Physicists Uncover Secrets of Neutron Star Interiors

New framework allows researchers to probe the extreme matter inside these cosmic wonders using gravitational waves

Published on Mar. 7, 2026

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Physicists have made a major theoretical breakthrough in understanding how inspiraling binary neutron stars respond to tidal forces, a key step in elucidating neutron stars' mysterious internal composition. The team has proven that the time-dependent tidal responses of such stars can be described in terms of their oscillatory behavior, or modes, extending an analogous result from Newtonian gravity to the relativistic setting. This research paves the way to probing the internal structure of neutron stars and some of nature's most extreme types of matter using gravitational waves.

Why it matters

Neutron stars harbor some of the most extreme environments in the universe, with densities exceeding those of atomic nuclei and gravitational fields rivaling black holes. Understanding the internal composition of these cosmic wonders could shed light on the nature of ultra-dense matter and even the early universe. However, much of neutron stars' interiors, especially their cores, remain a mystery. This new framework allows researchers to 'listen in' to the oscillations of neutron stars using gravitational waves, potentially revealing the presence of exotic states of matter.

The details

The researchers broke down the problem into simpler pieces, focusing on one star and viewing its partner as a tidal source. By dividing the interior and exterior of the star into distinct regions - a strong-gravity zone and a weak-gravity zone - and carefully stitching together the solutions, the team was able to impose the appropriate boundary conditions and eliminate the effects of radiation. This allowed them to find a complete set of harmonic-oscillator modes that describe the star's tidal response, just as in Newtonian theory.

  • The research was published as an Editors' Suggestion in the journal Physical Review Letters on February 18, 2026.
  • The LIGO collaboration's most recent data from 2017 did not have a high enough signal-to-noise ratio to detect the features captured in the new model, but newer generations of gravitational wave detectors expected in the next few years may provide the necessary sensitivity.

The players

Nicolás Yunes

Illinois Physics Professor who led the research team.

Abhishek Hegade

Former Illinois Physics graduate student and current Princeton University postdoctoral scholar who was a lead author on the study.

LIGO

The Laser Interferometer Gravitational-Wave Observatory, a collaboration that operates sensitive detectors to observe gravitational waves.

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What they’re saying

“It's very hard to study the physics of matter at such high densities and, relatively speaking, low temperatures. But the universe provides a natural lab to study this kind of matter through neutron stars.”

— Nicolás Yunes, Illinois Physics Professor

“As they get closer, tidal forces from one star begin to deform the other and vice versa. The amount of deformation depends on what's inside of the stars.”

— Abhishek Hegade, Former Illinois Physics graduate student and current Princeton University postdoctoral scholar

“If we can understand the mode frequencies of oscillation and their decay times, we might be able to determine the composition of neutron stars in a regime not accessible on Earth.”

— Nicolás Yunes, Illinois Physics Professor

What’s next

The researchers plan to extend their framework to include the effects of rotation and non-gravitational fields, such as magnetic fields, to further refine their models of neutron star interiors.

The takeaway

This breakthrough in understanding the tidal responses of binary neutron stars opens up new avenues for probing the extreme states of matter found in the cores of these cosmic wonders using gravitational wave observations. Insights into neutron star interiors could shed light on the nature of ultra-dense matter and even the early universe.