When you buy through links on our articles, Future and its syndication partners may earn a commission.An illustration of two neutron stars colliding and merging. | Credit: Robert Lea (created with Canva)We could soon be able to “see” inside a neutron star and learn what extreme matter governed by exotic physics lurks there, thanks to the imprint of tidal interactions on gravitational waves emitted by pairs of neutron stars spiraling toward an explosive merger.”One hope is that we’ll be able to get some information about the neutron-star equation of state at densities found in the inner core of a neutron star,” said Nicolás Yunes of the University of Illinois, who led the research, in a statement. “Is there really a quark core, as some have recently claimed? Are there phase transitions occurring inside that we don’t know about yet?”AdvertisementAdvertisementA neutron star is the compact remnant of a massive star that has gone supernova. With a diameter about the same as a large city, yet packing a mass several times that of our sun, neutron stars are incredibly dense. The pressure in their interior is so great that atoms are crushed and split apart into their constituent particles. The positively charged protons and negatively charged electrons are smushed together, forming a soup of neutral neutrons, which is why we call these objects neutron stars.However, deeper down inside a neutron star, close to its core, things could be even weirder. The gravitational pressure could be so extreme as to crush neutrons into their building blocks, which are fundamental particles called quarks and the gluons that ordinarily bind quarks together to form protons and neutrons.Scientists call this state of matter a quark-gluon plasma. This state of matter existed during the first fraction of a second after the Big Bang, and outside of particle accelerator experiments, the only other location in the universe where quark-gluon plasma may exist is inside neutron stars.If scientists could understand the interior of neutron stars, they could therefore learn more about the state of matter immediately after the Big Bang.AdvertisementAdvertisementBinary neutron stars have long been considered the best bet for deciphering what lurks within. These pairs of neutron stars spiral around one another in elliptical orbits, inching ever closer until they collide and merge in a kilonova. Crucially, their in-spiral sees the release of gravitational waves.Now, scientists led by Yunes and Abhishek Hegade of Princeton University think they’ve figured out how to decipher the frequency of these gravitational waves to interpret the interior structure of neutron stars.”As they get closer, tidal forces from one [neutron] star begin to deform the other and vice versa,” said Hegade. “The amount of deformation depends on what’s inside of those stars.”The problem is that the extreme gravity and high velocity (up to 40% the speed of light) of the neutron stars as they spin about one another means that scientists have to look toward Albert Einstein’s general theory of relativity for solutions. This is a complex endeavor, but Yunes and Hegade think they now have the answer.AdvertisementAdvertisementAs the binary neutron stars deform the shape and structure of each other through their gravitational tides, they trigger oscillations within their interior, like the ringing of a bell. The patterns of these oscillations are called modes, and the frequency of these modes is imprinted on the gravitational waves that the binary neutron stars radiate away.Neutron stars pack the mass of several suns into a city-sized sphere. | Credit: ESAA full set of modes is required to understand the binary system. Discerning these modes, however, is complicated by the fact that the tidal forces are dynamical: they change as the neutron stars orbit one another, and the effects of each neutron star overlap, making distinguishing what’s going on even more difficult.More in Science”Without a complete …
