Under the Microscope One of the most impressive characteristics of science is the way it can hold its nerve when it comes across things it doesn't yet understand. Dark matter is one example of this. This mysterious substance constitutes most of the matter in our universe but science has yet to determine what it is composed of, writes Prof William Reville.
Up until now the presence of dark matter in the universe could only be inferred but, just last month, August 2006, astronomers at last reported a direct observation of dark matter.
Dark matter is matter that neither emits nor reflects enough electromagnetic radiation (light, X-rays and so on) to be directly detected but whose presence may be inferred because of its gravitational effects on visible matter. The familiar ordinary matter that we see is the universe is called baryonic matter and is composed of particles - mostly protons, neutrons and electrons. Dark matter is non-baryonic, but its detailed composition is unknown. Ordinary baryonic matter constitutes only 5 per cent of the composition of the universe. Dark matter accounts for 25 per cent of the universe and a totally mysterious entity called dark energy accounts for 70 per cent.
For the past 70 years science has known that there is much more mass in galaxies than can be seen. In 1933, Fritz Zwicki discovered that spiral galaxies rotate at speeds that can only be explained if the total mass of the galaxy is several times greater than the sum of its visible stars and dust. The invisible mass is the dark matter - invisible because it neither emits nor reflects light. Dark matter doesn't interact with ordinary matter except through gravity and is not found alone but typically accompanies ordinary matter.
Marusa Bradac of the Kavli Institute for Particle Astrophysics and Cosmology at Stanford, Douglas Clowe of the University of Arizona, and others, decided to hunt for dark matter using a give-away visible effect called gravitational lensing - mass curves space around it and light coming at us from behind a large mass is bent, stretching images of objects behind the mass.
The astronomers examined the after-effects of an event that happened 100 million years ago when two galaxy clusters three billion light years away passed thorough each other at about 10 million miles per hour. Each galaxy cluster is composed of ordinary baryonic matter and dark matter. The baryonic particles, unlike dark matter, interact with each other in several ways. It would be expected therefore that, when the two clusters collided head on, the movements of the baryonic components in each cluster would be slowed down by their interactions whereas the dark matter components would rush ahead relatively unimpeded. This would result in a relative separation of dark matter from its ordinary matter counterpart.
The astronomers predicted that they would find two dark-matter regions moving in opposite directions with two regions of hot baryonic gas lagging behind. And, when they compared images from NASA's Chandra X-ray Observatory, the Hubble space telescope and other instruments, this is precisely what they found. The researchers claim that their results are the first direct detection of dark matter.
The results will be published in a forthcoming issue of Astrophysical Journal. Consider the following analogy. Imagine that each of the two galaxies on collision course is composed of people. The people are of two sorts - gregarious people who talk and interact with each other and loners who just march ahead without interacting. The gregarious people represent ordinary baryonic matter and the loners represent dark matter.
When the two galaxies merge and move into each other the gregarious people will be slowed down by their interactions with gregarious people from the other galaxy, while the loners will continue to rush ahead regardless. This will cause a relative separation of the two types of people.
Some philosophers claim that science does not reveal the true nature of the universe. Science, they claim, is simply a way of talking about the world agreed among scientists, a way that happens to be handy for doing useful technological things. This claim is wrong. Science does reveal, at least partially, the true nature of reality.
Dark matter illustrates this. It is an awkward property of the world whose nature is not understood by science. If science were just an agreed way of talking, dark matter would be "talked away" by scientists. But no, science acknowledges dark matter despite the embarrassing fact that it is not understood. In time, science will uncover the fundamental nature of this mysterious matter.
The scientific investigation in the fundamental nature of light is a historical example of how science must sometimes live with a mystery until eventually it solves the puzzle. Isaac Newton (1643-1727) postulated that the fundamental nature of light is particulate while Christian Huygens (1629-1695) postulated that light is a wave. The light diffraction experiments of Thomas Young (1773 - 1829) seemed to settle the question - light is clearly a wave. However, in 1905 Albert Einstein showed that under certain circumstances light behaves as a stream of particles. This posed a mystery. The mystery was eventually solved by the British physicist Paul Dirac in the late 1920s when he invented quantum field theory. This explained how light gives you a wave-like answer if you ask it wave-like questions, and a particle - like answer if you ask it particle-like questions.
William Reville is associate professor of biochemistry and public awareness of science officer at UCC - http://understandingscience.ucc.ie