What are some sources of gravitational waves that aren’t black holes? How did inflation create gravitational waves? How can we possibly detect them?
How do we measure the sizes of stars? What are the biggest ones today, and how big could stars have gotten in the past? Is there any way for a star to cheat and get even bigger?
What powers Cepheid variable stars? What about Mira variables and pulsating stars? And are there variable stars that don’t actually vary at all?
What would the Sun sound like if we could hear it? Does any noise at all happen in the universe? What’s the biggest sound of all?
Why does the vacuum of spacetime have energy? How much energy does it have? What prevents us from using it to do anything useful?