Nobel Prize-winning physicist Adam Riess explained how he discovered the universe was expanding, and what it means for its past and future, at Memorial Union’s Shannon Hall on April 30.
As a part of the Wisconsin Union Directorate’s student-led Distinguished Lecture Series, Riess, who received the 2011 Nobel Prize in Physics for his work on the cosmological constant, lectured in front of a crowd of over 300 about his research into dark matter, energy and an ever-expanding universe.
Charting the cosmos
When Riess started as a researcher in the High-Z Team in 1994, an international research group tasked with analyzing supernovae, current scientific consensus held that the expansion of the universe was slowing down, or decelerating. Yet Riess came across a physically impossible discovery while using a program to calculate this deceleration factor.
Using a simplified equation where the deceleration speed of the universe equaled its mass divided by two, Riess used the distance of stars across the universe to input what he believed was its deceleration speed. But when he asked his program to perform a simple check and calculate the universe’s mass, it gave him a negative number.
“Now this doesn't make any sense. There's no such thing as negative matter — that isn't good physics. But, computers don't know physics,” Reiss said.
Riess realized the amount of matter in the universe wasn’t negative, the deceleration of the universe was. In other words, the universe was not just expanding, but expanding at an accelerating rate: a “shocking” discovery, according to Riess.
Riess persisted with his research, urging his colleagues to interpret the groundbreaking data “not with [their] heart or head, but with [their] eyes.”
“We are observers, after all,” Riess said. “The universe indeed was slowing before it began expanding, as though there was this matter — called dark matter — slowing the expansion.”
Riess and his team’s findings were first published in a 1998 paper. After nearly a decade and a half of continued research into the subject, the team won a shared Nobel Prize in Physics in 2011.
Uncovering the ‘bread’ of life
Riess likened the dark matter slowing down the universe’s expansion to dough in a rising loaf of bread, holding together the raisins — galaxies — throughout the bread.
“You can think of the universe expanding like a giant loaf of raisin bread rising in the oven. The galaxies are like raisins, all moving apart from each other,” Riess said. “And the further away a raisin is, the faster it will move away from [another raisin] because there's more dough between [the two].”
Yet if dark matter is the dough of the universe, dark energy — its equally mysterious sister phenomena — is like the air bubbles which permeate a loaf of bread, increasing its expansion by forcing dough and raisin alike even farther apart.
“As that [dark] matter diluted and the universe grew in size, it became less important,” Riess said. “And this dark energy, waiting in the wings, became the dominant thing.”
Since Earth itself is trapped in one of these “raisins,” expanding farther and farther away from its neighboring galaxies, researchers needed a unique approach to model not only our distance from other galaxies, but also the speed at which they continue to hurtle away from each other.
Supernovae, the result of a dying star collapsing and exploding under its own gravity, create some of the brightest phenomena observable in our night sky. Yet not all supernovae are created equal, and among the many different types in our observable universe, Type Ia supernovae have captivated researchers like Riess.
Because of the unique circumstances in which they form, Type Ia supernovae always explode at the same brightness, regardless of their age – making them perfect candidates for what Riess called “lighthouses” of space. Much like a traditional lighthouse, the brightness and color of these supernovae acted as distance markers. By analyzing the color and brightness of supernovae, Riess and his team measured the distance between Earth and other galaxies.
It was through this measurement of “lighthouses” that Riess and his team discovered evidence for the universe expanding. But there was a catch: because it can sometimes take billions of years for light to reach earth, all of the team’s data on the universe’s expansion actually came from ancient history.
“The universe doesn't really instant message us,” Riess said.
Although the measurement data could map a universe billions of years behind that of the present day, Riess and his team were able to plot a “trajectory course” for the universe, mapping both its ancient history and its far-flung future.
“It seems initially annoying that we can't get contemporary current information about the universe, but [the data] actually becomes a very powerful tool to see the past expansion history of the universe,” Riess said.
What’s next?
Even this map of universal history, charted by the discovery of over 3,500 Ia supernovae, has a glaring inconsistency, one Riess hopes to solve by continuing his new research.
When comparing Riess’s researched speed model for universal expansion to background radiation created by the Big Bang, an error of “about a billion year difference in the age of the universe” was observed, along with evidence that the universe should be expanding about five times as slowly as Riess’s research determined.
Riess believes the key to understanding such a mismatch is rooted in combining the fields of physics and quantum mechanics to shed light on the mystery of dark energy and dark matter.
“96% of the universe is dark matter and dark energy, but we don't really know, physically, what those are,” Riess said. “It's very hard to say we understand the universe if we don't understand its biggest part… [Understanding] will help us determine not only the fate, but the origin of the universe.”
