ONE OF the most remarkable and exciting developments in physics has been the interaction between particle physics and cosmology. Remarkable, because it combines the study of the very small, namely, particle physics, with the study of the very large, namely, cosmology. Exciting, because each is contributing immensely towards answering fundamental questions in the other.
Cosmologists have been groping to understand the universe in the very first second after the Big Bang, because much of what was to happen subsequently was determined then. However, their task has been difficult because of a lack in understanding the behaviour of matter under extreme conditions (temperatures of the order of 10 billion degrees Centigrade). Particle physics has proved to be extremely useful in providing an insight into phenomena at such high energies.
And, if particle physics has enhanced the understanding of the universe, cosmology has provided particle physics with a number of inputs. To understand the behaviour of matter, its properties must be studied at very high energies, something that is not achievable in particle accelerators -- the expensive laboratories of particle physicists. The only "laboratory" allowing such energy is the very early universe, when matter was subject to extreme conditions. Cosmology and particle physics, therefore, are locked in a symbiotic relationship.
In April 1992, data from the satellite COBE (Cosmic Background Explorer) caused a stir among cosmologists and particle physicists. Instruments aboard the satellite detected slight fluctuations in background microwave radiation, which was supposed to be the residual radiation from the Big Bang. These fluctuations provided the first evidence of how an initially smooth universe could have evolved into the "lumpy" universe that we can observe today, replete with stars and galaxies.
This observation was significant not only because it supported the Big Bang theory, but also because it provided strong support to several theoretical ideas that hitherto were unsupported by experiments. The inflationary theory, which predicts a radically different behaviour of the universe in the first second, was proposed as an elegant synthesis of particle physics and cosmology to get around some of the conundrums of the Big Bang theory. It is strongly favoured by COBE's findings as is the existence of dark matter.
Since the 1930s, astrophysicists have conjectured there could be a great deal more matter in the universe than was believed. The only observable effect of such matter would be gravity, whose role is central to the evolution of the universe and the formation of structures such as galaxies. Now, COBE seems to be telling us that there is a good chance that dark matter exists.
While cosmologists have gone back to their computers to simulate their favorite models of dark matter to explain COBE data, the experimentalists have accelerated detector development to "see" the dark matter. The only problem seems to be no one knows just what to look for.
Till the 1980s, the candidates for dark matter were few and their properties well known. Then came a veritable explosion in particle physics. Theories named Supersymmetry, Grand Unified and String were proffered, each with its own baggage of particles. Soon there was a zoo of particles featuring exotic names and even more exotic properties. Axions, photinos, quark nuggets, WIMPs, Machos and a host of other hypothetical entities were added to the list of possible hopefuls for dark matter.
Detection of such matter would have a tremendous impact on both cosmology and particle physics for this would answer several questions regarding the evolution of the universe. Furthermore, conjectured, speculative theories in particle physics would either be confirmed or ruled out.
The philosophical implications, too, would be far-reaching, for detection of such matter would be a fitting response to anthropocentrism (regarding humankind as the centre of the universe). Not only would we not be at the centre of the universe, we wouldn't even be made of the same kind of matter as the vast majority of the universe!