Penny corded Sheldon’s fingers between her fingers, and said, “Tell me about the big bang.”

Sheldon knew about worlds. He knew about universes. Neutron stars collided and high-mass suns collapsed in brilliant gamma flashes several times a week, invisible to the naked eye. Penny, if she'd been in his head at that moment, might have thought the analogy was something romantic, like imagining that each of those exploding stars were their relinquished futures disappearing as one moment turned into the next. Sheldon didn't think like Penny, and he knew that whatever he decided now, the rest of those universes would continue on without him; he was forever grounded in this one, and he had no time machine to change the past.