THE EYE’S THE LIMIT

We can’t see most of the universe, but that doesn’t make it invisible

THE naked eye is not a reliable guide to the world. If you believed your eyes, you might conclude that the head louse is pretty much the smallest thing in existence. Fortunately, we can overcome this sensory limitation. The invention of the microscope, for example, allowed Robert Hooke and others to document hitherto unseen aspects of nature – details of the flea and other disease-bearing parasites, and of course the existence of microbes such as bacteria. Without that knowledge, we wouldn’t be living as long as we do. If we had stuck with our natural vision alone, we would also have very little conception of what is going on at the other end of the size scale. The invention of the telescope changed our view of the universe. And eventually, we learned to use spectrographs to split light into its component parts and so reveal the chemical composition of stars and distant planetary atmospheres. And yet, for all our technological sophistication, there is still more invisible stuff to uncover. Dark matter and dark energy together make up 95 per cent of the universe. Neither shines in the conventional way, so we have had to be ingenious to discern what our eyes cannot see. That’s how dark matter came to light in the first place: Swiss astronomer Fritz Zwicky was watching galaxy clusters in the 1930s and realised that their motion could only be accounted for by the gravitational pull of invisible mass.  Today, we suspect dark matter is made of particles that could be detected by a series of ever more sensitive experiments. The front runner is LUX, a vast vat of xenon atoms wired to equipment to detect the energy released by a collision between a dark matter particle and a xenon nucleus. Its trial run last October turned up nothing, but the researchers are undaunted. “Finding the Higgs boson took almost 50 years,” says LUX team member Henrique Araujo of Imperial College London. Dark energy remains another unseen, inexplicable component of the universe. The latest instrument on the case is the Dark Energy Survey, which will analyse light from over 300 million galaxies to try to uncover what dark energy really is. Our assault on the invisible universe also involves observing how galaxy clusters evolve, as well as looking at how structures in the cosmic microwave background radiation have changed in the last 13 billion years and how light’s path warps as it travels across the heavens. With that multi-pronged approach, we should avoid being fooled by a distortion in any one of our cosmic “If you believe your eyes, the head louse is the smallest thing in existence” microscopes. Josh Frieman, director of the Dark Energy Survey sees great progress in the coming years. “As Abe Lincoln might have said, you can perhaps fool one dark energy probe, but not all of them,” he says. Our technology for shedding light on hidden realities has come a long way since Robert Hooke’s efforts in the 17th century. Today, a scanning transmission electron holography microscope, for instance, allows us to see down to a resolution of 35 trillionths of a metre. Meanwhile the Large Hadron Collider allows us to probe things that don’t even have a physical presence in the everyday world. Eventually the dark energy and dark matter searches should complete the picture, bringing the invisible essence of the universe within our sights.  ■