Heh, I had to go off and read some stuff to get my head around what "negative" potential energy would even mean (high school physics was decades ago).

So... the negative is just a convention to represent work done against the "field", and positive is work done by the field? ie more of a vector than actually being negative energy? I think I get that bit now.

So now I'm wondering if that still applies to an expanding universe vs eg a rocket leaving earth. If things aren't moving further apart by work (ie force x distance) being done against their gravitational fields so much as space time expanding, is there an increase in potential energy? And then if objects are moving apart faster than escape velocity, could that still be seen as increasing potential energy?

I think I'm confusing myself further...