TWO FLUCTUATIONS. “On one hand, there are obviously vacuum fluctuations of matter fields, and on the other hand, there should also be quantum vacuum fluctuations of spacetime itself if we accept that the basic quantum principles we are already familiar with apply as well to a quantum theory of gravity.”
https://link.springer.com/content/pdf/10.1140/epjc/s10052-018-6433-5.pdf