The year was 1991 and the place Ahmedabad. The occasion — an international meeting on gravitation and cosmology. Cosmology is the science of the origin and evolution of the universe. By 1991, of course, a consensus was emerging that the universe originated in a super-explosion, often referred to as the big bang. A discussion was on concerning the mysterious dark matter and its relevance to the popular idea of the big bang model of the universe.
Before coming to the modern query as to what the dark matter is made of, let us go back more than two millennia to the Greece of Archimedes and Pythagoras. The followers of the latter who called themselves Pythagoreans maintained that the Earth went round a central fire and the Sun lay away from the central region. The critics asked the natural question: Why don’t we see the central fire? To silence them, the Pythagoreans argued that our view of the fire was blocked by a “counter-Earth” that also moved round the central fire. In due course this explanation was challenged by the query: Why don’t we see the counter-Earth? To this the reply given was that on a globe Greece was on the opposite side facing away from the counter-Earth. Some sceptics then shot down this explanation by voyaging to the other side where, of course, they failed to find the counter-Earth.
With this introduction, let us now return to dark matter. Dark matter was being discovered since the 1970s and its significance for cosmology was being appreciated as more and more of it was being found. One could say that the dark matter to the cosmologist was like black money to the economist. Although black money is not declared to the taxman, its amount can be estimated from the economic activity it generates. In the same way, the dynamic activity in the cosmos gives a quantitative estimate of how much matter is around to generate it. Since most of it is invisible, it is called “dark” matter. The quantitative estimate of dark matter puts it at around some five to six times the density of visible matter. This is like telling the astronomer that what matter he is able to observe with all his telescopes is a small fraction of what there really is. In the early days the principal candidates for dark matter used to be black holes, planet-like objects, low mass objects that could not become stars, etc.
However, the matter does not end there! There is another constraint that the big bang theory imposes. The theory is expected to explain how the light atomic nuclei like deuterium, helium, etc came into existence in the universe. The answer is that they were formed when the universe was a few seconds old. They were formed by the fusion of free particles like neutron and protons. But the process works only if the total density of the neutrons and protons is less than a certain fixed limit. And that limit is exceeded if we assume that all dark matter is made of neutrons and protons. In short, the dark matter we find is not made of chemical elements that we observe everywhere around us. So like the astronomers, the physicists and chemists also do not have a clue as to what this matter is made of.
What then is this matter made of? The answer given is: It is made of some strange type of particles different from neutrons and protons whose observation or creation in the laboratory has not been possible so far. This is given the name of non-baryonic matter. Non-baryonic because neutron and proton are collectively called baryons.
This situation is bizarre from a scientific point of view. What one is asked to believe has not yet been witnessed in the laboratory but one must believe in its existence if one wishes to keep the big bang theory alive. Indeed I am reminded of the Hans Andersen story The Emperor’s New Clothes in which the emperor was given a suit to wear which could allegedly be seen only by those who have not committed a sin. The suit in reality was composed of nothing and the emperor was actually naked. But neither he nor any of his courtiers would admit so for the fear of being declared sinful! Only a child who had no stake in the issue asked his mother: “Why is the king naked?”
In science the tradition has been to advance physical theories and check their predictions against a suitable test. If the prediction is vindicated by the test, well and good. The theory survives and may be further tested by more sophisticated tests. If the prediction fails, the theory is abandoned or seriously modified. But it is rare to keep the theory intact and change some crucial assumption. The above example is of this kind. Rather than give up the big bang theory, most cosmologists want to introduce the hypothesis of non-baryonic matter.
How deeply the big bang theory is now entrenched in the mindset of most cosmologists was brought home to me at the Ahmedabad meeting when in the course of various arguments, I put in the above example. How is it, I asked, that most cosmologists have uncritically accepted the hypothesis of non-baryonic matter to account for dark matter? Why not try the alternative of the more familiar baryonic option, with which physicists are familiar and which is known to astronomers through their observations? Why must we go for an esoteric option that has neither yet been observed in the cosmos nor detected in a terrestrial laboratory?
I thought my intervention was consistent with the traditional scientific approach. But I was in for a shock! One of the younger generation of cosmologists shot his hand up to object. If we are allowed to accept, he said, only the familiar forms of matter, then why not assume that dark matter is all made of television sets?
While this remark may have some kind of twisted logic, it illustrates the limits to which one may go in order to defend the popular dogma.
