Dr. Steigman was one of the ringleaders of cosmology in an era in which astronomy and particle physics were merging. It was a time when scientists were asking giant questions about the cosmos — like why there are matter and galaxies — and seeking answers in the relationships between quantum particles, formed when the universe was a split-second old and ablaze with energies beyond the dreams of earthly particle accelerators.
The universe, Dr. Steigman and his colleagues liked to say, was the poor man’s particle accelerator.
The 1980s saw an explosion of ambitious new ideas about the universe, and Dr. Steigman’s work put him at the center of it — though with something of a calming effect.
As Michael Turner, of the University of Chicago, put it, “During the halcyon days of a new theory a week, when young scientists were having too much fun, Gary often provided the adult supervision and wise guiding hand.”
Gary Steigman was born in New York City on Feb. 23, 1941, to Charles and Pearl Steigman and grew up in the Bronx, a fan of the New York Giants baseball team across the Harlem River. He passed up taking the entrance exam for the selective Bronx High School of Science because he wanted to be “normal,” he once said.
Nevertheless, science and the universe beckoned. He graduated with a bachelor’s degree in physics from the City University of New York in 1961 and then obtained a Ph.D. from New York University in 1968. It was there, in his thesis, working under Malvin A. Ruderman, that Dr. Steigman made his first contribution to cosmology.
According to all the laws of physics, when the universe was born in the Big Bang, elementary particles and their antimatter evil-twin opposites — with opposite charges and spins — should be produced in equal, counterbalancing amounts. But all that astronomers could see in the present-day universe was matter. Where did the antimatter go? Were there antimatter stars and galaxies hiding out there?
In his thesis, Dr. Steigman showed that a universe with equal amounts of matter and antimatter would not work. The universe, he concluded, must have become unbalanced in favor of matter in its earliest moments.
Cosmologists are still struggling to understand how that happened.
After research stints at Cambridge University and the California Institute of Technology, Dr. Steigman taught at Yale and at the Bartol Research Foundation, part of the University of Delaware, before joining the faculty at Ohio State.
He and his Great Pyrenees, Holly, spent many summers at the Aspen Center for Physics in Colorado, where scientists talk through problems over picnic tables under the trees and on hikes in the Rockies. Dr. Steigman was a longtime trustee of the center and a member of its advisory board.
“This is a way you can enjoy a high lifestyle without a high salary,” Dr. Steigman told an interviewer one day over canapés at the home of a center benefactor.
He was recruited to Ohio State in 1986 to found a cosmology center, which would include both astronomers and physicists.
Around the same time, he began a romantic, interhemispheric relationship with a Brazilian astronomer, Sueli Viegas, from the Institute of Astronomy, Geophysics and Atmospheric Sciences at the University of São Paulo, whom he had met at a conference in Rio de Janeiro. They married in 2004, after Dr. Viegas had retired from São Paulo.
Besides Dr. Viegas, Dr. Steigman is survived by a stepdaughter, Cibele Aldrovandi; a stepson, Leonardo Aldrovandi; and two nieces. A previous marriage had ended in divorce.
Dr. Steigman became an expert in the study of the nuclear reactions that took place in the first three minutes of creation. In those moments, the universe converted primordial hydrogen, the simplest element, into heavier elements like helium and lithium, which made up the first stars. (The rest of the elements needed to make planets and people would be manufactured in stars.)
It was in Aspen one day that he and a former office mate, David N. Schramm, of the University of Chicago, discovered that they had both made the same breakthrough: According to the Big Bang equations, the amount of primordial helium produced was crucially dependent on how many kinds of ghostly, nearly massless elementary particles there were in the universe.
At the time, particle physicists had suspected that there were three kinds, or generations, of neutrinos, each representing a different family of the elementary particles that make up nature. But as their particle accelerators had gone to higher and higher energies, they had discovered more and more generations.
Dr. Steigman and Dr. Schramm combined forces with James Gunn, now at Princeton, to write a paper declaring that based on helium abundances, there could be no more than seven families of elementary particles.
“The trend was for more numbers of neutrinos as accelerators went to higher energies,” Dr. Schramm recalled in an interview before his death in 1997. “We said the trend wasn’t going to continue. There was no statement from particle physics on the number of generations. It could be a thousand. For the first time cosmology was giving something back to physics.”
The physicists were at first amused at the astronomers’ invasion of their realm. But as the measurements of primordial helium got better and better, their prediction on the number of neutrino families shrank to about three, the number known today — a result confirmed by experiments at particle accelerators.
In collaboration with Dr. Schramm and others, Dr. Steigman continued to refine the Big Bang calculations and investigate their potential consequences for the universe.
One important result of their calculations was a determination that the amount of atomic matter in the universe fell far short of the amount needed to reverse its expansion and cause it to fall back together some day in a Big Crunch. This contradicted reigning theories that the universe was right on the border between eternal expansion and eventual collapse — a so-called flat universe.
In 1980, Dr. Steigman wrote a paper suggesting that massive neutrinos left over from the Big Bang might comprise the missing mass needed to flatten the cosmos. It was one of the first proposals for what became known as dark matter.
He, Dr. Turner and Lawrence Krauss, now at Arizona State, went on to write a prophetic paper in 1984 suggesting that all problems in cosmology could be solved by adopting an old idea — invented by Einstein in 1917 and later abandoned by him — known as the cosmological constant, a long-range cosmic repulsion force.
In 1999, two teams of astronomers discovered that the universe was expanding faster and faster with time, not slowing down, under the influence of some “dark energy” that appeared to behave exactly like Einstein’s cosmological constant.
In 2011, they won the Nobel Prize in Physics.