Here is the beginning of a discussion with Freeman Dyson about whether gravity changes over time:
That question was going to be at the core of my PhD dissertation had I completed my studies in planetary physics at MIT.
Unfortunately, I was subject to the draft during the Vietnam War and only completed one semester toward that degree. By the time I left the military years later, it had been shown (to a first order approximation) that gravity in fact does NOT change over time.
I was never much of a mathematician anyway, and my hopes for being a theoretical physicist ended around 1970. That was a path I now sorely regret going down.