Speaker
Description
Axion-like particles (ALPs) are among the leading dark-matter candidates. In the minimal cosine potential, cosmology imposes a robust requirement: by the epoch when ALPs behave as matter, the potential height needs to exceed the ALP energy density. This translates into a lower bound on the decay constant. A plausible loophole is to store most of the energy in nonzero-momentum modes while keeping the momenta non-relativistic. Using lattice simulations, we show that this does not evade the bound. When the condition is violated, the system behaves effectively as relativistic radiation rather than matter. For sufficiently small typical momentum, it also undergoes a nonlinear transition accompanied by the formation of “Baumkuchen-like” domain walls.