What triggers pulsar glitches?
In plain words
Pulsars, spinning neutron stars, sometimes suddenly spin faster in a glitch. The superfluid inside is thought to dump its spin into the crust, but what starts the event, a cascade of released vortices (tiny whirlpools) or a crack in the crust, is unknown.
Precise statement
Glitches have fractional spin-up $\Delta \nu/\nu \sim 1e-11 \text{ to } 1e-5$, with power-law size distributions in most pulsars and quasi-periodic large events ($\Delta \nu/\nu \sim 1e-6$, about every 3 yr) in Vela. Determine the trigger: collective unpinning avalanches of superfluid vortices, crust quakes, or hydrodynamic instabilities of the superfluid-crust counterflow, and derive the observed size and waiting-time statistics and the sub-minute spin-up timescale.
What would settle it
A model of vortex-crust dynamics that reproduces glitch size and waiting-time distributions and the resolved spin-up of a Vela glitch.
Status in the literature
A single-pulse recording of the 2016 Vela glitch showed pulse-shape changes during the event (Palfreyman et al., Nature, 2018), constraining but not deciding the trigger.