The QCD axion was originally proposed nearly five decades ago as a solution to the strong CP problem, providing a dynamical mechanism to explain the preservation of CP symmetry in QCD. In this talk, I will examine the possibility of axion condensation within neutron stars, driven by the extreme baryon densities present in their cores.

The analysis employs Chiral Perturbation Theory alongside nuclear interaction models constructed from Chiral Effective Field Theory to evaluate the plausibility of axion condensation under such conditions. Preliminary results suggest that this phenomenon is theoretically well-motivated, though a more refined understanding of nuclear physics at a pion mass near 82 MeV is essential for establishing more definitive conclusions.

Should observational or theoretical evidence support—or rule out—the occurrence of axion condensation in neutron stars, it would offer a powerful probe of the axion parameter space. The resulting insights would have profound implications for both axion phenomenology and the microphysical structure of dense nuclear matter within neutron stars.