In the wikipedia page "Future of an expanding universe" it refers to the scenario of a future without proton decay.
The page talks about how processes would lead to stellar-mass cold spheres of iron, calling these objects "iron stars":
In 101500 years, cold fusion occurring via quantum tunnelling should make the light nuclei in ordinary matter fuse into iron-56 nuclei. Fission and alpha-particle emission should make heavy nuclei also decay to iron, leaving stellar-mass objects as cold spheres of iron, called iron stars.
Under the heading "Collapse of iron star to black hole" it then says :
Quantum tunnelling should also turn large objects into black holes. Depending on the assumptions made, the time this takes to happen can be calculated as from 101026 years to 101076 years. Quantum tunnelling may also make iron stars collapse into neutron stars in around 101076 years.
How would quantum tunnelling lead to the collapse of an iron star to a black hole?