Who or what started the multiverse concept and why?

According to a few Google searches it was a physisict Hugh Everett in the late 50s. I am sure there were others in more meta and philosophical concepts in the past but I get the idea he is the known physicist to come up with it.

Disclaimer I didn't read a lot but his name came up a few times and I assume he had a theory that could relate to physics at the time.

I still like that Flash was probably the first for pop culture, when weird time travel started happening in other shows (after deep into Flash recent series) I started joking it was just Barry messing up crap between the multiverse and time.