In the 1990s, Saul Perlmutter led a team of astronomers at Lawrence Berkeley National Laboratory trying to measure how quickly the universe’s expansion was slowing. Gravity, most cosmologists expected, would gradually restrain the outward motion of galaxies. His team’s observations pointed to something startling: the expansion was accelerating. “Dark energy” became the name for whatever was driving that acceleration. Its nature remains unknown.

The discovery earned Perlmutter the 2011 Nobel Prize in Physics, which he shared with Brian Schmidt and Adam Riess of the independent High-Z Supernova Search Team. It also left cosmologists with a larger question. Understanding the universe’s eventual fate would require understanding this unknown influence on its expansion.

I photographed Perlmutter on August 20, 2024, at Lawrence Berkeley National Laboratory, in the hills above the Bay. Once through security, I was directed to follow Perlmutter Avenue and meet him at the Perlmutter supercomputer. Perlmutter is a legend here.

He arrived with a smile and a violin in hand. I had asked him to bring anything that might help tell his story. I was expecting a candle or perhaps a model of a planet. The violin was perfect.

Inside the chilled server room, amid blinking lights and the steady hum of cooling equipment, he lifted it and began to play. I found it strangely moving. I had come to photograph him beside a machine bearing his name, and here he was offering something far more personal. For a few minutes, I listened as the sound of his violin carried through the room.

Afterward, we walked the campus, talking about dark matter, the allocation of telescope time, and how much remained uncertain about the universe. Perlmutter could take a difficult scientific idea and make it approachable without losing what made it difficult. His explanations left room for questions, and I kept asking them.

He talked about the early days of the Supernova Cosmology Project. It had been a long bet, requiring painstaking observations and years of international collaboration. The team measured light from distant Type Ia supernovae, exploding stars whose brightness can be calibrated to estimate their distances. Comparing those distances with the stretching of their light as it traveled through space allowed the astronomers to trace the history of cosmic expansion.

The supernovae appeared fainter than expected. They were farther away than a slowing universe would have led the team to predict. The finding challenged the assumption they had started with, and the independent team’s observations pointed to the same conclusion.

What struck me as we talked was how comfortable Perlmutter seemed with everything still unexplained. He had helped establish a discovery that changed cosmology, yet the cause of it remained elusive.

“It’s not that we want to believe in dark energy,” he said, smiling. “It’s just that the universe seems to insist on it.”

I left with more questions than I had brought. I liked that about the afternoon. We had talked about the fate of the universe, but I kept thinking of his arrival, violin in hand, and how little I had known what to expect.