Honestly, I get a kick out of how a lot of the so-called 'hard' sci-fi tackles this. Stephen Baxter's 'Manifold' series tries to root it in actual cosmological theories, like branes and quantum foam, but man, it can feel like reading a physics textbook with a thin plot veneer. It’s clever, but I often lose the characters in all the diagrams.
Then you've got something like Alastair Reynolds' 'Pushing Ice,' which plays with slower, generational travel between weird pocket universes. The science there feels more like archaeology—piecing together rules from ancient, alien tech—which I find way more engaging than pages of exposition about imaginary math. The tension comes from not knowing the rules, which mirrors how we'd probably actually experience it: stumbling in the dark.
For a left-field pick, I’d throw in 'The Space Between Worlds' by Micaiah Johnson. It frames multiversal travel as a brutal, corporate-controlled resource extraction, hinging on your alternate selves being dead. The mechanics are almost secondary to the societal and personal cost, which somehow makes the concept feel more real and grimy than any pristine theory ever could.