The gravitational attraction between two objects is dependent on the mass of the objects, the distance between them, and the gravitational constant (G).
The masses and distance can vary, but the constant, as the name implies, is always a fixed number.
It was first measured in 1798 by Henry Cavendish, with an accuracy of around 1%. Since then, many more attempts have been made, using very different methods.
The margin of error for the results is not yet clear - the lowest measurement being around 6.66 ×10−11 m3⋅kg−1⋅s−2 and the highest 6.68, or in other words with a margin of error of around 0.3%
Although the error margin might seem small, it has extremely far-reaching consequences for calculations on cosmological scales regarding the beginning and the fate of the universe.
The constant is currently agreed to be an average value, with an accuracy of 0.1%
A further complication is that recent measurements (i.e. in the last 50 years or so) have tended to give values which appear to vary in a cyclic way. Giving rise to speculations that the constant may not in fact be constant. If it is eventually proven that the constant is varying, most of cosmological theory will have to be re-written. Note that many other investigators put the variations down to errors of measurements.
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