Two billion years after the cosmos banged into existence, a mysterious force known as dark energy began shoving space outward, causing the universe to balloon faster and faster and threatening to one day rip apart everything within it, from clusters of galaxies to particles inside atoms.
Astronomers have taken on the behemoth task of figuring out the fate of our nearly 14-billion-year-old universe by understanding what dark energy is, and how exactly it works. But on Earth, they can do so only under the darkest of night skies. For a team of researchers, that meant setting up shop at the McDonald Observatory in the remote Big Bend region of West Texas, in the biggest dark-sky reserve on the planet.
“We wanted to go for the most distant objects that we could see on a telescope,” said Karl Gebhardt, an astronomer at the University of Texas at Austin and the father of the Hobby-Eberly Telescope Dark Energy Experiment, or HETDEX. Those objects, he explained, are galaxies as far away as 12 billion light-years, and they are faint enough to be obscured by the brightness of the full moon, or too many well-lit storefronts.
The dark skies of Big Bend enabled the HETDEX team to collect data from 2017 to 2024; now, Dr. Gebhardt and his colleagues are on the cusp of their first major result. But the region’s starry nights are valued for far more than their scientific merit. They have also inspired an ecosystem of dark-sky advocacy that extends beyond astronomers’ desire to decipher the universe, particularly as light pollution encroaches on the ability to see the stars.
Each year, tens of thousands of visitors, mostly Texans from the bigger cities, trek up the mountains to attend “star parties” thrown regularly by McDonald Observatory. Nearly 400 people signed up for the star party last Friday, one of several events celebrating International Dark Sky Week throughout the region.
According to Stephen Hummel, who coordinates McDonald’s dark-sky outreach, local ordinances exist across Big Bend to minimize light pollution at night. But much of the effort is voluntary. “I don’t think astronomy is the biggest motivator for adopting these practices,” Mr. Hummel said before Friday’s star party, timed to April’s new moon.
Access to the stars “is part of the landscape, like the mountains are,” he said. “For many people, you can’t think about the Big Bend region without thinking about the night sky. It’s integral to its identity.”
At the speed of night
Scientists first discovered that the universe was expanding ever faster in 1998 by observing a certain kind of supernova, or exploding star. These supernovas emit the same amount of light regardless of where they sit in the universe; this makes it possible to predict how bright such events should appear given their distances from Earth.
If gravity were slowing down the expansion of the universe, as astronomers believed would be the case, such supernovas should have looked slightly brighter than predicted. Instead, those supernovas appeared dimmer: The expansion of the universe was speeding up.
“‘Dark energy’ is the phrase we use to represent our ignorance of how the universe is expanding,” Dr. Gebhardt said. But “it may not be dark. It may not be energy.”
One way to investigate the nature of dark energy is to chart the spread of matter across the cosmos, a pattern that froze in place as the universe cooled after the Big Bang. That pattern is a bit like a cosmic fingerprint: Its ridges have stretched as the universe has grown larger. Astronomers can measure this expansion by mapping the positions of galaxies in different eras of cosmic time.
The HETDEX team is attempting to make a map of the universe as it was between 10 billion and 12 billion years ago, an earlier epoch than any dark-energy survey has yet reached.
“I didn’t want to observe the same region of the universe and just try to do a better job,” Dr. Gebhardt said. “I wanted to do something new.”
Star-forming galaxies in this era of cosmic time emit photons, or particles of light, at a specific ultraviolet wavelength. As the universe expands, that wavelength gets stretched out, and the light is in the visible range by the time it reaches Earth.
To capture this ancient light, HETDEX researchers employed the giant Hobby-Eberly Telescope, which consists of 91 hexagonal mirrors tiled together like a reflective honeycomb. Tens of thousands of cables feed any collected photons into a set of spectrographs, which split the light into a rainbow of different colors. This data helps astronomers identify which light came from distant galaxies and calculate the source’s distance from Earth.
Using the galaxies’ depth and position in the night sky, scientists can construct a three-dimensional map of the early universe.
But the galaxies targeted by HETDEX are so far and faint that sometimes only a couple of hundred photons make it to Earth. Even in as dark and remote a place as Big Bend, that meant the survey could be done only when the moon was not visible, lest it wash out the telescope’s view. According to Taft Armandroff, the director of McDonald Observatory, the site has some of the darkest skies on the continent.
“It is really, really critical for the astronomy we do,” Dr. Armandroff said in an interview in January.
Astronomers have been exploring the universe at McDonald since 1939. But the effort to legally protect the darkness of the region with outdoor lighting regulations began in the 1970s, as ranching communities around the observatory began to grow. The Greater Big Bend International Dark Sky Reserve was formally designated in 2022.
Today, the reserve spans more than nine million acres across Texas and Mexico and includes several small towns, historical sites, protected wildlife areas and natural parks. With the observatory’s encouragement, the darkness has become its own attraction. It has also become an inspiration for economic opportunity, environmental conservation and pride in a rural way of life.
A party under the stars
A winding road, flanked by yellow grasslands and accented by the occasional javelina or roadrunner, leads up to the three research telescopes, situated atop neighboring mountain peaks, of McDonald Observatory. Smaller telescopes, used for education and outreach, dot the area below the summits.
The sky blushed pink as the sun dipped behind the Davis Mountains late on the Friday afternoon of International Dark Sky Week. The silver dome of the Hobby-Eberly Telescope creaked open as operators prepared for nightly observations.
Below, stargazers crowded into the visitor center to learn about light pollution and dark-sky-friendly illumination. Bats swooped through the breezy twilight air, and visitors ambled toward the outdoor amphitheater for a guided tour of the heavens. Clouds that had rolled in at sunset suddenly dissipated, revealing a moonless night splattered with stars. Orion beamed in the western sky, the Big Dipper in the northeast — the two constellations around which HETDEX had focused its galactic survey.
“If y’all don’t know what dark energy is, don’t feel bad,” the star party’s host said. “Neither do we!”
The HETDEX survey completed observations two summers ago, and astronomers have been analyzing its data since. Their first measurement of dark energy in the early universe is expected to come out this year.
“I thought that I was going to get depressed or tired,” said Dr. Gebhardt, who conceived the project more than two decades ago. “But I’ve never been more excited.”
Already, HETDEX scientists are thinking bigger. They hope to use the Hobby-Eberly to scan the entire night sky, increasing their pool of data to further refine their knowledge of dark energy in distant cosmic time.
But for now, Hobby-Eberly has lighter fare to study: the atmospheres of stars, planets circling faraway suns, the gravitational influence of galaxies central to other experiments. And on this Friday night, the star-party attendees had their own observations to make, including of cloud bands on Jupiter and a nursery of baby stars just south of Orion’s belt, 15,000 light-years away.
Julian Muñoz, a theoretical astrophysicist who joined HETDEX in 2023, vividly recalls the first time he saw the night sky at McDonald Observatory. “In a way, it’s like discovering the universe,” he said. Through the eyepiece of a telescope, he examined a cluster of ancient stars that astronomers have used to better understand how galaxies form.
“Not only is it there, but it was there when Newton was alive,” Dr. Muñoz said. “And it’ll be there when I’m gone. And we’ll get to understand the universe through it.”
