Dark energy: UT astronomers take us closer to understanding the greatest mystery in physics

Cason Hunwick

Armed with an upgraded telescope aimed at the dark West Texas sky, UT astronomers are peering billions of years back in time to search for answers to dark energy.

Modern science understands a lot about the universe. Astronomers know how planets and stars move from Newton’s laws, and physicists understand how gravity works from Einstein’s theory of relativity. But in 1929, astronomer Edwin Hubble discovered that the universe expands at an increasing rate, something no modern theory can explain.

“The universe is expanding faster,” said UT astronomy professor Karl Gebhardt. “This (discovery) caught everyone off guard. This has a term: dark energy.”

Dark energy is a term astronomers use to refer to the mechanism behind the continual expansion of the universe. They don’t know exactly what causes it. It could be energy associated with empty space, according to Gebhardt. It could be that our laws of gravity are wrong or that dark matter adds extra mass to galaxies.

“Dark energy has to do with our gross misunderstanding of the way the universe is expanding,” Gebhardt said. “It may not be dark, and it may not be energy. That is the term we use to represent our ignorance. It is a placeholder for what’s going on.”

Gebhardt helped orchestrate the plan behind the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX), an experiment at UT’s McDonald Observatory that is trying to figure out how dark energy changes our observations of the universe.

As the universe expands, the galaxies move away from each other, resulting in an effect called redshift.

When an ambulance drives by, its siren sounds lower as it speeds away. The sound waves get stretched out as the source of the sound recedes. The same phenomena happens with light from distant galaxies.

As the space between the Milky Way galaxy and distant galaxies grows due to expansion, the light that reaches Earth gets stretched out. Yellow light turns orange, purple turns blue. Essentially, the colors are shifted toward the red side of the color spectrum. By measuring the redshift, astronomers can measure the expansion.

Dark energy is a mystery that requires experiments on the scale of HETDEX in order to find answers.

“(We are) trying to explore these huge mysteries,” Gebhardt said. “That is one of the biggest mysteries out there. Why is the universe expanding the way it is? What is dark energy? (Astronomers) go after the most fundamental questions there are. Where does matter come from? Where does the universe come from? Why does gravity work?”

The Hobby-Eberly Telescope (HET) located at the McDonald’s Observatory, collects data that theorists can use to start to answer some of these questions.

Systems engineer Jim Fowler works at HET. Fowler is part of a team of electrical engineers, mechanics, programmers and software engineers that ensures that the telescope remains operational. They handle the dirty work to make sure the team of scientists working on the dark energy project get the data they need to measure expansion.

His team worked hard to install Visible Integral-Field Replicable Unit Spectrographs, or VIRUS instruments. These form an array of instruments that collect spectral data from the light of distant galaxies in order to measure their position and velocity.

Light bounces off the telescope’s mirror and enters fiber cables. These cables feed the data into spectrographs located in black casings on either side of the mirror. The spectrographs reduce raw information, the wavelengths of light from every point in the sky, from the cables into a usable spectral data.

Guided by software, the entire structure moves in unison and tracks across the sky every night, collecting crucial data for researchers back on campus.

Research scientists use this data to measure the relative positions and velocities of faint, ancient galaxies. The fainter and further the light, the older the galaxy. If something is ten billion light years away, that means the light comes from a galaxy that is ten billion years old.

“We want to measure the expansion rate of the universe 10 billion years ago and compare it to the rate 4 billion years ago. The difference will allow us to better understand the physics behind it,” Gebhardt said. “That’s the fundamentals of HETDEX.”

Instead of producing high-quality images, the goal is to collect lots of data points in a short period of time so data scientists can form a complete map of the sky.

“Basically, we built a light bucket,” Fowler said. “You’ll never see a pretty picture come from this telescope. What we’re doing is collecting data in a cost-effective way, and we’re getting the most science per photon possible.”

Fowler said as they install more spectrographs and collect data, they communicate and troubleshoot with scientists on campus. They’re developing their data analysis routine, getting the wheels turning so the data collection runs smoothly.

Erin Cooper is a research fellow in the department of astronomy. She analyzes the data collected on-site and writes computer programs to pinpoint useful points in the spectral data, which will ultimately form a map.

“We are trying to find specific points in the data where there are star-forming galaxies,” Cooper said. “They are easy to spot, and we know the wavelengths well. So it’s easy to measure the redshift. That redshift can be translated to distances.”

The team needs millions of data points to produce a full map. Data scientists have to come up with ways to isolate spectral data that correlates to these star-forming galaxies. Basically, they need the computer to do what a person couldn’t and observe millions of galaxies and determine if they are useful.

Physics senior Yaswant Devarakonda writes programs that comb through these piles of spectral data.

“The goal is to create a program that can draw the line between usable and unusable data,” Devarakonda said. “We’re using machine learning to try and automate the data selection process.”

Devarakonda said it’s an exciting time to work on the project. Researchers now receive new data every night.

“The spectrographs are working properly, and they’re slowly adding more and more so now we’re really in the brunt of it,” Devarakonda said. “It’s really picking up right now.”

Once they finish installing the spectrographs, Fowler said they could complete the survey in five years.

“We are going to build the largest spectrograph that takes largest amount of data by a factor of 100,” said Gebhardt. “We’re taking one of the largest surveys in terms of time. And we’re using one of the most powerful computers (at the Texas Advanced Computing Center) on the planet. All those things separate are pretty profound, when you put them together it is kind of insane.”