked to understand the galaxy rotation rates. Ultimately, Rubin’s research propelled humanity forward. Her research about galaxy rotations led to the discovery of dark matter. Her data provided some of the first evidence for dark matter, which was theorized by Fritz Zwicky in the 1930s, Dunkley said.

“We make computer simulations of what universe looks like without galaxies,” Dunkley said. “Where does the dark matter sit? Not only around galaxies, we think it links together and acts as a network.”

Dunkley described the dark matter as a web that’s spreading throughout space of the invisible matter — galaxies tend to form where there is dark matter.

“There’s five times as much as invisible dark matter than the stuff we can see,’ Dunkley said. “But we don’t know what it is. We don’t even know what it looks like, but we can guess.

“We look at light from distant galaxies is bent by invisible matter on the way to us,” Dunkley continued. “The galaxies get distorted as their light bends around the dark matter.”

To further this research, there is a Large Synoptic Survey Telescope in Chile. It will scan the whole sky every few nights for ten years starting in 2020, Dunkley said.

“It’s the biggest camera in the world,” Dunkley said. “It will measure millions of galaxies to reveal [a] web of dark matter. We’ll be able to infer what the invisible stuff looks like. We’ll be able to map universe out.”

Stephen Dyer, a retired professor of electrical engineering, said Dunkley’s lecture made him think about the role of women in physics as well as other areas like engineering and the arts.

“There are forces within one’s group that inadvertently limit group’s progress,” Dyer said.

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