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Dark matter and energy

Dark matter and energy

Physics gets dark and exotic

06 Jan 2000

Two of the outstanding challenges in physics identified in our millennium survey last month were the nature of “dark matter” and a proper understanding of nuclear structure. This month we look at these challenges in greater detail.

Dark matter is matter that does not interact with electromagnetic radiation: it cannot be seen with telescopes and only reveals itself through its gravitational interactions. Astronomers first became aware in the 1930s that our Milky Way galaxy was rotating faster than could be explained by the gravitational influence of the stars and dust that it contained. It later became clear that more than 90% of the matter in the universe – and possibly as much as 99% – might be dark.

Some of the dark matter might be in the form of ordinary or “baryonic” matter made primarily of neutrons and protons – such as failed stars or black holes. A small fraction might be in the form of neutrinos, but the majority of dark matter is non-baryonic and beyond the Standard Model of particle physics. Neutrinos with mass are also beyond the Standard Model but not as far beyond as neutralinos, axions and other exotic particles. Another possibility for this missing mass is “dark energy” in the form of a cosmological constant.

In this issue Nigel Smith and Neil Spooner of the UK Dark Matter Collaboration describe efforts to detect dark matter in underground experiments. Although the experiments sound easy – connect some photomultiplier tubes to a crystal and count the flashes – the challenge of isolating a dark-matter signal amongst a myriad of other background effects is formidable. However, dark-matter searches can be performed by teams much smaller than those found in traditional particle-physics experiments. The unequivocal detection of a dark-matter particle would be a tremendous boost for particle physics and cosmology, although there would doubtless be calls for it to be confirmed in an accelerator-based experiment.

Back in the baryonic world, meanwhile, nuclear physicists are probing more and more exotic nuclei. Over 7000 different nuclei are thought to exist, but only 3000 of these have been created and studied in the laboratory, and only 260 of the total are stable. As Isao Tanihata explained last month, there are various models of the nucleus that work well in different regions of the chart of the nuclides – something that is a source of both pride and frustration to nuclear physicists. In this issue Paddy Regan and Bertram Blankdescribe attempts to synthesize and study nuclei at the very limits of stability – the proton and neutron driplines.

These exotic species can be detected among the debris of collisions between stable nuclei. However, if enough of these short-lived nuclei could be isolated and then re-accelerated in a radioactive beam, it would be possible to create and study even more exotic nuclei. Plans for new facilities to perform such experiments are well advanced in various nuclear-physics labs around the world.

As we embark on the 21st century, dark matter and nuclear structure could be two of the outstanding challenges in physics to be overcome first.

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