News Physics 17 October 2016
3 minute read 

Spinning galaxies question existence of dark matter

New 'law of nature' predicts galaxy rotation using visible matter alone, but don’t turf out the idea of dark matter just yet, writes Cathal O'Connell.

X-rays and visible light from the radio galaxy 3C31, located 240 million light-years from Earth. Calculating how fast such galaxies spin doesn't need to take dark matter into account, according to a new paper.
X-ray: NASA / CXC / Univ. of Bristol / M. Hardcastle et al; Optical: NASA / STScI

Some 80 years after dark matter was first theorised, we still have no idea what it is. Now, a new study casts doubt on its existence altogether.

According to the standard model of cosmology, the immense gravity of dark matter is crucial for explaining why galaxies can spin so fast without tearing themselves apart.

But in work just accepted by Physical Review Letters, a team of American astronomers found a striking correlation between the visible matter (the stars and dust in galaxies) and its rotation speed. That means they can predict the rotation of galaxies – without invoking the dark stuff at all.

“Nothing in the standard cosmological model predicts this and it is almost impossible to imagine how that model could be modified to explain it, without discarding the dark matter hypothesis completely,” said David Merritt, an astrophysicist at Rochester Institute of Technology in New York and who was not involved in the research.

Gravity at the level of the solar system is a piece of cake. Simple laws, written by Johannes Kepler in the 1600s, tell us that a planet’s orbital speed depends precisely on its distance from the sun.

So while Mercury whizzes around the sun at an average of 47 kilometres per second, Pluto shuffles along at just a 10th that speed.

The weird thing is, galaxies don’t behave this way at all – stars don’t slow the further they are from the galactic centre. In some cases, they speed up.

According to our current understanding of gravity, stars at the edges of galaxies, such as our sun, should be flung out into deep space.

In the 1970s the American astronomer Vera Rubin, picking up an idea pitched by Swiss astronomer Franz Zwicky a few decades before, suggested an answer: there must be extra matter in and around the galaxies – perhaps 10 times more than what we can see – holding everything together.

But after 40 years of fruitless searching, some physicists think the hunt for dark matter has been a wild goose chase.

In the new work, a team led by Stacy McGaugh at Case Western Reserve University in Ohio found a direct relationship between the distribution of regular matter in a galaxy and its speed of rotation. This distribution held, even for galaxies thought to be dominated by dark matter.

McGaugh’s team pored over data from 153 galaxies collected by NASA’s Spitzer Space Telescope. Imaging in the infrared, Spitzer can see both the stars and the immense clouds of dust between them – and these components allowed McGaugh’s team to calculate the mass of the visible matter in each galaxy more accurately than ever before.

Then they compared these masses against the actual rotation speeds of each galaxy, which were clocked by astronomers for decades.

Surprisingly, for dark matter advocates at least, the measurements showed a tight correlation. This means the team could look at a galaxy’s visible matter and predict its rate of spin.

The relationship is strong enough to be termed a new law of nature, “a sort of Kepler's law for rotating galaxies,” the authors write.

The result was consistent over 2,693 data points across 153 galaxies with a huge variation in body shape – from dwarfs to giants, from spiral-armed or irregular beasts, some with central bulges and others without – all without accounting for dark matter.

The question is, why?

Perhaps our understanding of gravity is fundamentally wrong. One theory waiting in the wings is modified Newtonian dynamics which says that gravity behaves differently at very large distances. It has been successfully applied to galaxy rotation too.

But it is far too early to throw dark matter out.

For starters, there is plenty of evidence beyond galactic rotation that tells us dark matter must be out there.

While McGaugh admits the findings could stem from modified gravity, it could also be telling us something about the nature of dark matter itself.

For instance, the spread of dark matter and regular matter could be more intimately linked than previously thought. Besides, the new work shows correlation, rather than causation, but one that, in the words of the author, “demands explanation”.

"Most importantly, whatever theory you want to build has to reproduce this," says McGaugh.

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Cathal O'Connell is a science writer based in Melbourne.