If a field is rolling slowly and hasn't got n to the minimum yet, then the value of the second derivative where it is acts as the mass. But for a typical field that is just generically rolling, the magnitude of theSecond derivative has a value that is comparable,. same order of magnitude. So therefore, talking about the mass is sort of a proxy, sort of a quick and dirty way of getting at how strongly the potential is changing as the field value changes. That's all. The short answer is, we're just being sloppy. Sorry about that. We have two more questions left. James nancaro says if there are many bubble universes as an eternal inflation, and