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New theory of ‘dark gravity’ passes first test, but Einstein’s still on top

A Dutch theoretical physicist's alternative to the general theory of relativity has been supported by a study of more than 30,000 galaxies. Cathal O'Connell reports.

Gravitational lensing caused by the Abell 2218 cluster of galaxies. Its incredible mass magnifies and distorts galaxies lying behind into long arcs. Such lensing was used to test a new theory of gravity.
Science Photo Library / Getty Images

Theoretical physics is a bit like professional boxing. Everybody wants to have a crack at the champ.

In one corner, we have our white and wispy-haired champion, Albert Einstein, undefeated in the cosmology ring since he proposed his general theory of relativity in 1915. It beautifully describes the universe at its grandest scales, at least when the mysterious dark matter is factored in.

And in the other corner, we have the latest contender: Dutch theoretical physicist Erik Verlinde who put forward a rival theory that claims to explain the universe without the dark stuff.

Now, Verlinde's theory has been tested for the first time. European and Australian astronomers performed galactic measurements which appear to back up his ideas, albeit tentatively. The research is published in the Monthly Notices of the Royal Astronomical Society.

So are Einstein and dark matter on the ropes?

Well, no.

“The cosmology community is not going to throw up its hands, with wailing and gnashing of teeth, at the death of dark matter based on this study,” Geraint Lewis, an astrophysicist at the University of Sydney in Australia and who was not involved in the work, says.

According to general relativity, mass warps spacetime, bending the path of light through it like a lens. More mass means more bending, so one way to measure the mass in a region of space is to measure how much light bends around it.

While this works perfectly for individual objects, such as our sun, it tends to falter at larger scales.

The problem is, when you add up all the visible matter in galaxies – the stars and gas and dust – general relativity doesn’t work out. You see a lot more bending than Einstein’s equations predict.

To get the right answer, astronomers assume there must be extra stuff out there – dark matter – which we cannot detect. Once dark matter is worked in, the calculations work out splendidly.

The same story plays out for other tests of gravity at cosmic scales such as those involving the rate of galaxy rotation or the orbits of galaxies around one another. The success of the dark matter theory to solve each of these problems convinces most physicists that dark matter is real.

But Verlinde would beg to differ. To him, gravity “emerges” from the statistics of microscopic interactions, a bit like how heat emerges from the shaking of individual atoms or molecules.

the study should be taken seriously, but with a grain of salt

“For me, gravity doesn’t exist,” Verlinde told The New York Times soon after he proposed the idea in 2010.

At first, his idea simply reproduced the equations of Newton and Einstein, so was seen more as a new interpretation of gravity rather than a new theory.

But last month, Verlinde went a step further. In a paper published on the Arxiv, he described how the idea gives rise to a “dark gravity force”, which gave gravity extra strength at a very large distance.

He suggested this might explain the apparent extra bending of light by galaxies (as well as galaxy rotation and so on) without the need for dark matter at all.

Now astronomers have tested this dark-matter-killing idea for the first time.

In the new work, Margot Brouwer at Leiden Observatory in the Netherlands and colleagues used weak gravitational lensing to measure the mass of 33,613 galaxies picked up by the Galaxy And Mass Assembly (GAMA) and Kilo-Degree Survey (KiDS).

They then ran the numbers for Verlinde’s theory and the conventional dark matter. Both theories came out with the right answer.

But while the dark matter theory only works if you sprinkle in just the right amount of extra stuff – a bit like correcting your answer after the fact – Verlinde’s theory works without any extra input. The authors describe the performance of the new theory “remarkable”.

“This does not mean we can completely exclude dark matter, because there are many observations that Verlinde’s theory cannot yet explain,” Brouwer says. “However, it is a very exciting and promising first step.”

Lewis says the study should be taken seriously, but with a grain of salt.

One problem is that Verlinde’s theory is still a bit sketchy on the details. In its current form, it can on be applied to the most basic situations, such as spherical objects far apart from each other.

So Brouwer and her crew made a series of approximations involving clumps of thousands of disc-shaped galaxies. For instance, they only analysed a sub-population of the most isolated galaxies, and for these they also needed to factor in the gravity from neighbouring galaxies. They also had to make assumptions about the distribution of visible matter within the galaxies themselves.

Although they didn't need to add any matter of the dark variety, each of these steps added an extra element of uncertainty to the final result.

On the other hand, the dark matter model also may also not be realistic, since it makes assumptions about how dark matter is spread between galaxies.

Both sides rely on approximations and guesswork, making it difficult to draw any conclusions from this work. Of course in science it's not really a case of pitting one theory against another – rather, it's theory versus data.

Lewis calls it a draw: “[Verlinde’s theory] doesn’t do terribly, so lives to fight against better data another day.”

Cathal 2016.png?ixlib=rails 2.1
Cathal O'Connell is a science writer based in Melbourne.
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