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Neutron stars from string theory

Published: 25 February 2022
String theory and neutron stars
String-theoretic methods can be used to understand ultra-dense matter in neutron stars

Researchers have combined two of the core strengths of STAG - string theory and astrophysics - to understand observable properties of neutron stars from an underlying string-theoretic model.

Aaron Poole and Andreas Schmitt from Mathematical Sciences together with former STAG member Nicolas Kovensky, now a postdoctoral researcher at the Université Paris Saclay, have used the so-called holographic principle to describe ultra-dense nuclear matter inside neutron stars. Their far-reaching results have just been published as an article in Physical Review D and were highlighted as an Editor's Suggestion.

Matter in the core of neutron stars is expected to be several times denser than in large nuclei. At these extreme densities, the details of the fundamental interactions become highly relevant for macroscopic properties of the star such as its mass and radius. Although the underlying fundamental theory is known - quantum chromodynamics - our understanding of such ultra-dense matter is very limited because the calculations within quantum chromodynamics become notoriously difficult.
The reason is that the coupling between the particles (quarks and gluons on the most fundamental level) is strong and therefore simple approximations fail. It has been known for a while that strongly coupled theories can be approached by the holographic principle: perform your calculations in a suitable string theory in higher dimensions and translate the results via certain rules to a strongly coupled field theory in lower dimensions, i.e. in the real world. While this theoretical approach had been applied to neutron star matter before, Kovensky, Poole and Schmitt managed for the first time to construct the entire neutron star from a single holographic model. This includes the crust of the star, consisting of a crystalline structure, not unlike solids on earth. The key breakthrough for this work had been made earlier by Kovensky and Schmitt in showing how the holographic model has to be improved to implement neutron star conditions. Despite the relative simplicity of the model, the calculations remarkably show that "holographic neutron stars" reproduce all the basic properties of neutron stars recently measured by astrophysicists. This includes properties extracted from the detection of gravitational waves from the merger of two neutron stars. A systematic comparison with astrophysical data and resulting predictions from the string-theoretic calculations have already been put forward as a follow-up study.

The work opens up the door to several future research directions at the interface of string theory and astrophysics. The researchers are planning to further improve and refine their approach to increase our understanding - in an interplay with more traditional methods - of ultra-dense matter and their fascinating realisation in neutron stars.



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