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INTO THE DARK

INTO THE DARK

They make up 95 per cent of the cosmos, and yet we know next to nothing about them. Meet dark energy and dark matter. They’re what the universe is made of.

Words Becky Allen Images Ollie Macdonald Oulds

Humans have long looked to the skies and wondered. Wondered what’s out there. What the cosmos is made of, whether we would ever get to explore it, what our part in it is. In the intervening thousands of years, some small questions have been answered, but many – many – remain. Astronomers agree that the stuff we can see and understand accounts for just five per cent of what’s out there. That means the other 95 per cent is left for us still to explore. Within that 95 per cent there are two distinct groups – cold dark matter and dark energy – but exactly what they are continues to baffle the biggest brains in the business.

Despite not knowing what it’s made of and being unable to ‘see’ it directly with our most powerful tools, most experts agree dark matter must exist because so much circumstantial evidence points to it. Only with this huge amount of extra mass in the system can astronomers explain their observations of galaxies’ rotational velocity, gravitational lensing and cosmic microwave background radiation. Unless our laws of physics are wrong, dark matter must be there.

Professor Ofer Lahav, Perren Chair of Astronomy, likes to explain this by rewinding to the 1840s, when his predecessors were struggling to align theory with what they saw through telescopes. Astronomers studying Uranus and Mercury noticed oddities in their orbits. In the case of Uranus, the discovery of another planet (Neptune) ironed things out. But despite looking for a new planet between Mercury and the sun, resolving Mercury’s strange orbit anomaly required a whole new theory. “If you see something strange, you can either blame the observation or add something into the model that removes the discrepancy,” says Lahav. “Discoveries usually appear when there is such a gap between observations and the current model.”

His own gap is dark energy, something that wasn’t even a twinkle in an astronomer’s eye when Lahav began his PhD in 1985. “We thought 95 per cent of the universe was cold dark matter. The paradigm shift was due to two teams looking at supernovae; they realised that by interpreting light from these exploding stars in the ordinary way, they appeared too dim,” he says. “The solution was to insert lambda – a forgotten parameter invented by Einstein for a different reason.”

From UCL, Lahav is leading the UK’s contribution to the Dark Energy Survey (DES), an international effort helping uncover the nature of dark energy by measuring the 14-billion-year history of cosmic expansion. Over the past five years, DES has surveyed and recorded information from 300 million galaxies billions of light years from Earth.

A key part of DES is a cutting-edge camera – built at UCL before being installed in the giant 4m Blanco telescope at the Cerro Tololo Inter-American Observatory high in the Chilean Andes. As well as bagging 300 million galaxies, it’s survived several earthquakes and, says Lahav: “It’s working very successfully on the mountain and produced beautiful results.”

If the efforts to understand dark energy seem extreme, experiments to pin down dark matter require particle physicists like Dr Chamkaur Ghag to go to equal lengths, albeit in the opposite direction. Studying the stars requires telescopes at the highest places on Earth, or in space itself, whereas looking for dark matter must be done deep underground where experiments can be shielded from the background noise created by the massive amount of radiation that bombards the Earth.

Whereas Lahav travels 2,200m above the Chilean coast, Ghag descends 2,400m below the Black Hills of Dakota to work in the Sanford Underground Research Facility (SURF), where scientists and engineers have transformed Homestake – once the largest, deepest and most productive gold mine in America – into cutting-edge laboratories. It’s here that UCL is part of the LZ Dark Matter Experiment, a US-UK project that’s building exquisitely sensitive detectors designed to find the minuscule sub-atomic particles that make up dark matter.

Humans have long looked to the skies and wondered. Wondered what’s out there. What the cosmos is made of, whether we would ever get to explore it, what our part in it is. In the intervening thousands of years, some small questions have been answered, but many – many – remain. Astronomers agree that the stuff we can see and understand accounts for just five per cent of what’s out there. That means the other 95 per cent is left for us still to explore. Within that 95 per cent there are two distinct groups – cold dark matter and dark energy – but exactly what they are continues to baffle the biggest brains in the business.

Despite not knowing what it’s made of and being unable to ‘see’ it directly with our most powerful tools, most experts agree dark matter must exist because so much circumstantial evidence points to it. Only with this huge amount of extra mass in the system can astronomers explain their observations of galaxies’ rotational velocity, gravitational lensing and cosmic microwave background radiation. Unless our laws of physics are wrong, dark matter must be there.

Professor Ofer Lahav, Perren Chair of Astronomy, likes to explain this by rewinding to the 1840s, when his predecessors were struggling to align theory with what they saw through telescopes. Astronomers studying Uranus and Mercury noticed oddities in their orbits. In the case of Uranus, the discovery of another planet (Neptune) ironed things out. But despite looking for a new planet between Mercury and the sun, resolving Mercury’s strange orbit anomaly required a whole new theory. “If you see something strange, you can either blame the observation or add something into the model that removes the discrepancy,” says Lahav. “Discoveries usually appear when there is such a gap between observations and the current model.”

His own gap is dark energy, something that wasn’t even a twinkle in an astronomer’s eye when Lahav began his PhD in 1985. “We thought 95 per cent of the universe was cold dark matter. The paradigm shift was due to two teams looking at supernovae; they realised that by interpreting light from these exploding stars in the ordinary way, they appeared too dim,” he says. “The solution was to insert lambda – a forgotten parameter invented by Einstein for a different reason.”

From UCL, Lahav is leading the UK’s contribution to the Dark Energy Survey (DES), an international effort helping uncover the nature of dark energy by measuring the 14-billion-year history of cosmic expansion. Over the past five years, DES has surveyed and recorded information from 300 million galaxies billions of light years from Earth.

A key part of DES is a cutting-edge camera – built at UCL before being installed in the giant 4m Blanco telescope at the Cerro Tololo Inter-American Observatory high in the Chilean Andes. As well as bagging 300 million galaxies, it’s survived several earthquakes and, says Lahav: “It’s working very successfully on the mountain and produced beautiful results.”

If the efforts to understand dark energy seem extreme, experiments to pin down dark matter require particle physicists like Dr Chamkaur Ghag to go to equal lengths, albeit in the opposite direction. Studying the stars requires telescopes at the highest places on Earth, or in space itself, whereas looking for dark matter must be done deep underground where experiments can be shielded from the background noise created by the massive amount of radiation that bombards the Earth.

UCL’s crucial contribution to the project has been making sure they can account for every single source of radiation in the system. This is essential in these so-called ultra-low background rare event searches. Ghag sought out the ultra-pure titanium the team needed for the vacuum flask – a task that took two years – and then applied the same painstaking precision to each component.

“We’ve mapped exactly how much radiation comes from every nut and bolt to build up a background model of what we see in the absence of dark matter,” he says. “We need to be incredibly precise so that when we detect a tiny change we know we’ve detected dark matter.”

With the UK part of the project nearing completion, one of Ghag’s PhD students spent the autumn in South Dakota helping put the detector together, a process that will take more than a year, before entering a commissioning phase so the team can monitor its safety, stability and data quality.

“Safety’s critical,” Ghag adds. “Ten tonnes of xenon costs $10m, so springing a leak would be costly.” Then, in 2020, they’ll turn it on and wait for the data – which could answer one of the biggest questions in modern science.

Is he optimistic? “That the technology will run to design specification? Absolutely. The UK has pioneered this xenon technology and we’re world leaders,” he says. “Will we find WIMPs? I don’t know, but there’s always hope. There are other candidate particles, and WIMPs are low-hanging fruit. But if we detect them, it will open up a whole new era for understanding the universe.”

Which leaves one final question. So what? For 300,000 years, humans have existed knowing nothing about dark matter or dark energy. Although there are spin-offs such as the cutting-edge technology and new techniques for big data, which have applications in medicine and finance as well as communications and astrophysics, this is fundamental research about the universe we inhabit.

“We all look at the sky and ask big questions – they appear in ancient manuscripts and are still asked by children today – how big is the universe and what is our place in it?” concludes Lahav. “Understanding dark matter is fundamental, and if dark energy is there, it’s a fundamental change in how we think about physics – because how can we live in a universe when we don’t understand 95 per cent of it?”

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Portico Issue 5. 2018/19
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