- Email: [email protected]
Dark matter no-show spells trouble for WIMPs
In this section n Ancestor of all life was only half alive, page 12 n The fightback against racist technology begins, page 18 n Microbes that recycle what you flush, page 24
Dark matter no-show spells trouble for WIMPs
to make a difference. This means that, for now, Ali’s team is focusing on improving their gene therapy treatment. By contrast, Editas has chosen to target a very rare form of LCA that can be treated merely by disabling a gene – a much easier task. Most eye diseases, however, would only be treatable by gene editing if it could be improved to correct mutations efficiently. As things stand, CRISPR will be of limited use for treating eye diseases even if it proves safe and effective in human trials. But the technology is advancing rapidly, so this could soon change. A modified CRISPR method was recently announced that could provide an alternative approach for correcting genes. Watch this space. n
C.H. Faham
THE HUNT lasted nearly two years, but there was not even a sniff of the quarry. One of the world’s leading dark matter detectors has failed to find any candidate particles, suggesting that the dominant model of the stuff may be on its last legs. Dark matter does not emit light and scarcely interacts with normal matter except through gravity, yet appears to account for around 85 per cent of the universe’s mass. Without its gravitational pull, galaxies would spin themselves apart. Theorists have settled on some of its basic characteristics: its particles have some mass and interact a bit with other matter via the weak –A prime target for CRISPR– nuclear force. So they called these mysterious entities “weakly Ali at University College London, interacting massive particles”, or WIMPs. who led the team. WIMPs’ occasional glancing But there’s a catch. This would blows with normal matter are what mean correcting mutations in a allow experiments like the Large gene, instead of just disabling it, Underground Xenon detector in as in Hewitt’s experiment. Lead, South Dakota, to detect their Correcting genes is much harder presence and measure their in non-dividing cells like those properties. But on 21 July, the LUX in the retina. It is not yet possible team told the Identification of Dark to correct a gene inside a high enough proportion of retinal cells Matter conference in Sheffield, UK,
that its final 20-month run had failed to make a single detection. That means LUX, dormant since May, has ruled out a large number of possible characteristics for WIMPs – to the point where some argue it might be time to abandon WIMPs altogether. “I think we are getting to the point where the limits are excluding so much of the parameter space that we should rethink,” says Avi Loeb at Harvard University. “Perhaps the dark matter is not WIMPs.” Richard Gaitskell at Brown University in Rhode Island, who worked on LUX, is still optimistic. Plans are already under way for an upgraded detector called LZ that will be 70 times more sensitive. Gaitskell says that the technology to detect flashes in pools of liquid xenon, caused by WIMPs striking atoms, is improving exponentially. That’s a good sign for those hoping we can still catch WIMPs. “To avoid everybody dying of boredom and running out of money, you have to do it as fast as you can,” he says. But even he acknowledges that sooner or later, WIMPs will be out of hiding places. “On a 15-year view
you have to be ready to admit that, if we fail to see anything.” If dark matter isn’t WIMPs, what is it? There’s no shortage of alternatives, from lightweight particles called axions to tiny black holes left over from the big bang. Some renegades want to do away with dark matter altogether. Mordehai Milgrom at the Weizmann Institute in Rehovot, Israel, has fought a 30-year battle to explain away the need for extra matter in the universe. He accomplishes this by changing the
“We are getting to the point where we should rethink. Perhaps the dark matter is not WIMPs”
way gravity works on galactic scales – a theory called modified Newtonian dynamics (MOND). “I am certainly not surprised when I see negative reports coming from dark matter search enterprises,” says Milgrom. “Not finding dark matter at higher and higher sensitivity will only strengthen the case for MOND.” However, he wants the WIMP hunt to continue – not searching at all would leave both WIMP and MOND fans at a standstill. WIMPs have endured in part because theorists are unwilling to give up on decades of work around the idea. It’s always possible for them to tweak their models to save them from the latest data, says Loeb. “The good news about physics is that experiments set the agenda,” he says. “Theorists that have no connections with experiments miss being wrong, and that’s not physics in its actual sense.” Gaitskell isn’t bothered by playing whack-a-mole with different models. He’s no stick-in-the-mud either – he quit a previous experiment when it seemed as if it wouldn’t be able to deliver evidence for its preferred flavour of WIMPs. He maintains that there is still plenty of room for exploration. “I don’t think at this stage there has been any build-up in the science that says it’s not WIMPs,” he says. “I think it’s more that people would rather talk –It shed no light on dark matter– about something new.” Jacob Aron n 30 July 2016 | NewScientist | 9
Dark matter no-show spells trouble for WIMPs
to make a difference. This means that, for now, Ali’s team is focusing on improving their gene therapy treatment. By contrast, Editas has chosen to target a very rare form of LCA that can be treated merely by disabling a gene – a much easier task. Most eye diseases, however, would only be treatable by gene editing if it could be improved to correct mutations efficiently. As things stand, CRISPR will be of limited use for treating eye diseases even if it proves safe and effective in human trials. But the technology is advancing rapidly, so this could soon change. A modified CRISPR method was recently announced that could provide an alternative approach for correcting genes. Watch this space. n
C.H. Faham
THE HUNT lasted nearly two years, but there was not even a sniff of the quarry. One of the world’s leading dark matter detectors has failed to find any candidate particles, suggesting that the dominant model of the stuff may be on its last legs. Dark matter does not emit light and scarcely interacts with normal matter except through gravity, yet appears to account for around 85 per cent of the universe’s mass. Without its gravitational pull, galaxies would spin themselves apart. Theorists have settled on some of its basic characteristics: its particles have some mass and interact a bit with other matter via the weak –A prime target for CRISPR– nuclear force. So they called these mysterious entities “weakly Ali at University College London, interacting massive particles”, or WIMPs. who led the team. WIMPs’ occasional glancing But there’s a catch. This would blows with normal matter are what mean correcting mutations in a allow experiments like the Large gene, instead of just disabling it, Underground Xenon detector in as in Hewitt’s experiment. Lead, South Dakota, to detect their Correcting genes is much harder presence and measure their in non-dividing cells like those properties. But on 21 July, the LUX in the retina. It is not yet possible team told the Identification of Dark to correct a gene inside a high enough proportion of retinal cells Matter conference in Sheffield, UK,
that its final 20-month run had failed to make a single detection. That means LUX, dormant since May, has ruled out a large number of possible characteristics for WIMPs – to the point where some argue it might be time to abandon WIMPs altogether. “I think we are getting to the point where the limits are excluding so much of the parameter space that we should rethink,” says Avi Loeb at Harvard University. “Perhaps the dark matter is not WIMPs.” Richard Gaitskell at Brown University in Rhode Island, who worked on LUX, is still optimistic. Plans are already under way for an upgraded detector called LZ that will be 70 times more sensitive. Gaitskell says that the technology to detect flashes in pools of liquid xenon, caused by WIMPs striking atoms, is improving exponentially. That’s a good sign for those hoping we can still catch WIMPs. “To avoid everybody dying of boredom and running out of money, you have to do it as fast as you can,” he says. But even he acknowledges that sooner or later, WIMPs will be out of hiding places. “On a 15-year view
you have to be ready to admit that, if we fail to see anything.” If dark matter isn’t WIMPs, what is it? There’s no shortage of alternatives, from lightweight particles called axions to tiny black holes left over from the big bang. Some renegades want to do away with dark matter altogether. Mordehai Milgrom at the Weizmann Institute in Rehovot, Israel, has fought a 30-year battle to explain away the need for extra matter in the universe. He accomplishes this by changing the
“We are getting to the point where we should rethink. Perhaps the dark matter is not WIMPs”
way gravity works on galactic scales – a theory called modified Newtonian dynamics (MOND). “I am certainly not surprised when I see negative reports coming from dark matter search enterprises,” says Milgrom. “Not finding dark matter at higher and higher sensitivity will only strengthen the case for MOND.” However, he wants the WIMP hunt to continue – not searching at all would leave both WIMP and MOND fans at a standstill. WIMPs have endured in part because theorists are unwilling to give up on decades of work around the idea. It’s always possible for them to tweak their models to save them from the latest data, says Loeb. “The good news about physics is that experiments set the agenda,” he says. “Theorists that have no connections with experiments miss being wrong, and that’s not physics in its actual sense.” Gaitskell isn’t bothered by playing whack-a-mole with different models. He’s no stick-in-the-mud either – he quit a previous experiment when it seemed as if it wouldn’t be able to deliver evidence for its preferred flavour of WIMPs. He maintains that there is still plenty of room for exploration. “I don’t think at this stage there has been any build-up in the science that says it’s not WIMPs,” he says. “I think it’s more that people would rather talk –It shed no light on dark matter– about something new.” Jacob Aron n 30 July 2016 | NewScientist | 9