Gary Steigman died on 9 April 2017 from complications of a fall on the campus of the Ohio State University. He joined the faculty there in 1986, became a Distinguished Professor of Mathematical and Physical Sciences in 2005 and advanced to emeritus status in 2012. Gary will be remembered for his important role in bringing together the fields of astrophysics/cosmology and particle physics and thereby changing the course of both. Particle physicists – who often joked about the errors being in the exponents in astrophysics and cosmology – have now incorporated big chunks of astrophysics and cosmology into their field, and ideas from particle physics have revolutionized cosmology and underpin the current paradigm, Cold Dark Matter + Cosmological Constant (ΛCDM), much to the surprise of the older generation of cosmologists.
Born on 23 February 1941 to Charles and Pearl Steigman, Gary grew up in the Bronx. He received his BS from City College of New York (CCNY) in 1961 and his PhD, with Mal Ruderman, from New York University (NYU) in 1968, both in physics. Friends, colleagues, and even scientists with whom he engaged in brief conversations will tell you that he never lost his New York style and sensibility.
Gary went from NYU to Fred Hoyle’s Institute of Theoretical Astronomy in Cambridge, joining a remarkable group of postdocs that included Stephen Hawking, Joe Silk, and Martin Rees. While there, and early in his career, he made the case against the symmetric cosmologies that were being pushed by some particle physicists. Martin Rees remembers a seminar by the more senior (and more arrogant) particle physicist Roland Omnes who was advocating matter and antimatter domains separated by leiden-frost layers and disparaging astrophysical calculations such as Gary’s thesis work. Gary held his own with Omnes, and as we now know he was right. The Universe today is made of matter and needs an asymmetry between matter and antimatter of order 10-10 at early times to explain the existence of matter today. Particle physicists see the matter-antimatter asymmetry as a key feature of the Universe that must be explained by physics beyond the standard model (baryogenesis).
From Cambridge, Gary went on to another “dream” job. At Caltech, he joined fellow postdocs Jean Audouze, Catherine Cesarsky, and J. Craig Wheeler; graduate students Bob Kirshner, Bill Press, Paul Schechter and David Schramm, and young faculty Jim Gunn, John Bahcall, Kip Thorne, and Maarten Schmidt, with Willy Fowler and frequent visitor Fred Hoyle providing the inspiration and encouragement. At Caltech he also met his long-term collaborator David Schramm and began attending the summer sessions of the Aspen Center for Physics (ACP).
Gary would spend 20 summers at the ACP, serve as a Trustee and play an important role in getting astrophysics started there. His first summer in Aspen saw the inaugural NASA-sponsored astrophysics workshop – fittingly enough on the Early Universe, though in 1972 “early” had a much different meaning than it does now. This marked the official beginning of astrophysics at Aspen, and Gary became a member of the astrophysics organizing committee, which wrote the proposals, chose the workshop topics and kept astrophysics going until it was officially integrated into the ACP in the early 1990s. He and his beautiful Great Pyrenees dog Holly were a welcome sight at Aspen for many summers.
In 1977 Schramm and Steigman wrote the first of 46 papers together, one co-authored with Jim Gunn. They used the big-bang nucleosynthesis (BBN) production of He-4 to place a limit on the number of light neutrino species, less than 7 and later refined to less than 4. This paper caught the attention, ire and eventually respect of particle physicists. Their prediction was confirmed in 1989 when the number of neutrino species was measured at the SLC at SLAC and LEP at CERN. This influential paper helped to open up the new field of particle astrophysics and cosmology.
They went on to use the BBN production of deuterium to pin down the baryon density. In a series of papers with a handful of co-authors (including me), the argument was constructed and then refined. The growing gap between the BBN baryon density and that of dark matter became the linchpin for the existence of nonbaryonic dark matter. Around 2002, the BBN prediction was confirmed by a similarly high-precision determination of the baryon density from measurements of the Cosmic Microwave Background (CMB) anisotropy. I am still awe-struck that a measurement of the baryon density based upon nuclear reactions when the Universe was a second old agrees to better than 5% with a measurement based upon gravity-driven acoustic oscillations when the Universe was 380,000 years old – and that both are consistent with the amount of baryons we see today.
Other BBN papers by the Schramm-Steigman gang constrained almost every property of every new hypothetical particle proposed by theorists. BBN began a gate that every new particle theory had to successfully pass through to be seriously considered.
Steigman and Schramm helped to usher in the particle dark-matter era in cosmology with their 1981 paper, A Neutrino-dominated Universe, which won the 1980 Gravity Essay Competition. While hot dark matter – neutrinos – started the revolution, it lost out to cold dark matter, which has survived until today. Gary stuck with hot dark matter for a long time, often playing the crucial role of CDM skeptic, but finally gave in and wrote important papers on the detection of CDM particles. Particle dark-matter did more than change the math of the mass budget, it changed the way we think about structure formation in the Universe and led to the detailed and testable theories thereof, and their paper was one of the first to realize that.
A paper that Gary wrote with Lawrence Krauss and me in 1984 was similarly prescient. It identified the “Ω-problem” of inflation: namely the growing evidence that there is not enough dark matter to make the Universe flat, as required by inflation. Aiming to save inflation, our paper suggested that the cosmological constant took up the slack, and of course, the current paradigm is ΛCDM, with its inflationary beginning and one-third dark matter and two-thirds Λ mass/energy budget.
Gary played many leadership roles in the particle cosmology and astrophysics revolution: organizing early, influential workshops at the Institute for Theoretical Physics at UC Santa Barbara and Aspen, connecting astronomers with particle physicists (from 1972 to 1978 he was an Assistant Professor of Astronomy at Yale and so had astronomy bona fides), and helping to mentor many of today’s leaders, including myself, Rocky Kolb, Keith Olive, and Katherine Freese, who were much younger then and often needed wise counsel. (As Dennis Overbye’s Lonely Hearts of the Cosmos recounts, he wasn’t always successful, finding himself in the middle of a food fight with his younger charges at the Texas Symposium in Chicago in 1986.) His human legacy is the thriving group he created at Ohio State, which has become the Center for Cosmology and Astro-Particle Physics.
Gary’s pursuit of science took him to many institutions around the world, through long-term positions, visiting appointments, and conferences, leading to a global web of friendships. He was a loyal friend, a great collaborator and fun to be around because of wide interests and strong views on almost everything. His close friends had the pleasure of watching his romantic relationship with Brazilian astrophysicist Sueli Viegas blossom in the mid 1980s. Sueli and Gary would marry in 2004, and for many years, they split their time between Sao Paulo and Columbus. Sueli and her children, Cibele and Leonardo, were truly the loves of his life.
Science has lost a pioneer and many of us have lost a valued colleague and dear friend.