From a particle physics perspective, lambda wasn’t just a number. It was a property of space. And space, in particle physics, wasn’t empty. It was a quantum circus, a phantasmagoria of virtual particles popping into and out of existence. Not only did those particles exist, as experiments had shown, but they possessed energy. And energy, in general relativity, interacts with gravity. The result of quantum particles possessing energy that interacts with gravity was what physicists called the Casimir effect, after the Dutch physicist Hendrik Casimir.
