Sean Carroll, while speaking about the history of quantum field theory, has said a couple of times (without being completely sure of its factuality) that Richard Feynman was motivated by his trying to solve the cosmological constant problem.
That's the vacuum energy. They're the same thing. Okay, just different languages used to describe the same idea, the difference is that long after Einstein, by the '50s and '60s, people realize there could be different contributions to the cosmological constant. So in fact, Feynman I think I understand this correctly, but it's not a common fact, so maybe I'm hallucinating a little bit. One of Feynman's motivations for inventing Feynman diagrams was to get rid of quantum field theory. Back in the '50s, you could be ambitious like that, right now, quantum field theory works too well, it's a harder ambition to have, but in the 1950s, you could have said, Well, maybe I can get all the benefit of quantum field theory without all the downsides, like the infinites and one of the things that he knew was a problem Feynman was that if you have a field that fills all of empty space, even if there's no particles, even if it's in the lowest energy state in the vacuum state, it can still have energy.
From a previous episode, Tim Maudlin on Locality, Hidden Variables, and Quantum Foundations
I don't know if you know this, 'cause I only recently learned it, but Richard Feynman, when he was inventing Feynman diagrams, part of his motivation was to get rid of quantum field theory and replace it with another theory, particles again, and we now think of Feynman diagrams is a tool for understanding quantum field theory, so he changed his mind, but his motivation, I'm told, was the cosmological constant problem.
I could not find any confirmation about this, even if there are plenty of quotes of Feynman on the cosmological constant problem. Was something that Feynman said himself?