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Merging neutron stars and the gravity waves they produce CREDIT NASA/Goddard Space Flight Center
Imagine taking a star twice as massive as the Sun and crushing it to the size of Manhattan. The result would be a neutron star, one of the densest objects found in the Universe, exceeding the density of any material found naturally on Earth by a factor of tens of trillions.
Neutron stars are extraordinary astrophysical objects in their own right, but their extreme densities could also allow them to function as laboratories to study fundamental questions of nuclear physics, in conditions that could never be replicated on Earth.
Because of these exotic conditions, scientists still don’t understand what exactly neutron stars themselves, their so-called “equation of state” (EoS), are made of. Determining this is a major goal of modern astrophysical research. A new piece of the puzzle, limiting the range of possibilities, has been discovered by two IAS researchers: Carolyn Raithel, John N. Bahcall Fellow in the School of Natural Sciences; and Elias Most, School Fellow and John A. Wheeler Scholar at Princeton University. Their work was recently published in The Astrophysical Journal Letters.
Ideally, scientists would like to peek inside these exotic objects, but they are too small and distant to image with standard telescopes. Instead, scientists rely on indirect properties they can measure — like the mass and radius of a neutron star — to calculate EoS, much like one might use the length of two sides. of a right triangle to find its hypotenuse. However, the radius of a neutron star is very difficult to measure accurately. A promising alternative for future observations is to use a quantity called the “maximum spectral frequency” (or f2) instead.
But how is f2 measured? Collisions between neutron stars, which are governed by the laws of Einstein’s theory of relativity, lead to strong bursts of gravitational wave emission. In 2017, scientists directly measured these emissions for the first time. “At least in principle, the maximum spectral frequency can be calculated from the gravitational wave signal emitted by the oscillating remnant of two merged neutron stars,” says Most.
It was previously expected that f2 would be a reasonable approximation of the radius, as until now researchers believed there was a direct or “near-universal” correspondence between them. However, Raithel and Most demonstrated that this is not always true. They showed that determining EoS is not like solving a simple hypotenuse problem. Instead, it’s more like calculating the longer side of an irregular triangle, where you also need a third piece of information: the angle between the two shorter sides. For Raithel and Most, this third piece of information is the “slope of the mass-radius relationship”, which encodes information about EoS at higher densities (and therefore more extreme conditions) than the radius alone.
This new discovery will allow researchers working with the next generation of gravitational wave observatories (the successors to the LIGO currently in operation) to make better use of the data obtained as a result of neutron star mergers. According to Raithel, these data could reveal the fundamental constituents of neutron star matter. “Some theoretical predictions suggest that in the cores of neutron stars, phase transitions could dissolve neutrons into subatomic particles called quarks,” Raithel said. “This would mean that stars contain a sea of free quark matter inside. Our work can help future researchers determine if such phase transitions actually occur.
About the Institute
The Institute for Advanced Study has served the world as one of the leading independent centers for theoretical research and intellectual inquiry since its inception in 1930, pushing the frontiers of knowledge across the sciences and humanities. From the work of IAS founding professors such as Albert Einstein and John von Neumann to that of today’s greatest thinkers, IAS is dedicated to enabling curiosity-driven exploration and fundamental discovery.
Each year, the Institute hosts more than 200 of the world’s most promising postdoctoral researchers and scholars, selected and mentored by a permanent faculty, each of whom are preeminent leaders in their field. Current and past faculty and members have included 35 Nobel Prize winners, 44 of 62 Fields Medalists, and 22 of 25 Abel Prize winners, as well as numerous MacArthur Scholars and Wolf Prize winners.
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