Neutron stars are some of the strangest objects in the universe, the ultra-dense remnants left behind after massive stars explode. Made almost entirely of protons and neutrons packed so tightly that a teaspoon would outweigh mountains, they begin their lives unimaginably hot. But over millions of years, these stellar cores slowly release their heat into the darkness. Their cooling is steady, predictable, and deeply informative.
For a team of researchers from Deutsches Elektronen Synchrotron DESY, the Italian National Institute of Nuclear Physics, the University of Sydney, and the University of Padua, that cooling held a mystery worth chasing. Hidden within those fading temperatures might be the fingerprints of something the universe has long kept secret a new kind of particle and a possible fifth force of nature.
When the Universe Whispers About New Laws
The Standard Model of particle physics describes the four forces that govern ordinary matter. As co author Edoardo Vitagliano explained, “All the phenomena pertaining to the particles making up ordinary matter (i.e., protons, electrons, and neutrons) are determined by four fundamental forces the gravitational, electromagnetic, strong nuclear, and weak nuclear forces.”
Yet theorists have long suspected that this list might be incomplete. Many extensions of modern physics propose the existence of scalar particles particles without spin that could link to a new, previously undetected force. If real, such a particle would violate two of the deepest principles in gravity, the equivalence principle and the inverse square law. That would mean something profound that our understanding of the universe’s architecture is missing an essential piece.
“The existence of an additional fifth force can signal a paradigm shift in physics, and many experiments have been devoted to the quest for such a fifth force,” Vitagliano said. The challenge is that, on Earth, any deviations from normal gravity at the middle ground between the microscopic and macroscopic worlds are exceptionally hard to measure.
But the researchers realized that nature had already built the perfect testing ground thousands of light years away.
Stars That Could Reveal What Our Instruments Cannot
Certain neutron stars, including the group known as the Magnificent Seven and the pulsar PSR J0659, seemed ideal for the hunt. These stars pack nucleons so densely that if scalar particles existed and interacted with protons or neutrons, they would be produced in enormous quantities inside the stellar cores.
“Just like the electromagnetic interaction is due to the exchange of photons, the novel force would be mediated by a new scalar particle,” Vitagliano said. This new particle, according to theoretical predictions, might carry clues about extra dimensions or even play the role of dark matter.
If such particles were real, they would not sit quietly inside the neutron star. They would escape, carrying energy with them. And like a hot meal cooling faster in a drafty room, the neutron star would lose heat far more rapidly than expected.
“Thanks to their extremely large density, the evolution of neutron stars would be dramatically affected by the existence of new forces,” Vitagliano explained. “Neutron stars are like a warm meal that cools down over time, and new particles would cool them down faster.”
The question was simple. Are any observed neutron stars cooler than they should be.
The answer required an enormous amount of computational work.
Following Heat Through Millions of Years
The team created detailed simulations that tracked neutron stars from their birth to their current observable state. They added hypothetical scalar particles to their models, allowing those particles to be produced inside the star and escape into space. Then they compared these altered cooling curves with astronomical data.
The results were striking. If scalar particles carried energy away strongly enough to reveal a fifth force, the neutron stars we observe today would be much colder than what telescopes actually measure.
“Simulations accounting for the emission of scalar particles mediating a novel fifth force lead to neutron stars which nowadays would be way colder than what our telescopes observe,” said co author Alessandro Lella. “Therefore, if a fifth force exists it must be weak enough to avoid modifying the observed neutron star evolution.”
Because the stars matched normal cooling patterns, the team set new limits on how strongly scalar particles, if they exist at all, can interact with nucleons. These limits are the most stringent ever achieved.
“We reached the striking conclusion that, if a new force acts between particles at a distance smaller than a hair’s thickness, astronomical observations are actually the best probe to find it,” Vitagliano said. “We found that any existing fifth force needs to be way feebler than what was thought before.”
What the Cold Hearts of Stars May Still Reveal
The new bounds established by Vitagliano, Lella, and collaborators Damiano Fiorillo and Ciaran O’Hare do not rule out scalar particles entirely. Instead, they push the frontier tighter than ever, refining theories that predict violations of gravity and guiding future investigations.
Lella emphasized that neutron stars still hold many secrets. “The physics governing the interiors of neutron stars is still far from fully understood,” he said. “It is likely that we have yet to grasp the full range of opportunities that studying these compact objects can offer. Upcoming astronomical observations may hold some surprises. If previously unknown phenomena are discovered in neutron star data, they could open new pathways towards the understanding of the most recondite mysteries of our universe.”
Why This Research Matters
This work matters because it pushes on one of the most profound questions in physics whether the four known forces of nature truly tell the whole story. If a fifth force exists, it would reshape our theories of matter, gravity, and the cosmos. Neutron stars, in their extreme density and slow burn toward darkness, offer a natural way to test ideas that are otherwise unreachable.
By showing that these stars cool exactly as expected, the researchers have tightened the boundaries of what new physics can look like. In doing so, they have turned distant stellar remnants into precise cosmic laboratories, capable of probing forces far smaller than any experiment on Earth can detect.
Sometimes the universe hides its secrets not in explosive events or brilliant beams of radiation, but in the quiet fading heat of ancient stars. And by listening closely to that fading warmth, scientists are learning where the limits of reality truly lie and where the next big discovery may wait.
More information: Damiano F. G. Fiorillo et al, Leading Bounds on Micrometer to Picometer Fifth Forces from Neutron Star Cooling, Physical Review Letters (2025). DOI: 10.1103/tlqz-713s
