When I photographed John Mather at the National Academies in the spring of 2015, he had already helped measure the afterglow of the Big Bang. He was also working on a telescope that would look back toward the first galaxies. What struck me was how understated he seemed beside the immensity of those projects. He was thoughtful and precise.
His work on COBE rested on an extraordinary fact: the early universe is still visible. Its light has been traveling through space for billions of years. As the universe expanded, that light stretched to longer wavelengths and cooled. It reaches us as a faint microwave glow across the sky, carrying evidence of a time before stars.
NASA’s Cosmic Background Explorer, known as COBE, was designed to examine that glow. John led the instrument that tested a fundamental prediction: if the universe had once been hot and dense, its surviving radiation should have a particular distribution of energy across wavelengths. Physicists call this a blackbody spectrum. Expansion would cool the radiation while preserving that distinctive shape.
The experiment required an exacting comparison between the sky and a reference source carried aboard the satellite. A discrepancy could have exposed a problem with the instrument, or with the account of cosmic history it was testing. The measurements had to be trustworthy enough to tell the difference.
In January 1990, John presented a graph based on the first nine minutes of observations at a meeting of the American Astronomical Society. The measurements followed the predicted curve so closely that the audience rose in a standing ovation. Those nine minutes had taken years of work to obtain. They supplied remarkably strong evidence that the universe had evolved from a hot, dense beginning.
In 2006, John shared the Nobel Prize in Physics with George Smoot, whose COBE team had measured tiny variations in the background radiation. Together, the results helped turn questions about the early universe into a science of precise measurements.
By then, John was deeply involved in another long undertaking. He served as senior project scientist for the James Webb Space Telescope from 1995 to 2023, helping guide its scientific ambitions through decades of design and construction.
Webb launched in 2021 and released its first scientific images in 2022. It observes infrared light, allowing astronomers to study ancient galaxies whose light has stretched as the universe expanded. It also examines the atmospheres of planets around other stars. The instrument John had spent so much of his life helping bring into existence was finally gathering light.
When we met in 2015, John had already spent twenty years working on Webb. Six more would pass before launch. I find that span remarkable: so much of a scientific life devoted to an instrument before its first observations could begin.
