Gravitational waves could reveal exotic matter inside neutron stars
Under extreme pressures, even neutrons might fall apart
What happens inside heavy stars like neutron stars remains a mystery in astrophysics. Fabian Gittins of Utrecht ľϸӰ argues that gravitational waves may help uncover what’s going on in the cores of these ultra dense objects. Together with scientists at the ľϸӰ of Southampton and Los Alamos National Laboratory, he concludes that it’s possible that a new form of matter exists deep inside neutron stars and that we might detect it by listening to gravitational waves. Their findings were .
If you could scoop a teaspoon of matter from a neutron star, it would weigh about ten billion tons. These stars are among the densest known objects in the universe. Deep inside of them, physicists suspect, lies an exotic state of matter that is not naturally found anywhere else. Deconfined quark matter is thought to arise only under the most extreme physical conditions.
When two neutron stars orbit each other and eventually collide, they send out ripples in space-time: gravitational waves. Scientists have been able to measure these ripples since 2015, using large, terrestrial observatories like LIGO and Virgo.
Particular vibration
In this new study, Gittins and his team show that neutron stars begin to vibrate internally just before they merge, during their final orbits. One particular vibration, the interface mode, arises at the boundary between two distinct forms of matter: the familiar nuclear matter made of protons and neutrons, and a possible new phase deep inside the star, deconfined quark matter. In this state, even neutrons and protons dissolve into their fundamental building blocks, quarks, which move freely within the star’s core.
The interface mode subtly affects the gravitational wave signal. “We still know very little about what actually goes on inside neutron stars,” says Gittins. "But gravitational waves could one day help us tell the difference between ‘normal’ nuclear matter and something that we’ve never directly observed before.”
Stellar models
Rather than reporting an observation, the article presents a theoretical development. The researchers built computer models that mimic what happens deep inside neutron stars. In these stellar models they added possible regions of quark matter, changing their size and properties to see how the star would react.
Gravitational waves could one day help us tell the difference between nuclear matter and something that we’ve never directly observed before
They then calculated how these hidden layers would make the star vibrate and how that vibration would ripple out into gravitational waves. Finally, they compared the predicted signals with the sensitivity of current and future detectors and found that the effect should be large enough to spot..
Estimating and inferring
Until quite recently, the interior of neutron stars could only be studied indirectly. For example, by observing radio pulses emitted by spinning neutron stars, or by measuring how fast two stars orbit each other. “From that, you can estimate how heavy and large they are,” Gittins explains, “and from those numbers, you can infer the density. But that doesn’t tell us much about what the matter is actually made of.”
Gravitational waves, on the other hand, provide direct information about how deformable the stars are. “That is something that depends on how ‘stiff’ or ‘soft’ their material is. That leaves an imprint on the gravitational wave signal. With the right data, we can use that to learn more about the stars’ inner structure.”
Einstein Telescope
It’s still early days. So far, scientists have only observed and analysed two neutron star collisions. But future observatories, such as the Einstein Telescope, expected to cost around €2 billion and potentially built in the Dutch province of Limburg, will be far more sensitive. That would make it possible to detect even subtle signatures from the internal stellar vibrations.
Publication
, , F., , N. and , Physical Review Letters volume 135, issue 8.