The force of gravity has not changed over nine billion years, based on studies of supernova explosions old and new. The finding may seem obvious – after all the measured value, G, is called the gravitational constant. It's hardly surprising that it is actually constant.
However, just last year evidence emerged that G might be varying. Moreover, the same department that has now found G to be stable has previously contributed to that at least one of the other fundamental constants of the universe is changing. If that previous research is right questions would be asked about how constant any of these “constants” really are. As a result the universe feels a slightly more stable place with the publication of this finding in the Publications of the Astronomical Society of Australia.
Dr Michael Murphy of Australia's Swinburne University has co-authored a series of papers indicating the fine structure constant α is changing. α controls the strength of electromagnetic interactions, and is formed from the relationship of three of the most important numbers in physics – for it to change at least one of these must change as well. Murphy studied way light from distant quasars interacts with gas in closer, but still very distant, galaxies, to get a value for α at the time when the light passed through these galaxies.
Other research groups are still checking these reports, but in the meantime Murphy's colleague at the Center for Astrophysics and Supercomputing Professor Jeremy Mould investigated whether the attraction force between massive objects might be undergoing a similar change.
To do this Mould and his PhD student Syed Uddin investigated 580 Type Ia supernova explosions. Type Ias occur when a white dwarf draws so much material off a companion star that it is reaches a critical mass and explodes. There is a consistent relationship between the way Ia supernovae diminish in brightness with time, and how intrinsically bright they are, allowing us to measure their distance. It was this fact that lead to the discovery that the universe's expansion is accelerating as a result of what is known as dark energy, leading to the 2011 Nobel Prize.
"This critical mass depends on Newton's gravitational constant G and allows us to monitor it over billions of years of cosmic time – instead of only decades, as was the case in previous studies." says Mould. "Looking back in cosmic time to find out how the laws of physics may have changed is not new. But supernova cosmology now allows us to do this with gravity."
Mould and Uddin could find no evidence for any variation in the mass at which Ia supernovae occur, whether close to us in time and space, or as distant as we are able to detect.
Since the 1960s the Lunar Laser Ranging Experiment has been bouncing lasers off reflectors left on the moon by the Apollo astronauts (so much for the cranks who say they never got there). This has allowed measurements of the gravitational attraction between the Earth and Moon to an exceptional degree, and no change has been detected in four decades. However, having a span of billions of years to work with might have produced a different result. Instead, Mould and Uddi have concluded that if G is changing it is doing so by less than 0.00000001% per year, or we would be able to see the difference in the measurements they made.