Dark Universe Debate
“One way of thinking about reality is to say that something exists if, when you shake it, it answers. Science is about finding ways to shake the universe and trying to understand the answers.”
Those were the words of Prof Jean-Philippe Uzan of the Institute of Astrophysics in Paris, one of the participants at a public lecture at the University of the Western Cape (UWC) addressing the question, Does the Dark Universe Exist?
Prof Uzan was joined for the lecture by Associate Professor Gianfranco Bertone from GRavitation AstroParticle Physics Amsterdam (GRAPPA) at the University of Amsterdam, Geneva University's Prof Ruth Derrer and the University of the Western Cape's Prof Romeel Dave. The lecture, held on 20 November 2014, included crash courses in the basics of cosmology, and why we know what we think we know.
(If you’re a bit iffy on the details, you might want to check out this handy Cheat Sheet, put together by two of UWC’s brightest stars, Astrophysics post-docs Jonathan Zwart and Prina Patel. No simpler way to learn all about Big Bangs, changing ideas of gravity and the repulsive force of empty space...)
When it comes to cosmology, our main method of shaking the universe and drawing conclusions is through our study of its light – and through that study, astrophysicists have discovered that most of the universe is dark. The stuff we can see with our eyes makes up only 5% of the whole thing, with another 24% being composed of “dark matter” (stuff with mass, but that doesn't interact with light) and 71% being “dark energy” (the mysterious energy responsible for the accelerating expansion rate of the universe). But astrophysicists actually don't know what either of these substances actually are – and in fact, many would argue that the dark sector doesn't exist, and that it's all just a misunderstanding of the laws of physics.
“We’ve learned a lot about our universe in terms of light,” Prof Uzan said, “but now we need something new for this new century. One of the great successes of science is constructing new eyes, devising new ways of looking at things - and now we can use gravity to shake the universe in a new way...and the universe answers with new pictures and new interactions.”
The debate took place on the anniversary of the birth of one of astronomy's great figures – Edwin Hubble, who provided evidence that the galaxies were moving apart. His observations of redshift (the way light is shifted towards the red end of the spectrum when a light source is moving away from an observer) helped lead to much of modern cosmology. It would have been Hubble's 125th birthday.
“The universe is full of surprises,” noted Prof Dave. “The idea that our universe is expanding was a big one - and the idea that our universal expansion is accelerating was definitely not what we were expecting. And when we have no idea what something is, we first give it a fancy name.
Prof Bertone explored a few of the ways astrophysicists are currently investigating dark matter. Xenon detectors, for example: the probability of dark matter particles interacting with regular matter are astronomically small - so large detectors are built with a lot of regular matter to interact with, and machines are conceived to detect flashes of light that may occur when dark matter interacts with electrons. And the ATLAS experiment will search for particles of dark matter among the collisions occurring in the Large Hadron Collider.
“Some of these ideas have been around for years, but it’s only now that we have the experiments to collect the appropriate data - and while we may never have the final answer, we’ll keep trying and get closer in the next ten to twenty years.”
“Of course we all want more evidence, and the idea of what dark energy and dark matter could be has occupied some of the smartest cosmologists over the last few years. So we need new minds working on it in new ways,” concluded Prof Derrer. “That’s why we need young people from Africa to study physics and go out and find new things, and solve our problems. We’re really asking for help here, people.”
The lecture was organised by UWC’s Profs Jarita Holbrook, Romeel Dave and Roy Maartens, formed part of the annual Dark Side of the Universe conference, hosted this year by UWC and the University of Cape Town, and and UCT’s Prof Peter Dunsby - a gathering of leading cosmologists and astrophysicists with an interest in the “dark sector” of the universe. Find out more about UWC’s Astrophysics Group here.