Scientists find new way to measure dark matter

measure dark matter, alternative gravity
© Yury Morozov

Scientists reveal that billions of stars at the centre of the Milky Way are spinning more slowly – they believe it is being counterweighted by dark matter, slowing by 24% since it was created

Researchers at University College London (UCL) and the University of Oxford have measured the spin of the bar, made up of billions of stars, at the centre of the Milky Way for the first time in history.

This discovery led to the first measure of dark matter in terms of inertial mass.

‘Must be dark matter’, confirms Dr Schoenrich

Co-author Dr Ralph Schoenrich (UCL Physics & Astronomy) said: “Astrophysicists have long suspected that the spinning bar at the centre of our galaxy is slowing down, but we have found the first evidence of this happening.

“The counterweight slowing this spin must be dark matter. Until now, we have only been able to infer dark matter by mapping the gravitational potential of galaxies and subtracting the contribution from visible matter.

“Our research provides a new type of measurement of dark matter – not of its gravitational energy, but of its inertial mass (the dynamical response), which slows the bar’s spin.”

Why is the spinning bar so important?

Because of how the bar has slowed down, the scientists are able to estimate that there is dark matter there, at the heart of our own universe.

These stars are gravitationally trapped by the spinning bar. If the bar’s spin slows down, these stars would be expected to move further out in the galaxy – keeping their orbital period matched to the bar’s spin.

The researchers found that the stars in the stream carry a chemical fingerprint.

They are richer in heavier elements (called metals in astronomy), proving that they have travelled away from the galactic centre, where stars and star-forming gas are about 10 times as rich in metals compared to the outer galaxy.

The researchers found that this crucial bar had slowed down its spin by at least 24% since it first formed.

This revelation dismiss alternate gravity theories

Alternative gravity theories such as modified Newtonian dynamics reject the idea of dark matter, instead seeking to explain anomalies by messing around with Einstein’s theory of general relativity. They do not believe it possible to measure dark matter at all – as to them, it does not exist.

Co-author and PhD student Rimpei Chiba, of the University of Oxford, said: “Our finding offers a fascinating perspective for constraining the nature of dark matter, as different models will change this inertial pull on the galactic bar.

“Our finding also poses a major problem for alternative gravity theories – as they lack dark matter in the halo, they predict no, or significantly too little slowing of the bar.”

Read the full study here.


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