Mapping the cosmos: UT researchers search for dark energy

Laura Zhang

The University of Texas research project HETDEX will shine a light on dark energy, the mysterious force that might account for the expansion of the universe. 

HETDEX, which stands for the Hobby-Eberly Telescope Dark Energy Experiment, is the first major experiment to search for dark energy by measuring the expansion rate of the universe. 

Scientists have been working on HETDEX for the past decade, but only since November have they begun collecting data using the Hobby-Eberly Telescope. 

The Hobby-Eberly Telescope, located at the McDonald Observatory in West Texas, is one of the largest optical telescopes in the world. It can observe over 70 percent of the sky, according to UT astronomy professor Karl Gebhardt.

Gebhardt said that although several institutions share the telescope, UT owns over 70 percent, which allows UT researchers to spend significantly more time on the telescope. 

In order to enhance the telescope’s optical range, UT researchers attached 150 spectrographs to the telescope with over 34,000 optic fiber cables. Spectrographs are devices that break up light into individual wavelengths. They can reveal how far apart galaxies are from each other and the rate at which they are moving away from Earth. 

While other optical telescopes use one big spectrograph, the Hobby-Eberly Telescope used more cost-effective spectrograph replications that allowed researchers to start collecting data sooner.  

“Normally in astronomy, a lot of the cost is in the engineering of an instrument. What we did is something very different,” Gebhardt said. “Ultimately, we do a blind shot of the sky, go through and find galaxies, measure how far away they are, and then make a map of the universe.”

According to Gebhardt, the motivation behind this project stems from the fundamental misunderstanding of the universe, a misconception scientists coin “dark energy.”

“Dark energy is not an entity — it may not be dark, and it may not be energy, but it is the phrase we use to represent our ignorance about how the universe is expanding,” Gebhardt said. 

Greg Zeimann, a UT postdoctoral researcher, said the universe is accelerating at a rate that cannot be explained by current models. 

“If the universe is a car, right now someone is hitting the gas pedal, and there is no law of gravity that includes a gas pedal. It only includes a brake. Something else is hitting the gas pedal — that’s the expansion rate,” Zeimann said. “So, by measuring how much someone is hitting the gas pedal, you know how much dark energy there is.”

Understanding dark energy is essential to comprehending how the universe is growing, according to Niv Drory, a research scientist at the McDonald Observatory.

Drory said that if scientists model the expansion rate of the universe in an equation, the equation suggests a missing element, called dark energy.

“The expansion of the universe is governed by all the constituents in the universe that contribute to gravity. If both sides [of this equation] are equal, great, we explained the expansion rate,” Drory said. “If they’re not, then we’re missing something, which we deem dark energy.” 

Researchers hope that data collected from HETDEX will provide insight into how the universe works, according to Gebhardt.

“I love the fact that we’re so insignificant and yet we can still try to understand how the universe came to be and what it’s doing,” Gebhardt said.