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, for example, the beginning and the eventual 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 investigators put the variations down to errors of measurements.

See Wikipedia

Also see Gravityplugin-autotooltip__plain plugin-autotooltip_bigGravity

Although three of the four (known) fundamental forces have been unified by one paradigm called the Standard Model (SM) Gravity remains outside.

This doesn't necessarily mean that Standard Model has failed in some way. Perhaps Gravity is independent from and i… and The Fine Structure Constantplugin-autotooltip__plain plugin-autotooltip_bigThe Fine Structure Constant

The Fine Structure Constant, identified by the Greek letter α is a fundamental number used in quantum physics calculations. The current estimate is that it's around 0.00729735256 - or roughly 1/137.

It has been called the and Physical constantsplugin-autotooltip__plain plugin-autotooltip_bigPhysical constants

Constants variability :

Many physics calculations rely on the assumption that the ‘physical constants’ e.g. light-speed, gravity, etc etc are, in fact, constant. Extremely accurate experimental procedures strongly suggest that they (mostly) are. But…

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