To Hunt for Dark Energy, Swipe Left or Swipe Right

Physics is awash in large collaborations. The gravitational wave hunters in the LIGO Scientific Collaboration include 1,200 members in 18 countries. The ATLAS particle physics collaboration encompasses 6,000 members from 42 countries. But the size of Lindsay House’s team is an entirely different magnitude. Since 2021, House, a graduate student in astrophysics at the University of Texas at Austin, has recruited 17,000 people from 85 different countries to contribute to her dark energy research.

House’s 17,000 collaborators are “citizen scientists” — volunteers who have signed up for an online program she created called Dark Energy Explorers. “I definitely didn't expect there to be this much interest,” says House, who is in the fourth year of her program.

The volunteers contribute to the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX), a collaboration that researches the nature of dark energy, a hypothesized form of energy causing the universe’s expansion to accelerate. To that end, they are using the eponymous telescope, located 450 miles west of Austin at the McDonald Observatory, to create a 3D map of a projected 1.2 million galaxies, ranging from 9 to 11 billion years old. “We’re mapping the large-scale structure of the universe,” says House. By analyzing the spatial distribution of the galaxies, they can determine the universe’s expansion rate at different points in time. Many competing hypotheses about dark energy imply certain expansion rates, so these measurements will help them rule out incorrect theories.

The citizen scientists contribute to the research by sorting through telescope images paired with spectral data. After reading a brief tutorial, they classify which sets of images contain a galaxy and which do not. Images containing galaxies resemble black dots in a sea of gray noise and come with a telltale spike in their spectra at the right wavelength.

On a smartphone, Dark Energy Explorers’ design appears to borrow from online dating apps like Tinder: Swipe right if you think the given images contain a galaxy; swipe left if the images just look like noise. Unlike a dating app, the interface reminds you to take it easy. “Do not stress yourself on thinking you have misidentified the images,” the instructions read. “Each classification is distributed to ~10 users and averaged by HETDEX astronomers before analyzing their data.”

The HETDEX team then uses the classifications to aid their analysis. “It's really helpful,” says House. “They are classifying these galaxies and artifacts so well that we can use it in our science.”

In particular, the researchers pair the human classifications with machine learning to remove false detections that arise from instrument or calibration issues. According to a study published in The Astrophysical Journal in 2023, House and her collaborators found that the citizen scientists helped them remove 7,781 false detections. As astronomy collaborations rely on ever larger data sets, citizen science projects like House’s have made data processing much more efficient, she says.

To find volunteers, House’s team wrote press releases, which led to some podcast appearances and some coverage in local news. They also relied on the publicity engine of Zooniverse, the citizen science platform that hosts Dark Energy Explorers, which has an existing online presence and which enables volunteers to communicate with each other on message boards. The team has also partnered with public libraries and museums to reach wider audiences.

To stay connected with this far-flung community, House herself has played an active role in organizing events. She has put on a live meet-and-greet on YouTube, where HETDEX scientists chat with volunteers about the research, as well as a virtual tour of the telescope facility they use. She has presented her work through the APS Joint Network for Informal Physics Education and Research (JNIPER), and participated in school visits over Zoom to teach about the project.

Because of overwhelming interest from schools, House’s team has put together a suite of materials, including videos and worksheets, for teachers to use in their classrooms. Her team also organized a logo design competition among their citizen scientists, and eventually printed the winning design on stickers.

However, sustained publicity and engagement over multiple years has been a challenge because the team is so small, says House. She manages Dark Energy Explorers largely by herself, with support from team members. For researchers interested in starting their own citizen science projects, “some sort of longevity plan for driving traffic to the project would be really valuable,” she says. Many citizen scientists in her project are enthusiastic, which is “cool and exciting,” she says, but it can be hard to keep up with the demand.

With the volunteers’ help, HETDEX has identified 200,000 galaxies, about a sixth of what they expect to find by the project’s end. HETDEX will stop taking data this summer, but it will still take a while to sort through all of it. “We’ll still have plenty of data for Dark Energy Explorers,” says House.

House feels motivated to do this work because she believes everyone should have a personal relationship with the cosmos. She grew up in the mountains near Asheville, North Carolina, and remembers having access to dark night skies and watching the moon with her family. “It should be an educational right for people to actually reflect and understand their own place in the universe,” she says.