The recent discovery of primordial gravitational waves has brought to mind the possibility that gravity operated in a very different way in the early universe, and with this possibility comes a logical question: if gravity itself expanded with the universe, how long has gravity—as we know it—existed in a stable and more-or-less predictable form?
At least 9 billion years, according to the pioneering work of Australian cosmologists Jeremy Mould and Syed A. Uddin. By looking at records of 580 supernovas (recorded at various distances, and subsequently—given the amount of time it takes light to arrive on Earth—at various points in ancient history), Mould and Uddin determined that gravity hasn't changed much, if at all, between the oldest observable supernovas and the most recent. They put the upper limit of any absolute change to gravitational force at one tenth billionth per year (about 0.00000001%), but that reflects the limitations of our instruments more than it reflects any indication that gravity has in fact changed. As far as they can tell, it hasn't. At all.
This isn't to say that gravity is especially well understood, as SciShow explains:
I'm reminded of the words of the Athanasian Creed, which in its explanation of the Trinity declares rather impenetrably that God is "one incomprehensible, not three incomprehensibles." In a similar spirit, Mould and Uddin have shown us that while we still don't know exactly what gravity is, its mysterious effect doesn't seem to have changed very much over the universe's recent lifetime. As we try to understand what gravity is and how it operates on a cosmic level, this is important information to have.