Old Supernovae Show Gravitational Constant Remains Unchanged

March 26, 2014

By comparing 580 type Ia supernovae, astronomers found that Newton's gravitational constant and gravitation didn't change over the last 9 billion years.

A type Ia supernova in a distant spiral galaxy, artist's impression.

A type Ia supernova in a distant spiral galaxy, artist's impression. Credit: CAASTRO/Swinburne Astronomy Productions

Newton's gravitational constant, denoted by letter "G", is used when calculating the gravitational force between two objects, together with objects' masses and distance between them. It has a very small value and is hard to measure accurately. For this reason, there was always a chance the gravitational constant, and with that the strength of gravitational force, changed over time.

A new study by a team of astronomers from Swinburne University of Technology, Melbourne, Australia, compared 580 type Ia (Roman numeral "1" followed by letter "a") supernovae and their findings suggest that the gravitation has remained unchanged over at least the last 9 billion years.

"Looking back in cosmic time to find out how the laws of physics may have changed is not new," said Professor Jeremy Mould of Swinburne University, "but supernova cosmology now allows us to do this with gravity."

Type Ia supernova explosion involves a white dwarf star – a star that is already a remnant of a Sun-like, main- sequence star – and another star in a binary system. A white dwarf is very dense, with the mass comparable to that of the Sun, but compressed to Earth's volume. This enables it to accrete the material from its companion star and if it reaches critical mass, it explodes in a type Ia supernova.

Since all type Ia supernovae have the same peak luminosity, they are used for measuring distances of their host galaxies. In this study, the farthest galaxy where supernova took place was 9 billion light-years away. This also means it happened 9 billion years ago – the time it took the light to reach us.

"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," Mould added.

If the gravitational constant was changing with time, e.g. decreasing, the Earth's gravitational pull would be getting weaker and the Moon would be moving away from us. The Moon actually does move away about 4 centimetres a year, but not because of gravitation getting weaker, but because of tidal friction that transfers energy from Earth to the Moon.

The Lunar Laser Ranging Experiment has been measuring the distance between the Earth and the Moon for over five decades, taking into account the tidal friction mentioned above, and found no evidence of gravitational constant changing. The study by Australian team confirms this, but including much longer periods of time.

"Our cosmological analysis complements experimental efforts to describe and constrain the laws of physics in a new way and over cosmic time," Says Syed Uddin. He is Professor Mould's PhD student and the co-author of the paper published in the Publications of the Astronomical Society of Australia.

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