meaning that the galactic yawn is the timeline of the universe, ie, planets and stars and pretty much ALL heavenly bodies keep getting further and further away from each other, kinda like how during a yawn, the bottom and upper lip widen. It would eventually come to the point that planets would drift so far away from their stars that they'd never stay in orbit anymore. I'm not sure if "galactic yawn" is the right word for it, but pretty much it means that the Galaxy is ALWAYS expanding, and never stopping