The search for dark matter and dark energy in the Universe

Philosophical transactions a November issue

Organised and edited by Carlos Frenk, George Kalmus, Nigel Smith and Simon White

What is the universe made of? How is it expanding? What is the origin of galaxies and other cosmic large-scale structures? These questions and some tentative answers were the focus of the discussion meeting on The search for dark matter and dark energy in the Universe, held at The Royal Society on 22-23 January 2003.

Astronomers have known for many years that the predominant form of mass in the universe is dark matter, that is, matter that does not emit detectable electromagnetic radiation at any wavelength. Only recently, however, has it become possible to measure how much dark matter there is. These measurements are based on surveys of unprecedented numbers of galaxies combined with careful studies of tiny irregularities in the residual heat left over from the Big Bang, the cosmic microwave background radiation. These irregularities are imprints left in the radiation by the precursors of today’s galaxies. By contrasting the properties of the primeval irregularities with the large-scale distribution of galaxies, physicists can infer the amount of dark matter in the universe. The surprising result is that the density of dark matter falls short, by about a factor of three, from the critical value required to ensure that the cosmic expansion would eventually come to a halt.

Although we now know how much dark matter there is, its identity is still unknown. It is clear, however, that the dark matter cannot be the same sort of matter that we see in stars, planets and people. Such ordinary matter makes up only about 10 percent of the total. The search for the remaining 90 percent of the cosmic mass is intense and is taking place deep underground in well shielded laboratories around the world.

The combination of microwave background and galaxy data has recently produced another, perhaps even more perplexing result: our universe must contain not only dark matter but also a new form of energy which, in the absence of a better name, is often called “dark energy.’ This gives rise to a cosmic repulsive force which counteracts the effect of gravity. Dark energy has dominated the overall evolution of the universe for the past 8 billion years or so, causing it to expand at an ever increasing rate. The origin of the dark energy is a profound mystery.

The Royal Society Discussion meeting brought together foremost world experts in cosmology and particle physics. They discussed the evidence for dark matter and dark energy, their effect on the expansion of the universe and on the properties of galaxies, speculate on the origin of the dark energy and describe experimental searches for the dark matter particles using ultra-high-technology devices. The meeting was pervaded by the certain knowledge that these are really exciting times in the quest for understanding the origin of our universe.