suppose the universe started shrinking. because the density vs mass is a 1/M^2 factor, density can be made arbitrarily small given high enough mass. so it's not hard for all the mass in the universe to quickly reach black hole density. when critical density of the universal is reached, what would it look like? i just still don't understand what the flipped space and time signs in the schwarzchild metric mean, what would it look like to us inside the universe-black hole? does everything suddenly change?