Research, Science & environment

New center to focus on physics of ultra-dense neutron stars

Theoretical studies of the physics of neutron stars will leverage data from new gravitational wave detectors, which listen for the spacetime vibrations from merging neutron stars

computer simulation of merger of two neutron stars with purple cones on either side of green concentric circles

This rendering shows the density of matter in the aftermath of two merged neutron stars, resulting in the formation of a black hole. (Graphic courtesy of David Radice/Pennsylvania State University)

The National Science Foundation (NSF) has awarded UC Berkeley $10.9 million over five years to expand research on the exotic state of matter inside neutron stars — the dense remains of exploded stars — and what can be learned from the growing number of gravitational wave detectors now listening for the spacetime vibrations generated by the violent merger of two neutron stars.

The Network for Neutrinos, Nuclear Astrophysics and Symmetries (N3AS) is an NSF Physics Frontier Center led by Wick Haxton, Berkeley professor of physics and a theoretical nuclear physicist at Lawrence Berkeley National Laboratory (Berkeley Lab).

The N2AS center, involving 13 institutions, will focus on using the most extreme environments found in astrophysics — the Big Bang, supernovae and neutron star and black hole mergers — as laboratories for testing fundamental physics under conditions beyond the reach of Earth-based labs.

“We operate as a single team, combining our expertise in order to tackle the complex multi-physics problems the arise in astrophysics — problems that are beyond the capacity of a single investigator,” Haxton said. “The postdoctoral fellows we hire belong to the team and have the freedom to move among the sites, learning the physics they need to know to make progress from a variety of world experts.”

With three gravitational wave detectors now operating around the world, scientists are detecting ever more mergers of neutron stars or black holes that ring the fabric of spacetime like a bell.

The light – optical, infrared, X-ray, and gamma ray – emitted after the merger can tell scientists about the hot, radioactive debris produced in and ejected by the merger. Such observations in 2017 confirmed for the first time that elements heavier than iron were being manufactured and ejected by neutron star mergers. Berkeley N3AS members Dan Kasen, professor of physics, and Eliot Quataert, professor of astronomy, played a key role in the theoretical work that led to this conclusion.

artistic concept of a neutron star merger with two orbs in the middle of the drawing

Artist’s conception of a neutron star merger, which creates a whirling cloud of radioactive debris and a short gamma-ray burst (jets). (Image courtesy of National Science Foundation/LIGO/Sonoma State University/A. Simonnet)

The gravitational waves produced in a merger of two neutron stars provide rich information on the structure of these exotic objects, in which a mass roughly comparable to that of the sun is squeezed into a sphere with a radius of about 6 miles. How neutron stars deform during the merger reflects properties of nuclear matter at several times the density of an atom’s nucleus.

The new underground Kamioka Gravitational-Wave Detector (KAGRA) in Japan that began operating in February will be teamed with the existing Laser Interferometer Gravitational-Wave Observatory (LIGO) in the United States and the Virgo observatory in Italy.

“N3AS is really excited about the ‘Gravitational-Wave Era,’” Haxton said. “We can use the detectors to pinpoint new cataclysmic events in the cosmos.”

The center will be housed on the third floor of Old LeConte Hall, encouraging collaboration between faculty in the astronomy department in Campbell Hall and physics faculty in LeConte.

“Collaboration across disciplines is the key to progress in addressing the deep science questions the center seeks to answer,” said Randy Katz, Berkeley vice chancellor for research. “Through a unique pilot program, the provost and the campus research and finance offices are fronting resources to the center to allow it to renovate and create an exciting vision for state-of-the-art collaboration spaces in LeConte Hall. The center is not only doing new science at the edge of knowledge; it is itself an experiment in a new partnership between our researchers, the funding agency and campus to create new facilities that will enhance our ability to pursue cutting-edge research well into the future.”

The N3AS builds upon a research hub in nuclear astrophysics that was established in 2017 at Berkeley and Berkeley Lab. That hub, also directed by Haxton, was supported by the NSF and the Heising-Simons Foundation.

Haxton credits physics professor and dean of physical sciences Frances Hellman and James Symons, former associate laboratory director, for creating the initial partnership 10 years ago among Berkeley, Berkeley Lab’s Nuclear Science Division and the Department of Energy’s Nuclear Physics program that generated a nuclear physics effort on campus focused on neutrinos and astrophysics.

“In this new era of gravitational wave detection, this NSF center is well-positioned to connect the dots between some extremely interesting scientific questions, including the properties of neutron star mergers, the nature of neutrinos and dark matter and the formation of heavy elements,” said Symon’s successor, Natalie Roe, now Berkeley Lab’s associate laboratory director for the physical sciences.

In this 2017 video, astrophysicist Dan Kasen, a member of the N3AS, explains how the first detection of gravitational waves from merging neutron stars answered an age-old question: Where does all the gold and platinum in the universe come from? (UC Berkeley video by Roxanne Makasdjian and Stephen McNally)

The N3AS will focus on a number of research areas, including:

  • the description of the ultra-dense, neutron-rich nuclear matter found at the centers of supernovas and neutron stars;
  • neutrinos, which are responsible for most of the energy and particle produced in astrophysical explosions, and which are trapped and entangled quantum mechanically in neutron stars;
  • nucleosynthesis, the creation of new elements, particularly those heavier than iron, that were not produced in the Big Bang or by burning stars;
  • dark matter, which makes up about 85% of the mass of the universe, but has only been observed indirectly through its gravitational effects; dark matter can influence how supernovas and neutron stars cool; and
  • the use of high-performance computing to simulate mergers and supernovae and connect the microphysics of neutrinos and dense matter to astrophysical observations.

Aside from UC Berkeley, the other institutions in the collaboration are Los Alamos National Laboratory, North Carolina State University, Northwestern University, Ohio University, Pennsylvania State University, UC San Diego, University of Kentucky, University of Minnesota, University of New Hampshire, University of Notre Dame, University of Washington and University of Wisconsin, Madison.

The center will feature close ties with RIKEN, Japan’s largest research institution, and CNRS, the French National Center for Scientific Research. It will support a fellowship program that hires four fellows per year to conduct research for three years. The first two years are spent at an N3AS institution of the fellow’s choosing, and the fellow must move to a different institution for the third year of research.