Neutron stars are among the most mysterious objects in the universe.
Formed when massive stars collapse after exploding as supernovae, these incredibly dense objects pack more mass than the Sun into a sphere only about 20 kilometers wide.
Their gravity is so strong that only black holes surpass them.
Yet despite decades of study, scientists still do not fully understand what lies inside these cosmic giants.
Now, physicists from the University of Illinois Urbana-Champaign and several international institutions have made an important theoretical breakthrough that could help answer this question.
Their research shows how scientists might use gravitational waves—ripples in spacetime—to uncover the internal structure of neutron stars.
The study was published in Physical Review Letters and was selected as an Editors’ Suggestion.
Neutron stars are named because they contain huge numbers of neutrons. These particles form when extreme pressure squeezes protons and electrons together.
But scientists believe the story is more complicated. The outer layers of neutron stars may contain heavy elements, free electrons, and protons.
Deeper inside, researchers suspect the matter could enter exotic states, possibly forming superfluids or superconductors. Some theories even suggest that the cores of neutron stars may contain quark matter, a dense soup of the fundamental particles that make up protons and neutrons.
Studying such extreme matter on Earth is nearly impossible. Particle accelerators can briefly create similar conditions by smashing atoms together, but those experiments involve extremely high temperatures, unlike the relatively cooler but denser environment inside neutron stars. Because of this, astronomers rely on observations from space to learn more.
A promising new source of information comes from binary neutron stars—pairs of neutron stars orbiting each other. As they circle closer together, they gradually lose energy by emitting gravitational waves, which travel across the universe at the speed of light. Eventually the stars collide in a violent merger.
Before that final collision, each star pulls on the other through gravity, creating tidal forces similar to the way the Moon pulls on Earth’s oceans. These forces deform the stars slightly and cause them to vibrate. The vibrations create patterns known as oscillation modes, which influence the gravitational waves emitted during the inspiral.
Scientists believe these gravitational waves contain hidden clues about what the stars are made of. If researchers can identify the vibration patterns and how quickly they fade away, they may be able to determine the internal composition of neutron stars.
However, describing how neutron stars respond to tidal forces is extremely complicated. These objects are governed by Einstein’s theory of general relativity, and they move at enormous speeds—sometimes approaching 40 percent of the speed of light. Their intense gravity also warps the surrounding spacetime.
To solve this problem, the research team developed a new mathematical framework. They treated one star as the main object of study while the other acted as the source of tidal forces. Using equations that describe how matter and gravity interact, they divided the space around the star into two regions: a strong-gravity zone near the star and a weak-gravity zone farther away.
By analyzing these regions separately and then combining the results, the scientists were able to show that neutron star vibrations behave like a complete set of oscillating modes, similar to vibrating springs. This idea was already known for simpler systems described by Newton’s laws, but proving it works under Einstein’s theory of relativity had remained a major challenge.
With this new framework, researchers can now model neutron star vibrations more accurately and predict how these oscillations affect gravitational waves.
Although current detectors such as LIGO have already observed neutron star mergers, the signals so far are not strong enough to reveal all the details predicted by the new model. Future gravitational-wave detectors, which will be more sensitive, may be able to detect these subtle features.
If that happens, scientists could finally begin to answer long-standing questions about neutron stars, including whether their cores contain exotic forms of matter.
By “listening” carefully to gravitational waves from distant collisions, researchers may one day uncover what lies deep inside some of the universe’s most extreme objects.
Source: KSR.
