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.”

As nuclear talks restart in Pakistan this weekend, President Trump will confront the complicated legacy of his own decision, eight years ago, to cancel what he has called “a horrible, one-sided deal.”

That Obama-era agreement suffered from flaws and omissions. It would have expired after 15 years, leaving Iran free after 2030 to make as much nuclear fuel as it wanted. But once Mr. Trump withdrew from the deal in 2018, the Iranians went on an enrichment spree much sooner, leaving them closer to a bomb than ever before.

Now, Mr. Trump’s negotiators are dealing with the consequences of that decision, which he made over the objections of many of his national security advisers at the time.

Much recent attention has focused on Iran’s half ton of uranium that has been enriched to a level just shy of what is typically used in atom bombs. The majority of it is thought to be buried in a tunnel complex that Mr. Trump bombed last June. But those 970 pounds of potential bomb fuel represent only a small fraction of the problem.

Today, international inspectors say, Iran has a total of 11 tons of uranium, at various enrichment levels. With further purification, that is enough to build up to 100 nuclear weapons — more than the estimated size of Israel’s arsenal.

Virtually all of that cache accumulated in the years after Mr. Trump abandoned the Obama-era deal. That is because Tehran lived up to its pledge to ship to Russia 12.5 tons of its overall stockpile, about 97 percent. Iran’s weapon designers were left with too little nuclear fuel to build a single bomb.

Now, matching or exceeding that diplomatic accomplishment is one of the most complex challenges facing Mr. Trump and his two lead negotiators, his son-in-law Jared Kushner and his special envoy, Steve Witkoff, who are expected to leave for Pakistan on Saturday.

Mr. Trump is acutely aware that whatever he can negotiate with the Iranians will be compared with what Mr. Obama achieved more a decade ago. While the two countries are still exchanging proposals, and could well come up empty-handed, Mr. Trump is already judging his own, yet-to-be-negotiated agreement as superior.

“The DEAL that we are making with Iran will be FAR BETTER,” Mr. Trump wrote on his social media site on Monday. The Obama-era deal “was a guaranteed Road to a Nuclear Weapon, which will not, and cannot, happen with the deal we’re working on.”

Based on Mr. Trump’s often-shifting objectives for the conflict with Iran, Mr. Kushner and Mr. Witkoff face a daunting list of negotiation topics, many of which the Obama team failed to address. They have to find a way to limit Iran’s ability to rebuild its arsenal of missiles. (The 2015 deal never addressed Iran’s missile capability, and Tehran ignored a United Nations resolution imposing limits.)

They need to find a way to fulfill Mr. Trump’s mandate to protect anti-regime protesters, whom Mr. Trump promised to help in January when they took to the streets. In fact, those protests were among the triggers for the American military buildup that ultimately led to the Feb. 28 attack.

And they must negotiate a reopening of the Strait of Hormuz, which the Iranians shut down after the American-Israeli attacks, a move Mr. Trump was clearly unprepared for. Now Iran has discovered that a few inexpensive mines and threats to ships have given it huge leverage over the global economy, pressure it can dial up or down in ways that nuclear weapons cannot.

But it is the fate of the atomic program that lies at the negotiations’ heart. As in the 2015 talks, the Iranians declare they have a “right” to enrich under the Nuclear Nonproliferation Treaty, one they refuse to give up. But that still leaves room for “suspension” of all nuclear efforts for some number of years. (Vice President JD Vance demanded 20 years when he met his Pakistani interlocutors two weeks ago, only to have Mr. Trump declare a few days later that the right period was “unlimited.”)

William J. Burns, the former C.I.A. chief who played a lead role in the Obama-era negotiations, said in The New York Times on Friday that a good deal would require “tight nuclear inspections, an extended moratorium on the enrichment of uranium and the export or dilution of Tehran’s existing stockpile of enriched uranium in exchange for tangible sanctions relief.”

He also called for the Trump administration to delineate every term. “Unless the lines are clearly drawn and strictly monitored,” Mr. Burns said, “the Iranians will paint outside them.”

That is exactly what happened when Mr. Trump pulled out of the Obama agreement in 2018 and replaced it with nothing. At the time, Iran did not have a single bomb’s worth of uranium. Then it started enriching with a vengeance.

In the current war, Mr. Trump has spoken publicly about a possible raid to seize Iran’s half ton of near-bomb grade material, which could make roughly 10 weapons. But he has not talked about the overall 11-ton cache and the threat it poses to the United States and its allies.

It is hardly a new problem. In 2006, Iran began enriching uranium on an industrial scale. While it described its aims as peaceful and civilian in nature, its aggressive moves convinced experts that Tehran wanted to build a bomb.

The alarms rang louder in 2010, when Iran began enriching uranium to 20 percent. That level of purity marks the official dividing line between civilian and military uses. Iran said it wanted the 20 percent fuel for a research reactor at the University of Tehran.

The 20 percent enrichment alarmed the Obama administration. It put the Iranians on the road to the 90 percent fuel used to make a warhead light and compact enough to fit atop a missile. (It is possible to make a weapon from 20 percent fuel, but it would be so large and heavy that a truck, boat or aircraft would be needed to deliver it.)

In the Obama-era pact, the Iranians were prohibited from enriching fuel to a purity level greater than 3.67 percent, which is sufficient to fuel nuclear reactors for civilian power. The country’s entire stockpile was limited to about 660 pounds. The constraints were supposed to remain in place for 15 years, until 2030. But the Iranians were permitted to continue the low-level enrichment, and they built more efficient centrifuges.

That loophole turned out to set them up well for what happened after Mr. Trump scrapped the agreement three years later and reimposed economic sanctions. The Iranians responded by blowing past all those limits.

Early in 2021, just before Mr. Trump left office, Iran reinstituted its goal of raising the enrichment level to 20 percent.

Then a mysterious blast knocked out power at Natanz, which is Iran’s main enrichment complex. Iranian officials blamed it on Israeli sabotage, and retaliated by raising part of its stockpile to the 60 percent level, the biggest jump in the history of its enrichment program. That was just a hairbreadth away from the highest military grade.

From early 2021 to early 2025, the Biden administration tried, unsuccessfully, to negotiate new limits. Throughout the negotiations, Iran kept enriching, expanding its cache of 60 percent fuel.

Then, in June 2025, Mr. Trump bombed Iran’s enrichment plants at Natanz and Fordo, as well as uranium storage tunnels and other facilities at Isfahan. He declared that the nuclear program had been “obliterated.”

Officially the U.S. government was more circumspect, saying the program had been “set back.” But if “Operation Midnight Hammer” did, in fact, cripple much of Iran’s atomic infrastructure, the Trump administration said little or nothing about the survival of Iran’s cache of enriched uranium, which the International Atomic Energy Agency has estimated at 10.9 tons, with purity levels ranging from 2 percent to 60 percent.

One of the few officials who did discuss it was Mr. Witkoff, who called the stockpile “a move towards weaponization — it’s the only reason you would have it.” Iran, he added, could turn its most enriched fuel into roughly three dozen bombs.

While public discussion has focused on whether a U.S. commando team could retrieve Iran’s half ton of uranium enriched to 60 percent, nuclear experts say Tehran could turn the entire 11 tons into bomb fuel, if it can activate new centrifuges, probably underground, to boost its levels of enrichment.

Edwin S. Lyman, a nuclear expert at the Union of Concerned Scientists, said Iran’s stockpile could yield roughly 35 to 55 weapons depending on its skill in making not only the bomb’s fuel core but such nonnuclear parts as detonators that spark the chain reactions.

Thomas B. Cochran, a nuclear weapons expert who wrote an influential study on enrichment levels, concluded that Iran’s stockpile was sufficient for 50 to 100 bombs if further enriched.

For the United States, the location of the 11-ton stockpile is a major uncertainty. For Iran, it’s political leverage.

“Yes, a lot of their top scientists have been killed,” said Gary Samore, who advised the Obama White House on Iran’s nuclear program. “But they still have the basic industrial capacity to produce nuclear weapons if they decide to do that.”

Iran has another card in the nuclear game — uncertainty over the exact location of a new enrichment complex that Tehran was about to declare on the eve of the 12-day war with Israel last June. The International Atomic Energy Agency reported that a disclosure meeting set for June 13, 2025, was “canceled due to the commencement of the military attacks on that day.”

Analysts now believe that Iran may have set up the plant in the maze of mountain tunnels that adjoin its sprawling Isfahan industrial site. That’s near where Tehran is thought to store the bulk of its uranium stockpile, raising the possibility that Iran harbors a deeply buried industrial site that could conduct new rounds of fuel enrichment.

“We can’t bomb away their knowledge,” said Matthew Bunn, a nuclear specialist at Harvard. And since a plant to enrich uranium can be “comparable in size to a grocery store,” he added, the mountainous terrain of Iran offers many places to hide a clandestine bomb effort.