Research

Neutron stars are among the most exotic objects in the Universe. Born from the collapsed core of a massive star, they pack more mass than our Sun into a sphere barely 20 kilometres across. This makes them remarkably dense: a single teaspoon of neutron-star matter would weigh more than a billion tonnes.

But their strangeness doesn’t stop there. Neutron stars are extreme in every sense: they can spin hundreds of times per second, host magnetic fields trillions of times stronger than Earth’s and warp spacetime so strongly that Einstein’s theory of gravity—general relativity—is needed to describe them. Deep inside, matter may exist in exotic forms that we can’t recreate in any laboratory on Earth.

For decades, much of this physics has been somewhat out of reach. But now, with the discovery of gravitational waves—ripples in the fabric of spacetime—we have a new way to observe neutron stars. When a compact binary (two bodies that are either neutron stars or black holes) with at least one neutron star inspirals and merges, it emits gravitational waves that carry rich information about the star’s internal structure and composition. This was seen for the first time in 2017 (see the BBC’s coverage).

These signals give us a unique opportunity: to explore the behaviour of matter under the most extreme conditions in Nature, and to test fundamental physics in regimes that were once out of reach.

💥 Binary neutron-star mergers

When two compact objects orbit one another in a binary system, they gradually lose energy by emitting gravitational waves. Over time, these waves cause the stars to spiral inward until they eventually collide in a violent cosmic merger. These events are not only dramatic—releasing enormous amounts of energy—but also incredibly informative.

For binaries with neutron stars, the gravitational-wave signal encodes information about the internal stellar structure, such as how easily the star deforms under tidal forces. This makes binary neutron-star mergers a unique probe of ultra-dense nuclear matter, helping us understand what lies at the core of a neutron star and whether exotic forms of matter like de-confined quarks may exist there.

In my work, I develop theoretical models and computational tools to interpret these signals. This includes:

• Relativistic models of tidal interactions, which describe how the stars distort each other during the inspiral.
• Predictions of gravitational-wave signatures that could reveal exotic physics.
• Contributions to data-analysis pipelines that extract physical information from real gravitational-wave observations.

Although gravitational waves from compact binaries are the only type detected so far, we know that neutron stars can radiate gravitational waves in other ways.

🗻 Neutron-star mountains

Another mechanism through which a neutron star can emit gravitational radiation is by rotating while supporting a slight deviation from perfect axial symmetry. While their strong gravity tends to enforce symmetry, neutron stars can sustain tiny deformations—features known as “mountains”. These are supported by the star’s elastic crust and are unlike those we find on Earth: even the largest are just fractions of a millimetre tall, yet they can have significant astrophysical effects.

If a spinning neutron star carries such a deformation, it produces a steady, periodic signal known as a continuous gravitational wave. These waves are far weaker than those from binary mergers, making them much harder to detect.

In my research, I have

• Developed the first fully self-consistent relativistic models of neutron-star mountains, accounting for both the star’s elastic crust and its curved spacetime geometry.
• Found evidence for gravitational-wave emission by conducting a population-synthesis study and comparing to observed electromagnetic pulsars.

🌊 Oscillation modes

In addition to binary mergers and continuous radiation from mountains, neutron stars can emit gravitational waves through their internal oscillations. Much like a ringing bell, a neutron star can vibrate in a wide variety of patterns—known as oscillation modes—each governed by the star’s internal structure and physical composition.

These waves are sensitive probes of exotic physics. Different types of modes respond to different properties of the star, including temperature, composition, rotation and even phase transitions in dense matter.

I am particularly interested in how the vibrational modes contribute to the dynamical tidal response in binary systems. As neutron stars orbit one another, tidal forces can excite the oscillations and influence the emitted gravitational-wave signal. This offers a novel way to study neutron-star structure.

In my research, I have

• Investigated a broad range of oscillation modes—including g-modes, r-modes, interface modes and ocean waves—using both relativistic and microphysical modelling.
• Studied how these modes are shaped by rotation, composition, nuclear reactions and first-order phase transitions.
• Examined the reliability of numerical simulations of hot neutron stars formed in mergers, and how oscillation spectra can be affected by artificial heating.