Exploding stars prove Newton's gravity unchanged over cosmic time

Mar 23, 2014
Supernova
Credit: NASA

(Phys.org) —Australian astronomers have combined all observations of supernovae ever made to determine that the strength of gravity has remained unchanged over the last nine billion years.

Newton's gravitational constant, known as G, describes the attractive force between two objects, together with the separation between them and their masses. It has been previously suggested that G could have been slowly changing over the 13.8 billion years since the Big Bang.

If G has been decreasing over time, for example, this would mean that the Earth's distance to the Sun was slightly larger in the past, meaning that we wouldexperience longer seasons now compared to at much earlier points in the Earth's history.

But researchers at Swinburne University of Technology in Melbourne have now analysed the light given off by 580 supernova explosions in the nearby and far Universe and have shown that the strength of gravity has not changed.

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

A Type Ia supernova marks the violent death of a star called a white dwarf, which is as massive as our Sun but packed into a ball the size of our Earth.

Our telescopes can detect the light from this explosion and use its brightness as a 'standard candle' to measure distances in the Universe, a tool that helped Australian astronomer Professor Brian Schmidt in his 2011 Nobel Prize winning work, discovering the mysterious force Dark Energy.

Professor Mould and his PhD student Syed Uddin at the Swinburne Centre for Astrophysics and Supercomputing and the ARC Centre of Excellence for All-sky Astrophysics (CAASTRO) assumed that these supernova explosions happen when a white dwarf reaches a or after colliding with other stars to 'tip it over the edge'.

"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." said Professor Mould.

Despite these vastly different time spans, their results agree with findings from the Lunar Laser Ranging Experiment that has been measuring the distance between the Earth and the Moon since NASA's Apollo missions in the 1960s and has been able to monitor possible variations in G at very high precision.

"Our cosmological analysis complements experimental efforts to describe and constrain the laws of physics in a new way and over cosmic time." Mr Uddin said.

In their current publication, the Swinburne researchers were able to set an upper limit on the change in Newton's gravitational constant of 0.00000001% per year over the past nine billion years.

The ARC Centre of Excellence for All-sky Astrophysics (CAASTRO) is a collaboration between The Australian National University, The University of Sydney, The University of Melbourne, Swinburne University of Technology, the University of Queensland, The University of Western Australia and Curtin University, the latter two participating together as the International Centre for Radio Astronomy Research. CAASTRO is funded under the Australian Research Council Centre of Excellence program, with additional funding from the seven participating universities and from the NSW State Government's Science Leveraging Fund.

This research is published this month in the Publications of the Astronomical Society of Australia.

Explore further: Hubble monitors supernova in nearby galaxy M82

More information: Mould & Uddin "Constraining a possible variation of G with Type Ia supernovae" in PASA 2014. arxiv.org/abs/1402.1534

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User comments : 11

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Jizby
Mar 23, 2014
This comment has been removed by a moderator.
arom
1 / 5 (8) Mar 24, 2014
Australian astronomers have combined all observations of supernovae ever made to determine that the strength of gravity has remained unchanged over the last nine billion years.
Newton's gravitational constant, known as G, describes the attractive force between two objects, together with the separation between them and their masses. It has been previously suggested that G could have been slowly changing over the 13.8 billion years since the Big Bang.

This is interesting, anyway what seems more interesting is that how and why mass create attractive force between two objects (or create curved space – general relativity), knowing the mechanism maybe help to understand more about the gravity…

http://www.vacuum...=7〈=en
verkle
1 / 5 (8) Mar 24, 2014
A changing G could have helped explain the current dilemna of gravitation on a galaxy level. I still don't buy into the "dark" energy and matter nonsense.

We need to come up with better answers! Lots of science is still required.
Jizby
Mar 24, 2014
This comment has been removed by a moderator.
dav_daddy
5 / 5 (5) Mar 24, 2014
A changing G could have helped explain the current dilemna of gravitation on a galaxy level. I still don't buy into the "dark" energy and matter nonsense.

We need to come up with better answers! Lots of science is still required.


Dark matter & dark energy aren't meant as answers. They are place holders we use to describe 2 phenomena for which we currently have no explanations.
Jizby
Mar 24, 2014
This comment has been removed by a moderator.
Shitead
3.7 / 5 (3) Mar 24, 2014
"the Swinburne researchers were able to set an upper limit on the change in Newton's gravitational constant of 0.00000001% per year over the past nine billion years."

This is not a cosmology problem, it is a banking problem. If I invest an amount G at 0.00000001% interest per year (which is very close to current bank rates), how long will it take for G to double? By the "rule of 72" (look it up!) G will double about every 7 billion years, meaning that G could have quadrupled since the "Big Bang" and this model would not have accounted for it. Should'a known better!
Jizby
Mar 24, 2014
This comment has been removed by a moderator.
HannesAlfven
1 / 5 (6) Mar 24, 2014
Re: "In their current publication, the Swinburne researchers were able to set an upper limit on the change in Newton's gravitational constant of 0.00000001% per year over the past nine billion years."

See table titled "A Changing Constant" for a record of recent, actual, non-simulated change in value of G at http://www.nature...30a.html

One would think that the changing value of G would be topically relevant to this article. It's curious that they leave this information out.
CrossMan
5 / 5 (3) Mar 24, 2014
@Shitead, I think that it's 0.00000001% of the starting (or current) value per year, not of the compounded value per year, so not more variation than 10% in 10 billion years. This is, of course, not as good as lunar ranging bounds (100x better), but covers a much longer range of time.

@HannesAlfven, You're right, it would have been nice to point out the difficulty of accurately measuring G (the most poorly known of the fundamnetal constants). The SN data (also the ranging data) are only sensitive to variations in its value over time, not the value itself. This is bad because it doesn't help us determine its value, but it's good because we can be confident that its value doesn't change despite not being able to measure it. This in turns means that the disagreements between experiments trying to determine its value are most likely due to systematic measurement errors. See also http://phys.org/n...her.html
kelman66
4.5 / 5 (2) Mar 24, 2014
A 10% variation in G in 10 billion years would be massive methinks. That would be a major discovery and that would be the headline for the article.
HannesAlfven
1.5 / 5 (4) Mar 24, 2014
Re: "You're right, it would have been nice to point out the difficulty of accurately measuring G ... but it's good because we can be confident that its value doesn't change despite not being able to measure it. This in turns means that the disagreements between experiments trying to determine its value are most likely due to systematic measurement errors."

You seem to simply dismiss the conclusion of the researcher quoted at the end of the article:

"Faller says the fear of being wrong can cause investigators to wait many years before publishing results that don't agree with previous measurements. He and Parks ran their experiment in 2004, and have spent the time since then searching for effects they might have missed. But he's sure their measurement is sound: "I feel like I've checked everything and I have to wash my hands.""
Whydening Gyre
5 / 5 (1) Mar 24, 2014
Everything's subject to relativity, folks...
CrossMan
5 / 5 (2) Mar 25, 2014
@kelman66 -- Yes, a 10% change over cosmic history would still be big news. All we know is that if there is any change it's smaller than that. Let's keep pushing those bounds tighter or actually determine a variation!

@HannesAlfven -- What exactly did I dismiss? It's a tough measurement and *someone* has an unknown systematic error. Of course you may not want to publish a result that disagrees with someone else's precise measurement by several sigma -- what if you made an embarrassingly simple mistake? (Like claiming superluminal neutrinos before checking your cables...) But this is the value of having multiple methods of measuring G -- it lets us know there's a problem!
CrossMan
5 / 5 (2) Mar 25, 2014
Feynman relates a similar problem when measuring the electric charge, e. Millikan did it first, but had the wrong air viscosity, so his value was low. Later measurements came out high, but were renormalized down so as not to disagree too much with Millikan. Eventually the values leveled off at the *right* value. There are definitely psychological effects at play with humans do science. But the process carries on.

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