Early theoretical work at Illinois foreshadowed LIGO/Virgo announcement

Siv Schwink

U of I Professor of Physics and Astronomy Stuart Shapiro
U of I Professor of Physics and Astronomy Stuart Shapiro
The historic October 16 joint announcement by the U.S.-based Laser Interferometer Gravitational-Wave Observatory (LIGO) and the Europe-based Virgo detector of the first detection of gravitational waves produced by colliding neutron stars is doubly noteworthy. It’s also the first cosmic event observed in both gravitational waves and light—some 70 ground- and space-based observatories observed the colliding neutron stars. This is arguably the biggest moment to date in “multi-messenger astronomy.”

In a press release issued by LIGO and Virgo collaborations, National Science Foundation Director France A. Córdova comments, “It is tremendously exciting to experience a rare event that transforms our understanding of the workings of the universe. This discovery realizes a long-standing goal many of us have had, that is, to simultaneously observe rare cosmic events using both traditional as well as gravitational-wave observatories. Only through NSF’s four-decade investment in gravitational-wave observatories, coupled with telescopes that observe from radio to gamma-ray wavelengths, are we able to expand our opportunities to detect new cosmic phenomena and piece together a fresh narrative of the physics of stars in their death throes.”

Well before the development of today’s innovative technologies supporting this simultaneous gravitational-wave and optical observation, early research in numerical relativity at the University of Illinois at Urbana-Champaign helped to lay the theoretical foundation for it. In fact, many features of the discovery had been predicted in the early computational simulations of Professor of Physics and Astronomy Stuart Shapiro and his group.

"This discovery of merging binary neutron stars with counterpart electromagnetic radiation is particularly rewarding to me and to my University of Illinois colleagues. It means that some of the cosmic phenomena that we have been studying and simulating on supercomputers for many years actually occurs in nature, much in the way we predicted," Shapiro comments.

When Shapiro joined the faculty at Illinois in 1996, bringing with him his then-postdoc Thomas Baumgarte, Shapiro was immediately invited to join NCSA as a research scientist by then-Director Larry Smarr.

“Physics Illinois was a hotbed of research on the properties of neutron stars, and NCSA was a leading center for numerical relativity,” remembers Shapiro.

At that time, Shapiro was already an established expert in the physics of compact objects—including black holes, neutron stars, and white dwarfs—and was a co-developer of one of the very first computer codes that could calculate the structure of a spinning neutron star according to Einstein’s theory of general relativity. With Baumgarte and several other collaborators, Shapiro was also one of the builders of the first code in general relativity that modeled binary neutron stars moving in a close circular orbit about each other.

“While binary neutron stars had been observed at that time, all were in widely separated orbits,” Shapiro recalls. “But these orbits were known to be shrinking at the exact rate predicted by general relativity, due to their emission of gravitational waves.”

“Of course, gravitational waves from widely separated stars are too weak to be detected directly on Earth,” he adds. “Only after these orbits would decay, after millions or billions of years, and the stars collide and merge, could the waves be detected by LIGO/Virgo.”

In 1999, using the computer tools they’d devised, Shapiro and Baumgarte, along with another postdoc in the group, Masaru Shibata, constructed models of a single rotating neutron star with mass greatly exceeding the maximum mass of a non-spinning neutron star. They proposed that such a star was a possible short-lived remnant of a binary neutron star merger and coined the name “hypermassive neutron star” for this transient object. Their model suggested that it would be stable over many rotation periods, but predicted it would eventually collapse to form a spinning black hole.

The team predicted this delayed collapse would be attributable to gravitational radiation loss, turbulent viscosity, and/or internal magnetic fields, which would radiate away or transport outwards angular momentum (spin), the hypermassive neutron star’s principal source of support against gravitational collapse.

But solving Einstein's equations of general relativity to simulate the full evolution of a binary neutron star, beginning with a binary in circular orbit and through its late inspiral, merger, and eventual collapse, was proving to be quite a challenge for numerical relativists. The same was true for the inspiral and merger of binary black holes.

So in 2000, Baumgarte and Shapiro, building on earlier work by Shibata and his graduate advisor Takashi Nakamura, both of Kyoto University, designed a new, robust numerical algorithm that enabled simulations of the inspiral and merger of these compact binary stars to be performed in a stable fashion according to general relativity. The scheme they designed is now referred to as the Baumgarte-Shapiro-Shibata-Nakamura (BSSN) numerical relativity scheme and is the most widely implemented approach to solving Einstein's equations on a supercomputer.

Shapiro and his group spent part of the next several years studying exactly how a hypermassive neutron star eventually collapses to a spinning black hole, finding in 2006 that the black hole typically becomes embedded in a low-mass disk of magnetized gas of neutron-star tidal debris that accretes slowly onto the black hole.

“This accretion fuels a central engine for generating outward electromagnetic power,” shares Shapiro. “But a central question remained: could the merger of a binary neutron star, or a binary black hole-neutron star, actually be the engine that generates the powerful short gamma-ray bursts that have been observed by satellites since the 1970s?”

“Such an identification had been hypothesized by theorists, but a firm indication that this might be possible in general relativity was still lacking,” he recounts.

Shapiro and his group spent the next several years simulating the merger of binary neutron stars and binary black hole-neutron stars with magnetized neutron stars and spinning black holes, becoming one of the very few groups worldwide capable of simulating such an event according to the laws of general relativistic magnetohydrodynamics.

The key breakthrough came in 2015 when Shapiro and his postdocs Vasilis Paschalidis and Milton Ruiz were able to evolve a magnetized binary black hole-neutron star from its late inspiral, through tidal disruption of the neutron star and merger, to the formation of a magnetized gaseous disk orbiting the remnant black hole.

They found that in addition to the burst of gravitational waves emitted during this event, a jet of plasma was eventually launched above the poles of the black hole, and this jet also shot a collimated beam of electromagnetic radiation outward into space.

“Such a jet, confined by the walls of a tightly wound, twisted, helical magnetic field—picture field lines twisted as in a barber-pole—above the black hole poles, was long believed to be a crucial ingredient in generating a short gamma-ray burst. The jet produced in our simulations had the expected duration and typical luminosity of observed short gamma-ray bursts. This finding was a real first and generated a lot of excitement in the field,” Shapiro comments.

Next the group simulated the merger of magnetized binary neutron stars and, in 2016, reported that those systems that formed a transient hypermassive neutron star following merger and then underwent delayed collapse to a black hole also produced a jet.

“We determined that this model represented a good candidate for the short gamma-ray burst central engine and a counterpart to the gravitational wave burst,” Shapiro states. “This too was all very exciting.”

Most recently, in 2017, the team performed simulations for more massive neutron star binaries that had a total mass above a critical threshold that forced them to undergo prompt collapse following merger.

“These systems do not have time to form a hypermassive neutron star and, as a result, do not launch a jet. Thus, they are apparently not good short gamma-ray burst candidates,” Shapiro continues. “Determining whether or not a binary neutron star can become a short gamma-ray burst may thus depend on its total mass, as well as the nuclear equation of state that provides pressure support in a neutron star and determines the critical threshold mass below which delayed collapse can occur.”

According to Shapiro, details of the discovery announced today will dictate many further questions and future simulations. Shapiro and his U of I group hope to continue to play a significant role in this very exciting and ongoing scientific investigation.


Stuart Shapiro's research has been funded by NASA and by the National Science Foundation.


Recent News

  • Research
  • Condensed Matter Physics
  • Condensed Matter Experiment
  • Condensed Matter Theory

One of the greatest mysteries in condensed matter physics is the exact relationship between charge order and superconductivity in cuprate superconductors. In superconductors, electrons move freely through the material—there is zero resistance when it’s cooled below its critical temperature. However, the cuprates simultaneously exhibit superconductivity and charge order in patterns of alternating stripes. This is paradoxical in that charge order describes areas of confined electrons. How can superconductivity and charge order coexist?  

Now researchers at the University of Illinois at Urbana-Champaign, collaborating with scientists at the SLAC National Accelerator Laboratory, have shed new light on how these disparate states can exist adjacent to one another. Illinois Physics post-doctoral researcher Matteo Mitrano, Professor Peter Abbamonte, and their team applied a new x-ray scattering technique, time-resolved resonant soft x-ray scattering, taking advantage of the state-of-the-art equipment at SLAC. This method enabled the scientists to probe the striped charge order phase with an unprecedented energy resolution. This is the first time this has been done at an energy scale relevant to superconductivity.

  • Alumni News
  • In the Media

Will Hubin was one of those kids whose wallpaper and bed sheets were covered in airplanes and who loved building model airplanes. By the time he took his first flight in the late 1940s, he was hooked.

Now, he shares his passion for planes with children by taking them for their first flight, at no charge, in his four-seat 2008 Diamond DA-40 aircraft through the local Experimental Aircraft Association’s Young Eagles program.

“It’s a lot of fun and pretty rewarding. Anyone who loves flying likes to introduce others to it. It’s true of anything, any hobbyist. Some will talk constantly but they’re ecstatic,” said Hubin, a retired Kent State University physics professor.

Hubin learned to fly in 1962 when he was earning a doctorate in physics at the University of Illinois and has been flying ever since, adding commercial, instrument, instructor, multi-engine and seaplane ratings.

  • Research
  • Theoretical Biological Physics
  • Biological Physics
  • Biophysics

While watching the production of porous membranes used for DNA sorting and sequencing, University of Illinois researchers wondered how tiny steplike defects formed during fabrication could be used to improve molecule transport. They found that the defects – formed by overlapping layers of membrane – make a big difference in how molecules move along a membrane surface. Instead of trying to fix these flaws, the team set out to use them to help direct molecules into the membrane pores.

Their findings are published in the journal Nature Nanotechnology.

Nanopore membranes have generated interest in biomedical research because they help researchers investigate individual molecules – atom by atom – by pulling them through pores for physical and chemical characterization. This technology could ultimately lead to devices that can quickly sequence DNA, RNA or proteins for personalized medicine.

  • In Memoriam

We are saddened to report that John Robert Schrieffer, Nobel laureate and alumnus of the Department of Physics at the University of Illinois at Urbana-Champaign, passed away on July 27, 2019, in Tallahassee, Florida. He was 88 years old.

Schrieffer was the “S” in the famous BCS theory of superconductivity, one of the towering achievements of 20th century theoretical physics, which he co-developed with his Ph.D advisor Professor John Bardeen and postdoctoral colleague Dr. Leon N. Cooper. At the time that Schrieffer began working with Bardeen and Cooper, superconductivity was regarded as one of the major challenges in physics. Since the discovery of the hallmark feature of superconductivity in 1911—the zero resistance apparently experienced by a current in a metal at temperatures near absolute zero—a long list of famous theoretical physicists had attempted to understand the phenomenon, including Albert Einstein, Niels Bohr, Richard Feynman, Lev Landau, Felix Bloch, Werner Heisenberg and John Bardeen himself (who was awarded the Nobel Prize for his co-invention of the transistor at around the time that Schrieffer began working with him in 1956).