Could the solution to the most frustrating mystery in astronomy lie in the bottom of a mine in Yorkshire?
Physicists dreaming of the next generation of dark matter detector are hoping so.
The evidence that most of the matter in the cosmos is in a form that we don’t understand has mounted over the last 50 years.
Observations of galaxies, clusters of galaxies and the Universe as a whole all point to the existence of ‘dark matter’, an invisible substance we can only infer from its gravitational influence.
No alternative theory is as convincing as the idea that most of the matter in the cosmos is in this form that we don’t understand.
Still, it would be much more satisfying if we could say what these pesky dark matter particles actually are, and that likely means detecting them directly.
There are several experiments currently endeavouring to do so, the most sensitive of which involve placing tanks containing roughly 10 tonnes of xenon deep underground.
Here they are hidden from the cosmic rays that would bombard them on the surface.
Millions of dark matter particles must pass through the tank every second, most of them continuing onward without any effect at all.
Dark matter and Xenon atoms
A very tiny fraction of them should score a direct hit on a xenon atom, producing a burst of light that can be seen.
Such interactions are very rare (probably a good thing, as millions of dark matter particles are passing through you right now).
Experiments like XENONnT, situated in the majestic Gran Sasso lab under a mountain not far from Rome, hope to detect fewer than five events per year, not enough to help us find dark matter.
And so the quest to create a larger detector begins.
The proposed DARWIN/XLZD instrument – its clumsy name the result of a merger between different groups of experimenters – would contain up to 60 tonnes of xenon.
More xenon hopefully means more head-on collisions with dark matter.
This should allow us to detect dark matter particles which are lighter, or more reluctant to react with normal matter.
Experiments in Switzerland and Germany are already under way to prove that detectors sensitive enough to make use of the larger xenon tank can be built, but funding is still being sought and a location has yet to be selected.
The amazing Boulby Mine in North Yorkshire is as good a candidate as any, especially if the UK wants to make a major contribution to this audacious effort.
This next experiment will be worth the wait, as it will be the last of its kind.
Get much better than this at detecting collisions with ghostly particles and you’ll start seeing not dark matter but neutrinos.
We know these nearly massless, fast-moving particles definitely exist and can interact with normal matter.
They will swamp any dark matter signal in more sensitive detectors.
This is the final saloon for a set of experiments that offer us the best chance of directly detecting dark matter.
Frustratingly, a failure won’t prove that dark matter isn’t a major component of the cosmos, but a success would be the greatest physics discovery of the 21st century.
Fingers crossed.
Chris Lintott was reading DARWIN/XLZD: A Future Xenon Observatory for Dark Matter and Other Rare Interactions by Laura Baudis. Read it online at: arxiv.org/abs/2404.19524.
This article appeared in the July 2024 issue of BBC Sky at Night Magazine.
