In what episode does Homer rise from Earth, becomes the Cosmos and comes back to Earth?

Answer 1

I think you mean "Homer the Heretic". If not though, could you please explain in another way what you mean?

Answer 2

I think you mean Thank God It's Doomsday

Answer 3

It sounds like you're referring to the "Powers of Ten parody" couch gag. It's actually seen in three episodes:

 - "The Ziff Who Came to Dinner"

 - "On a Clear Day I Can't See My Sister"

- "Eternal Moonshine of the Simpson Mind"