A mystery lurks inside the corpses of dead stars. Neutron stars, formed when certain types of stars die in supernova explosions, are the densest form of matter in the universe; black holes are the only thing densest, but they have so completely escaped the limits of normal physics that they are no longer matter. The atoms in neutron stars were squeezed so tightly by gravity that they broke apart, the protons and electrons within them shattered together to create neutrons, leaving small-city-sized objects that contain masses larger than the sun. About 95 percent of a neutron star’s mass is made up of pure neutrons, but physicists are wondering what happens in the center, where the density peaks. Do neutrons break down further into their constituent quarks and gluons? Do some of the quarks transform from their normal “up” and “down” flavors to become stranger and heavier “weird quarks” not found in ordinary matter? Do the particles form an extreme state of matter called a superfluid that flows without viscosity, never slowing down?
Scientists have taken a step towards understanding the inner workings of these bizarre bodies by studying the light and gravitational waves that result when two neutron stars collide with each other and become a black hole. Gravitational waves are folds in spacetime that are sculpted when large masses move. Scientists only gained the ability to detect gravitational waves in 2015 and have so far only identified a handful of events involving neutron stars (the others have been collisions of black holes). But studying the properties of these waves – their frequency and how they change over time – can tell scientists a lot about the objects that created them. Physicists seek accurate measurements of the masses and radii of neutron stars, which would help reveal their “equation of state”: the relationship between pressure and density within these stars. Knowing the equation of state of neutron stars would in turn indicate what kind of matter is hiding inside them.
In a new study, an international team of researchers combined gravitational wave measurements from two neutron star collisions, as well as the light signals that came along with one of them (the other was dark), with estimates of the masses and rays of neutron stars from observation. fast-rotating neutron stars called pulsars. “The big advantage is that it’s a very coherent picture,” says study member Tim Dietrich of the University of Potsdam in Germany, who is co-author of a paper reporting the findings published today in Science. “We combine all the things we currently know, including gravitational waves and electromagnetic waves, information from individual neutron stars and theoretical calculations from nuclear physics.” The equation of state they derived predicted that a neutron star containing the mass of 1.4 suns would have a radius of about 11.75 kilometers, roughly .81 to .86 kilometers. That’s a little over half the length of Manhattan. “The size of the neutron star directly depends on the behavior of matter within the nucleus, so this gives us a better understanding of the properties of the neutron star’s material,” says Dietrich.
For example, if neutrons remain intact in the core of these stars, they would push themselves against the outer layers, potentially leading to a slightly larger radius. If, on the other hand, the neutrons break down into a quark soup, the nucleus would be softer and the whole star would collapse a little, resulting in a smaller radius.
The new measurement broadly agrees with previous studies that have looked at gravitational wave data and other ways to measure the size of neutron stars. “This paper is a nice joint reanalysis of previous studies and does not change the general impression that has been in place in recent years that a neutron star’s radius is around 11-13 km,” says Mark Alford, a physicist. at Washington University in St. Louis. Anna Watts, an astrophysicist at the University of Amsterdam, says that this kind of combined analysis “is clearly the way forward”, but that none of the measurements “are still good enough to truly define the nature of dense matter.” The field will have to wait for future data to truly understand what is happening inside the neutron stars.
“I think it’s a very nice analysis,” says physicist James Lattimer of Stony Brook University, who was not involved in the research. However, he warns that in modeling how well the different possible equations of state fit the data, the team may have mistakenly eliminated too many equations that produce neutron stars with large radii. “I think they underestimated their uncertainty. But in a way, it’s a matter of opinion and how much trust you place in the different statistical methods. “
In addition to revealing the secrets of neutron stars, the study also produced a measurement of the Hubble constant, which reflects the expansion rate of the universe. To derive the constant, the scientists used the amplitude of gravitational waves from one of the collisions to estimate the distance at which the crash occurred. They then compared the distance measurement with the known velocity of the host galaxy of the collision, which was measured by observing the galaxy’s redshift as its light drifted towards the red end of the spectrum. The Hubble constant they found, 66.2 kilometers per second per megaparsec, isn’t accurate enough to decide among competing measurements that already exist, but it adds another data point to the hotly contested question of how fast the cosmos is growing.
Scientists hope to apply the same type of analysis to future collisions of neutron stars that appear. “We have taken this first step and now we will move forward,” says Sarah Antier, a team member from the University of Paris, an astronomer who looks for light signals that accompany gravitational wave events. “My job is to connect several observers to provide a network for immediate observations” when gravitational wave detectors find a new signal.
Physicists are biding their time until the next generation of gravitational wave detectors, such as the Cosmic Explorer in the US and the Einstein Telescope in Europe, go online in the 2030s. These machines are expected to be much more sensitive, allowing them to acquire many more signals from more events and offer more accurate data. Future projects such as the Enhanced X-ray Timing and Polarimetry Mission (eXTP) and Athena’s X-ray Observatory are also expected to collect more accurate pulsar measurements.
Scientists have learned so much in the short term since gravitational wave data became available, the future promises to greatly expand our knowledge of extreme matter under intense pressure. “The past four years have been extraordinary,” says Lattimer. “It shows the potential we will have in the future. We should have a lot more measurements from gravitational wave events, and as we add each new event the results will converge. “