value, with non-overlapping standard uncertainty intervals. As of 2018, efforts to re-evaluate the conflicting results of measurements are underway, coordinated by NIST, notably a repetition of the experiments reported by Quinn et al. (2013). In August 2018, a Chinese research group announced new measurements based on torsion balances, and based on two different methods. These are claimed as the most accurate measurements ever made, with a standard uncertainties cited as low as 12 ppm. The difference of 2.7σ between the two results suggests there could be sources of error unaccounted for. Suggested time-variation A controversial 2015 study of some previous measurements of , by Anderson et al., suggested that most of the mutually exclusive values in high-precision measurements of G can be explained by a periodic variation. The variation was measured as having a period of 5.9 years, similar to that observed in length-of-day (LOD) measurements, hinting at a common physical cause that is not necessarily a variation in . A response was produced by some of the original authors of the measurements used in Anderson et al. This response notes that Anderson et al. not only omitted measurements, but that they also used the time of publication rather than the time the experiments were performed. A plot with estimated time of measurement from contacting original authors seriously degrades the length of day correlation. Also, consideration of the data collected over a decade by Karagioz and Izmailov shows no correlation with length of day measurements. As such, the variations in most likely arise from systematic measurement errors which have not properly been accounted for. Under the assumption that the physics of type Ia supernovae are universal, analysis of observations of 580 of them has shown that the gravitational constant has varied by less than one part in ten billion per year over the last nine billion years according to Mould et al. (2014). See also Gravity of Earth Standard gravity Gaussian gravitational constant Orbital mechanics Escape velocity Gravitational potential Gravitational wave Strong gravitational constant Dirac large numbers hypothesis Accelerating universe Lunar Laser Ranging experiment Cosmological constant References Footnotes Citations Sources (Complete report available online: PostScript; PDF. Tables from the report also available: Astrodynamic Constants and Parameters) External links Newtonian constant of gravitation at the National Institute of Standards and Technology References on Constants, Units, and Uncertainty The Controversy over Newton's Gravitational Constant — additional commentary on measurement problems Gravity Fundamental