Cavendish measured this constant with a (*) torsion balance.  
This value is thought to vary at low accelerations in MOND theory.  The hierarchy problem asks why its value is so small, which is explained in string theory, by having the gauge boson associated with this value not fixed to a brane.  Cavendish measured this constant with a (*) torsion balance.  Like Boltzman's constant, this fundamental constant does not occur in the formula for the fine structure constant.  For 10 points, name this constant found in an inverse square law describing the mutual attraction of matter.
This value is thought to vary at low accelerations in MOND theory.  
Universal {Gravitational} Constant [accept {big g}, accept logical equivalents such as {gravity}, prompt on "g", prompt on "Newton's constant"]