Under the Microscope/Prof William Reville:Science is the slave of observation. All hypotheses and theories must accommodate data acquired by observing the natural world no matter how awkward this data may be.
Science has learned a lot about the natural world during the past 400 years but yet it seems that an infinite amount remains to be discovered.
For example, science knows little or nothing about the composition of 95 per cent of the universe, which is made of dark matter (30 per cent) and dark energy (65 per cent). Science can make intelligent guesses about dark matter but the nature of dark energy remains entirely mysterious.
Science has known about dark matter since 1933. One important piece of evidence is the rate at which stars in spiral galaxies rotate about the galaxy centre. It is clear from such observations that galaxies contain far more matter than is visible. This is the dark matter of the galaxy, so called because it cannot be directly detected. It is electrically neutral and does not emit light or other electromagnetic radiation. However, it is attracted to other matter by the force of gravity.
The familiar world around us is composed of 92 different types of atoms. But only 5 per cent of the universe is composed of such atoms and the remaining 95 per cent is composed of dark matter and dark energy. No one knows the nature of dark matter. The atoms of our familiar world are made of fundamental particles called protons, neutrons and electrons. These cannot constitute dark matter because calculations show that particle synthesis during the big bang generated far too few such particles to account for dark matter. Until recently, many scientists felt that ghostly particles called neutrinos that are present in gigantic numbers in the universe and that interact very little with other forms of matter, could constitute the bulk of the dark matter. However, it is now estimated that neutrinos constitute no more than 0.3 per cent of the matter of the universe. Neutrinos are categorised as "hot" dark matter because in the early universe they moved too fast to become incorporated into settled cosmic structures.
The fundamental nature of dark matter must be "cold" - as yet undiscovered particles that moved relatively slowly when formed. The standard elementary particles model contains no candidate for cold dark matter but developments of the model suggest several plausible particles.
One development, called super symmetry, postulates a new family of heavier and more sluggish "super partners" for every known elementary particle. One super partner, called the neutralino, is postulated as the basic dark matter particle. It has no electrical charge and is unaffected by electromagnetic forces. Theoretical calculations estimate that the total mass of neutralinos formed in the big bang matches the mass of dark matter inferred to be present in the universe.
Nobody has yet detected dark matter. Currently several projects to detect dark matter are ongoing in different parts of the world. Although the great majority of hypothesised neutralinos will pass through a slab of detector matter without interacting, the occasional one will hit an atomic nucleus in the slab and transfer a small amount of its energy to that nucleus. The excited nucleus will quickly release the absorbed energy which will be recorded by the detector.
We can envisage the dark matter in our galaxy as a stagnant gas of neutralinos through which our solar system passes at a speed of 220 km per second. Although a billion dark matter particles pass through every square metre on Earth per second, the extent of interaction of nutralinos with ordinary matter is very low - 0.0001 to 0.01 interaction events per kg per day. Instruments capable of detecting such low interaction rates are only now coming on stream.
The main difficulty with detector design is screening out spurious background effects. The metals out of which detectors are built contain traces of radioactivity and the Earth is bathed in cosmic rays and these would register the same signal in the detector as neutralinos. In order to identify dark matter both of these unwanted backgrounds must be reduced a million fold. The radioactive background can be reduced by purifying the metals and cosmic rays can be screened out by locating the detectors deep underground.
It will be difficult to pick out a unique neutralino signal. Luckily, the signal strength should vary between summer and winter, which will help in identification. Our solar system rotates around the centre of our galaxy and our Earth rotates around our Sun. Our solar system experiences a "head wind" as it moves through the dark matter but the movement of our Earth around the Sun means that, for example, in the northern hemisphere the head wind will be greater in the summer than in the winter. The dark matter signal should therefore be greater in the summer than in the winter. Background noise will not change with the seasons.
Dark energy only entered the picture in 1998 when astronomers surveyed exploding stars in distant galaxies. They found that these supernovas were further away than they should have been. The new results implied that a mysterious force was acting against gravity and causing galaxies to recede from each other at ever greater speeds. They called this mysterious force dark energy and it is a force that no one understands.
One is reminded of the words of the famous British physiologist JBS Haldane when he said, "It seems to me that not only is the universe queerer than we suppose, but it is queerer than we can suppose."