Does gravity change with time?

The Newtonian gravitational parameter G is a constant in both Newton's law of gravitation $(\mathrm{force}{\mathrm{\hspace{0.17em}=\hspace{0.17em}-GM}}_1{M}_2{\mathrm{/r}}_{12}^2)$ and Einstein's general theory of relativity. However, over the last fifty years there have been numerous suggestions that G might in fact change with time. The detailed grounds for these suggestions have differed, but most variable‐G theories account for gravitation on a cosmological basis. As such, they usually involve a time‐dependent G: $\mathrm{G\hspace{0.17em}=\hspace{0.17em}G(t),}$ where t is a parameter that can, loosely speaking, be interpreted as the “age” of the Universe. A value of G that depends on time is a drastic departure from the established physics of gravitation (based on Newton's and Einstein's theories). Before looking at G‐variability in detail it is therefore logical to ask first: Why should G vary?