Global view of QCD axion stars

Abstract

Taking a comprehensive view, including a full range of boundary conditions, we reexamine QCD axion star solutions based on the relativistic Klein-Gordon equation (using the Ruffini-Bonazzola approach) and its non-relativistic limit, the Gross-Pitaevskii equation. A single free parameter, conveniently chosen as the central value of the wavefunction of the axion star, or alternatively the chemical potential with range −m < µ < 0 (where m is the axion mass), uniquely determines a spherically-symmetric ground state solution, the axion condensate. We clarify how the interplay of various terms of the Klein-Gordon equation determines the properties of solutions in three separate regions: the structurally stable (corresponding to a local energy minimum) dilute and dense regions, and the intermediate, structurally unstable transition region. From the Klein-Gordon equation, one can derive alternative equations of motion including the Gross-Pitaevskii and Sine-Gordon equations, which have been used previously to describe axion stars in the dense region. In this work, we clarify precisely how and why such methods break down as the binding energy increases, emphasizing the necessity of using the full relativistic Klein-Gordon approach. Finally, we point out that, even after including perturbative axion number violating corrections, solutions to the equations of motion, which assume approximate conservation of axion number, break down completely in the regime with strong binding energy, where the magnitude of the chemical potential approaches the axion mass.

Classical decay rates of oscillons

“…Gravitational Effects: We have ignored gravity in our calculations. Aspects of gravitational effects on the decay rates were considered in (for example) [52,[70][71][72][73][74][75] in the weak field limit, with a number of authors focussing on the cosine potential relevant for axions. For the cases we have considered here, it is quite plausible that inclusion of gravitational effects will allow for even longer-lived configurations.…”

Dipole radiation and beyond from axion stars in electromagnetic fields

“…Less compact configurations can also exist if the amplitude of the field is not so large, and R m −1 -they are referred to as dilute axion stars [52,55].…”

“…Axion stars are large amplitude, spatially localized and oscillating φ field configurations (also known as scalar solitons, oscillons, axitons etc. [42][43][44][45][46][47][48][49][50][51][52][53][54][55]). Such exploration is the main purpose of this paper.…”

The Dynamics of the Neutron Complexes: From Neutron Star to Black Hole

“…These phenomena were observed in numerical calculations of nonlinear Schrödinger equations [13][14][15]. The Gross-Pitaevskii equation over the years and until 2 If in the latter we put the chemical potential equal to zero recently has been used in the study of astrophysical phenomena, including describing the formation of structures of various scales -from stellar to galactic [16][17][18][19][20].…”