Boson Stars as Solitary Waves

TL;DR

This paper proves the existence, stability, and decay properties of traveling solitary wave solutions for a pseudo-relativistic boson star equation, highlighting the challenges due to lack of Lorentz invariance.

Contribution

It introduces a variational approach to construct boosted ground states and establishes their stability and decay, advancing understanding of boson star dynamics.

Findings

Existence of traveling solitary waves with speeds less than light.

Orbital stability of these solitary waves.

Exponential decay of the wave functions in space.

Abstract

We study the nonlinear equation $i \partial_t \psi = (\sqrt{-\Delta + m^2} - m) \psi - ( |x|^{-1} \ast |\psi|^2 ) \psi$ on $\RR^3$, which is known to describe the dynamics of pseudo-relativistic boson stars in the mean-field limit. For positive mass parameters, $m > 0$, we prove existence of travelling solitary waves, $\psi(t,x) = e^{i t \mu} \sol_{v}(x-vt)$, with speed $|v| < 1$, where $c=1$ corresponds to the speed of light in our units. Due to the lack of Lorentz covariance, such travelling solitary waves cannot be obtained by applying a Lorentz boost to a solitary wave at rest (with $v=0$). To overcome this difficulty, we introduce and study an appropriate variational problem that yields the functions $\sol_v \in \Hhalf(\RR^3)$ as minimizers, which we call boosted ground states. Our existence proof makes extensive use of concentration-compactness-type arguments. In addition to their existence, we prove orbital stability of travelling solitary waves $\psi(t,x) = e^{it \mu} \sol_v(x-vt)$ and pointwise exponential decay of $\sol_v(x)$ in $x$.