Imagine a layer of pennies laid flat across this table. The pennies can't be squeezed any closer together, like neutron degeneracy. But gravity is the equivalent of warping the table. Suddenly, the pennies can be closer together. See figure 14.12 on page 425 .
Pennies can flow through the hole and out of our universe.
Recall we covered escape velocity in section 2.9. The escape velocity is the speed necessary to propel an object into deep space. Heavy, more compact objects have higher escape velocities. Einstein taught us that nothing can go faster than the speed of light. A neutron star is already close to this situation.