Scientists from around the globe participated in a livestream event on 1 December to discuss the nuclear science impacts of the groundbreaking discovery of gravitational waves from two colliding neutron stars announced worldwide in October.
The scientists discussed how the discovery likely signals that many more similar observations are on the horizon. Gravitational waves open up a new window into the cosmos, and nuclear science will play an important role in turning the window into a precision tool for analyzing the fundamental properties of matter and the creation of elements. Advances in nuclear science – specifically in models, theory, and experiments at next-generation research facilities – will unravel the underlying mysteries of heavy-element creation.
More than 400 participants tuned in to the livestream, including participants in Greece, China, Switzerland, Sweden, the United Kingdom, and across the United States.
The Joint Institute for Nuclear Astrophysics – Center for the Evolution of the Elements (JINA-CEE), a multi-institutional Physics Frontiers Center funded by the National Science Foundation (NSF), hosted the livestream. The event brought together nuclear physicists, astronomers, and computational astrophysicists to discuss further research directions based on the gravitational-wave science discovery and follow-up observations.
In October, the Gravitational Wave Laboratories LIGO and VIRGO announced the first observation of gravitational waves from the merger of two neutron stars on 17 August, in an event called GW170817, as explained by Duncan Brown, Charles Brightman Endowed Professor of Physics from Syracuse University, who is also a member of the LIGO collaboration. Immediate follow-up observations revealed a short gamma-ray burst and a so-called kilo-nova associated with the same event. A kilo-nova is the weeklong afterglow of a neutron star merger and is thought to be powered by the radioactive decay of rare isotopes produced and ejected during the merger.
Livestream speaker Mansi Kasliwal, assistant professor of astronomy at California Institute of Technology, described detecting the neutron-star merger as “scientifically amazing,” both the detection itself and the global effort: 70 telescopes on every continent and more than 2,000 astronomers worldwide were involved, and data was collected in amounts never before seen for a single astronomical event.
The livestream speakers and panelists elaborated about how significant a discovery the GW170817 event is for nuclear astrophysics: It is the long-sought "smoking gun" observation that directly indicates a possible site for the rapid neutron capture process (r-process) thought to be responsible for many of the heavy elements in nature including most of the gold, platinum, and uranium found today. Merging neutron stars seem to eject enough material and happen often enough to account for the observed abundances of those heavy elements, explained James Lattimer, distinguished professor at Stony Brook University and co-author of the paper that first proposed mergers involving neutron stars as sources of r-process heavy elements in 1974.
However, the observations indicate the production of some heavy elements. Figuring out which elements are made translates into work for nuclear scientists. With reliable nuclear data, models can be matched to observations and predict in detail which elements are produced. “What we really want to know is the properties [of nuclei] very far from stability [and] that is what various experiments are working on including the Facility for Rare Isotope Beams,” emphasized Dan Kasen, associate professor of physics and astronomy at the University of California Berkeley.
Better nuclear and atomic physics data will also enable scientists to tease out more information from GW170817 and future observations. This could provide hints on the formation of specific element groups and may even reveal what is happening deep inside the neutron star collision. Commenting on the intriguing 1.7-second delay observed between the arrival of the gravitational wave and the gamma-rays that followed – which matches the nuclear-physics-based duration of heavy-element synthesis – Columbia University Assistant Professor of Physics Brian Metzger noted, “Maybe the r-process is more closely linked to what happens at the central engine.”
“We have now this pure sample of r-process right at its production site and we can try to analyze that,” Kasen said. ”It’s really just the start of an exciting time and it should be really interesting to see what happens in the years to come.”
MSU Associate Professor of Physics Artemis Spyrou wholeheartedly agreed, highlighting the research underway now at facilities like Argonne National Laboratory, Radioactive Isotope Beam Factory in Japan, the National Superconducting Cyclotron Laboratory, and planned for FRIB in the future. She explained how FRIB will give access to the extremely neutron-rich isotopes that have now been identified to play a key role in explaining the observed kilo-nova.
The new neutron star merger observations are also a boon for nuclear science because these events are essentially “astrophysical scale” nuclear colliders that probe dense matter in complementary ways to Earth-bound experiments, Jocelyn Read from California State University, Fullerton explained. The gravitational wave signal from GW170817 has already provided unprecedented information on the size, mass, and deformability of neutron stars, which directly informs the fundamental properties of dense nuclear matter. With future observations of maybe up to 40 neutron merger events per year, “We will get an amazing precision,” Read added.
This makes it particularly important for nuclear science to provide better and complementary laboratory data on dense nuclear matter that probe different temperatures and densities. “It will be fascinating if some of the constraints from the gravitational-wave events disagree with those that we infer from laboratory experiments,” said Metzger, as this may indicate possible new physics such as a phase transition.
Improved laboratory measurements are being pursued at various nuclear accelerator facilities. The Thomas Jefferson National Accelerator Facility will measure the neutron distribution in lead nuclei with unprecedented precision. Measurements planned at FRIB will be able to study asymmetric nuclear collisions between very neutron-rich and very neutron-poor nuclei, explained Spyrou. According to Astrophysicist Andreas Bauswein, livestream panelist from the Heidelberg Institute for Theoretical Studies in Germany, such experiments can provide some of the key pieces of nuclear information needed in his computer simulations of merging neutron stars.
University of Notre Dame Freimann Professor of Experimental Nuclear Physics Ani Aprahamian summarized the opportunity the gravitational-wave science discovery presents for nuclear scientists. She said the observation has made it even more important to measure the most neutron-rich nuclei. “The challenge is open and it’s really exciting for us to go there.”
Hendrik Schatz, JINA-CEE director and livestream organizer, said the livestream was organized to bring together astronomers and nuclear scientists to both share their excitement about the LIGO/VIRGO discovery while also looking ahead to outline future research directions at the intersection of nuclear science, astrophysics, and multi-messenger astronomy.
“This event provided a great kick-off for nuclear astrophysics in the era of gravitational wave astronomy and highlighted the tremendous research opportunities that this opens up for nuclear science. I am also excited that the format of the event enabled participation of a broad and very diverse community of researchers,” Schatz said.
Led by Michigan State University, JINA-CEE is dedicated to interdisciplinary research at the interface of nuclear physics and astrophysics. JINA-CEE scientists address fundamental questions about the evolution and properties of matter in the cosmos, and the origin of the chemical elements that make up our world. JINA-CEE consists of a unique network of four core institutions (MSU, Arizona State University, University of Notre Dame, and University of Washington) and 22 associated institutions in seven countries.