alters from time to time and is also allowed to differ from place to place.

Yet another variable-G cosmology, called scale-covariant theory, was proposed in the 1970s by Vittorio Canuto of the City College of New York. Canuto called for a reframing of Dirac’s LNH by means of modifying a number of physical laws, including general relativity and the principle of the conservation of energy. Then, the LNH enters the model as a special condition.

In 1989, Steinhardt and Daile La incorporated aspects of the Jordan-Brans-Dicke theory into extended inflationary cosmology, a variable-G version of the standard inflationary scenario. More recently, a changing gravitational constant has been suggested as a possible solution to the dark-energy conundrum. A diminution in gravity’s strength would offer a natural way of explaining the acceleration of the cosmos—a weaker hold allowing for faster expansion.

Researchers have developed numerous tests to distinguish the various contenders for a possible new theory of gravitation and to determine if standard general relativity requires modification. Each of the variable-G models offers specific predictions in the fields of astrophysics and geophysics, consequences that experimenters can readily assess.

ASTROPHYSICAL CLUES

Astrophysics provides us with myriad examples of gravitationally bound systems. Each would be profoundly affected if G happened to vary. Researchers would notice discrepancies on every scale—from individual stars (such as the Sun) to star clusters (such as Messier 67) to galaxies (such as Andromeda) to clusters of galaxies (such as the one in the constellation Hercules) and finally to superclusters (such as the Local Supercluster).

A slow decrease in the gravitational constant would engender a multitude of long-term consequences. Objects in orbit would



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