18th October 2014

The first direct detection of dark matter particles may have been achieved

Astronomers have detected what appears to be a signature of "axions" – dark matter particle candidates. If confirmed, this would be the first direct detection and identification of the elusive substance, which has been a mystery in physics for over 30 years.



XMM-Newton observatory. Credit: ESA

A landmark paper by Professor George Fraser – who tragically died earlier this year – and colleagues from the University of Leicester offers what is potentially the first direct detection of dark matter. This hypothetical form of matter comprises 85% of the Universe, but neither emits nor absorbs light or other electromagnetic radiation in any significant way. Its existence is only known because of the gravitational pull it has on objects. In other words, it is what holds everything together, and without it, galaxies would unravel and fly apart.

The study – to be published on 20th October in the Monthly Notices of the Royal Astronomical Society – looked at 15 years of measurements taken by the European Space Agency's orbiting XMM-Newton observatory; almost its entire archive of data. A curious signal was seen in the X-ray sky which had no conventional explanation, but is now believed to have been the result of axions. Previous searches for these particles, notably at CERN, and with other spacecraft in Earth orbit, have so far proved unsuccessful.

“The X-ray background – the sky, after the bright X-ray sources are removed – appears to be unchanged whenever you look at it,” says Dr. Andy Read from the University of Leicester's Department of Physics and Astronomy and now leading the paper. “However, we have discovered a seasonal signal in this X-ray background, which has no conventional explanation, but is consistent with the discovery of axions.”

As the late Professor Fraser explains in the paper: “It appears plausible that axions – dark matter particle candidates – are indeed produced in the core of the Sun and do indeed convert to X-rays in the magnetic field of the Earth.”



A sketch (not to scale) showing axions (blue) streaming out from the Sun, converting in the Earth's magnetic field (red) into X-rays (orange), which are then detected by the XMM-Newton observatory. Credit: University of Leicester

It is predicted that the X-ray signal due to axions will be greatest when looking through the sunward side of the magnetic field, because this is where the field is strongest. Each of these ghostly particles is extraordinarily light, with a vanishingly small mass just 1/100 billionth that of an electron or a million times less than a neutrino.

Dr. Read concludes: “These exciting discoveries, in George's final paper, could be truly ground-breaking, potentially opening a window to new physics, and could have huge implications, not only for our understanding of the true X-ray sky, but also for identifying the dark matter that dominates the mass content of the cosmos.”

President of the Royal Astronomical Society, Professor Martin Barstow: “This is an amazing result. If confirmed, it will be first direct detection and identification of the elusive dark matter particles and will have a fundamental impact on our theories of the Universe.”

We may know a lot more about dark matter in the coming years – thanks to a string of new observatories including the Euclid Space Telescope (2020), the European Extremely Large Telescope (2022) and the Advanced Technology Large-Aperture Space Telescope (2025). Dr. Read's team also plans to double the dataset from XMM-Newton and look at the results with more precision over the next few years.

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