Why our ignorance of the dark universe doesn't matter

BELIEVE IT or not, scientists do not understand the nature of 96 per cent of the fabric of the universe.

There are two components to this large mysterious fraction – dark matter and dark energy. In this article I will deal only with dark matter.

The fact that scientists are not fazed by not yet understanding such a large fraction of the universe is testament to the confidence we have in the scientific method.

Dark matter is so called because we cannot detect it directly. We know it exists only because of indirect evidence – the gravitational tug it exerts on ordinary matter, the stuff in the universe we can see. The concept of dark matter is not new. In 1933 the Swiss astronomer Fritz Zwicky proposed that distant galaxy clusters he was observing would fall asunder but for the extra gravitational pull of a mysterious invisible mass. Astronomers later verified Zwicky’s hypothesis by measuring the movements of stars around distant galaxies.

The Earth and all that we see on it, and all that we see through our telescopes when we look into space (stars, galaxies, dust etc), is made of ordinary matter, but ordinary matter accounts for less than 5 per cent of the fabric of the universe. It is now believed that invisible dark matter pervades the entire universe, forming a cosmic skeleton on which ordinary matter coalesces into galaxies, each composed of billions of stars and planets. In other words, dark matter gives the universe its overall structure. Astronomers estimate that the overall fabric of the universe is partitioned as follows: dark energy 74 per cent, dark matter 22 per cent, ordinary matter (made of atoms) 4 per cent.

Not very long ago, ordinary matter was presumed to make up the entire universe, but we now know it is relatively rare. The basic building block of ordinary matter is the atom, a combination of mass and forces that hold the mass together. The atom is composed mainly of subatomic particles called protons and neutrons (collectively known as baryons), and electrons (a member of the family of leptons).

The forces that hold these particles together are the strong nuclear force that holds the protons and neutrons together in the centre (nucleus) of the atom and the electromagnetic force that holds the electrons orbiting around the nucleus. The currently known elementary particles and their interactions are described in a model called the “Standard Model” (a simple explanation of elementary particles may be viewed in a film of a recent lecture at UCC by Cormac O’Raifeartaigh, of WIT, on understandingscience.ucc.ie)

The fundamental nature of dark matter is unknown, but physicists are now convinced that most dark matter must consist of non-baryonic particles and that it does not have an atomic structure. Physicists conceive of dark matter as a gas of weakly interacting massive particles (Wimps), yet to be discovered, that can pass through ordinary matter like ghosts through walls. If these particles are discovered, the Standard Model will have to be expanded to accommodate them.

Dark matter is likely to be made of a variety of particles whose nature will illuminate some of the most profound mysteries in science as well as explaining the “missing” mass in the universe eg, some dark matter particles may explain why most ordinary matter is not radioactive and others may explain why time always runs forward. An exciting experiment to detect dark matter is under way, buried deep in the earth beneath the forests of Minnesota. The project is called the Cryogenic Dark Matter Search (CDMS) experiment and it reported two events in December 2009 that could be the first direct detection of dark matter Wimps.

Although dark matter particles constantly stream through the earth without interacting with ordinary matter (except through the weak force of gravity) physicists predict that very occasionally a dark matter particle will physically nudge an atom of ordinary matter and this is what the CDMS experiment is trying to detect. The experiment is located deep within the earth in order to screen out the otherwise confounding background effect of cosmic rays. The detector itself is made of crystals of silicon and germanium. The crystal is chilled to a temperature close to absolute zero (near minus 273.15 degrees Celsius) in order to stop the silicon and germanium atoms from jiggling about and thereby making it possible to detect any slight nudge given by an interacting dark matter particle.

The scientists report that they are 75 per cent certain that the two events they recorded bear the signatures of Wimps and a 25 per cent chance they are false positives caused by stray background radiation. Dark matter particles are predicted to be quite big – 30 to 60 times the size of a proton. The experiment is continuing with the detector tuned to a higher level of sensitivity. The scientific world waits with bated breath.

William Reville is UCC’s associate prof of biochemistry and public awareness of science officer – understandingscience.ucc.ie

“Dark matter is likely to be made of a variety of particles, which will illuminate some of the great mysteries in science – as well as why most ordinary matter is not radioactive and why time always runs forward