[Paper Review] Universal behavior of rotating neutron stars in GR: even simpler than their Newtonian counterparts

George Pappas, Theocharis A. Apostolatos|arXiv (Cornell University)|Nov 21, 2013

Pulsars and Gravitational Waves Research1 citations

TL;DR

This paper demonstrates a universal relation among the first four multipole moments (mass, current, quadrupole, and current-quadrupole) of rotating neutron stars in General Relativity, independent of the nuclear equation of state. This simplifies the description of neutron star spacetime geometry and enhances their potential as probes for testing gravity theories beyond general relativity.

ABSTRACT

Recently it was shown that slowly rotating neutron stars exhibit an interesting correlation between their moment of inertia $I$, their quadrupole moment $Q$, and their tidal deformation Love number $\lambda$ (the I-Love-Q relations), independently of the equation of state of the compact object. In the present work a similar, more general, universality is shown to hold true for all rotating neutron stars within General Relativity; the first four multipole moments of the neutron star are related in a way independent of the nuclear matter equation of state we assume. By exploiting this relation, we can describe quite accurately the geometry around a neutron star with fewer parameters, even if we don't know precisely the equation of state. Furthermore, this universal behavior displayed by neutron stars, could promote them to a more promising class of candidates (next to black holes) for testing theories of gravity.

Motivation & Objective

• To investigate whether multipole moments of rotating neutron stars in General Relativity exhibit universal behavior independent of the equation of state.
• To determine if the first four multipole moments (mass, current, quadrupole, and current-quadrupole) are related in a way that simplifies spacetime geometry description.
• To explore the implications of this universality for testing alternative theories of gravity.

Proposed method

• Numerical relativity simulations of slowly and rapidly rotating neutron stars in General Relativity.
• Computation of the first four multipole moments: mass (M), current (J), quadrupole (Q), and current-quadrupole (S) moments.
• Analysis of the relations between these moments across a wide range of equations of state and rotation rates.
• Use of the I-Love-Q universality framework as a foundation, extending it to full rotation.
• Validation of the universality by testing the relations across diverse stellar models.

Experimental results

Research questions

• RQ1 Do the first four multipole moments of rotating neutron stars in General Relativity form a universal relation independent of the equation of state?
• RQ2 Can the geometry of rotating neutron stars be described with fewer parameters due to this universal behavior?
• RQ3 How does this universal relation compare to the I-Love-Q relation in the slowly rotating limit?
• RQ4 To what extent does this universality hold across different rotation rates and equations of state?
• RQ5 Can this relation enhance the use of neutron stars as probes for testing gravity theories?

Key findings

• The first four multipole moments of rotating neutron stars in General Relativity are universally related, independent of the nuclear equation of state.
• This universal relation simplifies the description of neutron star spacetime geometry, requiring fewer parameters even without knowing the exact equation of state.
• The relation extends the I-Love-Q universality to rapidly rotating configurations, confirming its robustness beyond the slow-rotation approximation.
• The observed universality suggests that neutron stars can be used as effective probes for testing gravity theories, similar to black holes.
• The findings support the potential of neutron stars as powerful tools for testing general relativity and alternative gravity models.