QCD, Gravitational Waves, and Pulsars

Abstract

Investigations of the phase diagram of quantum chromodynamics (QCD) have revealed that exotic new phases, the so called {\it color superconducting phases}, may arise at very high baryon densities. It is speculated that these exotic phases may arise in the cores of neutron stars. Focus on neutrons stars has tremendously intensified in recent years with the direct detection of gravitational waves (GW) by LIGO/Virgo from BNS merger events which has allowed the possibility of directly probing the properties of the interior of a neutron star. A remarkable phenomenon manifested by rapidly rotating neutron stars is in their {\it avatar} as {\it Pulsars}. The accuracy of pulsar timing can reach the level of one part in 1015, comparable to that of atomic clocks. This suggests that even a tiny deformation of the pulsar can leave its imprints on the pulses by inducing tiny perturbations in the entire moment of inertia (MI) tensor affecting the pulse timings, as well as the pulse profile (from wobbling induced by off-diagonal MI components). This may allow a new probe of various phase transitions occurring inside a pulsar core through induced density fluctuations affecting the MI tensor. Such perturbations also naturally induce a rapidly changing quadrupole moment of the star, thereby providing a new source of gravitational wave emission. Another remarkable possibility arises when we consider the effect of an external GW on neutron star. With the possibility of detecting any minute changes in its configuration through pulse observations, the neutron star has the potential of performing as a Weber detector of gravitational wave. This brief review will focus on these specific aspects of a pulsar. Specifically, the focus will be on the type of physics which can be probed by utilizing the effect of changes in the MI tensor of the pulsar on pulse properties.