the constancy of the gravitational constant, also known as Newton's gravitational constant (G), is assumed to be a fundamental constant of nature and has been considered to remain constant throughout the existence of the universe. However, it is worth noting that the constancy of physical constants is a topic of ongoing scientific investigation and scrutiny.
To date, there has been no experimental evidence or observational data suggesting that the gravitational constant has changed over time. Measurements of G have been carried out using various experimental techniques, such as the Cavendish experiment, and while there have been small uncertainties in the measurements, they have generally been consistent with a constant value for G.
That being said, theories beyond the standard model of particle physics, such as certain extensions to general relativity or theories involving extra dimensions, propose the possibility of the gravitational constant being variable. These theories often introduce additional scalar fields or modifications to the theory of gravity that could allow for variations in fundamental constants.
However, it is important to emphasize that these proposals are speculative and have not yet been supported by compelling empirical evidence. As our understanding of the universe evolves and as experimental techniques continue to improve, scientists will continue to investigate the constancy of fundamental constants, including the gravitational constant, in order to deepen our understanding of the laws of nature.