Dark matter: the mysterious physical substance that makes up the majority of our universe still cannot be identified. However, considerable effort is being made to try to identify it. To no avail. Dan Hooper is one of the world's leading experts on the subject. He is a research associate in theoretical astrophysics at the Fermi National Accelerator Laboratory and Associate Professor of Astronomy and Astrophysics at the University of Chicago. In this article, he tells us his researcher's blues...
Ahe last few decades have ushered in an astonishing era in the science of cosmology. A wide range of high-precision measurements has allowed us to reconstruct the history of our universe in great detail. And when we compare different measurement-the rate of expansion of the universe, patterns of light released by the formation of the first atoms, the spatial distribution of galaxies and galaxy clusters, and the abundance of various chemical species-we discover that they all tell the same story, and that they all describe the same series of events.
Frankly, this line of research has been more successful than I think we would have liked. We know more about the origin and history of our universe today than we would have imagined we would learn in such a short period of time a few decades ago.
But despite these considerable successes, there is still much to learn. And in some respects, the discoveries of the past few decades have raised as many new questions as they have answered. One of the most vexing mysteries lies at the heart of what makes up our universe. Cosmological observations have determined with great precision the average density of matter in our universe. But this density is much greater than that of ordinary atoms.
After decades of measurement and debate, we are now convinced that the overwhelming majority of the matter in our universe -- about 84 per cent -- is not composed of atoms or any other known substance. While we can track its gravitational pull, which allows us to say clearly that it is there, we simply don't know what it is. This mysterious thing is invisible, or at least almost invisible. For lack of a better name, we call it "dark matter", but naming something is very different than understanding it.
Astronomers map dark matter indirectly, through its gravitational attraction to other objects. NASA, ESA and D. Coe (NASA JPL/Caltech and STScI)
For almost as long as we have known that dark matter exists, physicists and astronomers have been devising ways to learn what it is made of. They have built ultra-sensitive sensors, deployed in underground mines deep, to measure the light impacts of individual dark matter particles colliding with atoms.
They have built exotic telescopes - sensitive not to optical light but to gamma raysto cosmic radiation and to neutrinosThis is to search for the high-energy radiation that would be generated by the interactions of dark matter particles.
And we looked for signs of dark matter using incredible machines that accelerate beams of particles - typically protons or electrons - to the highest possible speeds and then crush them together in an attempt to convert their energy into matter. The idea is that these collisions could create new and exotic substances, perhaps including the types of particles that make up the dark matter of our universe.
Just ten years ago, most cosmologists - including myself - were reasonably confident that we would soon begin to solve the dark matter puzzle. After all, there was an ambitious experimental program on the horizon that would allow us to identify the nature of dark matter and begin to measure its properties. That programme included the world's most powerful particle accelerator - The Large Hadron CooliderThe Large Hadron Collider, the Large Hadron Collider - and a host of other new experiments and powerful telescopes.
CERN's experiments are trying to focus on dark matter - but so far nothing has been found. CERN
But things didn't turn out the way we expected. Although these experiments and observations were carried out as well or better than we could have hoped, the discoveries did not come.
Over the past 15 years, for example, experiments designed to detect individual particles of dark matter have become a million times more sensitive, yet no sign of these elusive particles has emerged. And although the Large Hadron Collider has, with the exception of the Higgs particleThe new particle, which has been found to have excellent technical performance, no new particles or other phenomena have been discovered.
The persistence of dark matter has left many scientists both surprised and confused. We had what seemed to be very good reason to expect dark matter particles to be discovered now. And yet the hunt continues, and the mystery deepens.
In many ways, we have no more outstanding issues today than we did 10 or two years ago. And sometimes it seems that the more accurately we measure our universe, the less we understand it. Throughout the second half of the 20th century, theoretical particle physicists often succeeded in predicting the types of particles that would be discovered as accelerators became increasingly powerful. That was really impressive.
But our foreknowledge seems to have come to an end - the long-anticipated particles associated with our favourite and most motivated theories have stubbornly refused to appear. Perhaps the discoveries of these particles are on our doorstep and our confidence will soon be restored. But at the moment, there seems to be little support for such optimism.
In response, physicists are flocking back to their boards, reviewing and revising their assumptions. With bruised egos and a little more humility, we are desperately trying to find a new way to make sense of our world.
Dan HooperAssociate Scientist in Theoretical Astrophysics at the Fermi National Accelerator Laboratory and Associate Professor of Astronomy and Astrophysics, University of Chicago.