We will conclude this series with a description of what most people consider to be the deepest mystery in theoretical physics today. Cosmological observations suggest a preposterous composition for our universe in which nearly 90 per cent of the matter does not emit any form of radiation, and of which nearly two-thirds is in an exotic form that exerts negative pressure! The normal matter — of which you and I are made up of — makes up only about 5 per cent of the universe. The universe is Copernican in every sense of the term — not only are we not at the centre of anything, we are not even made of the most dominant form of matter.

The evidence for this weird composition comes from using different techniques and observations but the basic principle is to look for gravitational effects which are unaccounted by the visible matter. Consider, for example, a class of galaxies known as disc galaxies in which most of the visible matter (stars, gases, etc.) is contained in a plane in the form of a circular disc. In the outskirts of such galaxies there exist tenuous hydrogen clouds orbiting the galaxy in the plane of the disc. If we see such a galaxy edge-on, then the hydrogen clouds at two sides of the galaxies will be moving in opposite directions — one towards us and the other away from us. Using spectroscopic techniques, it is possible to determine the speed of these clouds. Knowing the speed and the location, one can estimate the amount of gravitating matter present in the galaxies influencing the motion of the cloud.

Such studies have led to a remarkable result. Disc galaxies seem to contain five to ten times more matter than the visible matter. This dark matter also seems to be distributed over sizes at least three to four times bigger than that of the visible matter. One gets the impression that the visible matter is only a tiny speck embedded in the middle of a vast dark matter structure. Similar results are obtained from the study of bigger objects. Clusters of galaxies, for example, seem to contain at least 10 times more matter than is visible. From the clustering properties of the dark matter, one can infer that these are most probably made of weakly interacting massive particles, which are yet to be discovered in the lab.

The total amount of matter, including this dark matter, constitutes a cosmic energy density which is about 30 per cent of what is required to eventually halt the expansion of the universe and make it recontract again. The latter density is called the critical density and is about 10-29 gram per cubic centimetre. There is, however, very compelling evidence from the study of cosmic microwave background radiation that the total energy density in the universe is actually close to the critical energy density to within a few per cent. In fact, this study also shows that the unaccounted difference between the energy density of dark matter and the critical density is provided by some form of an exotic fluid permeating the universe that is nowadays called the dark energy. It differs from dark matter, in two vital details.

First, it is unclustered at astrophysical scales and is distributed very smoothly while the dark matter clusters around, say, galaxies. Second, it exerts negative pressure. To understand the concept of negative pressure, you need to recall that the familiar gas pressure is always positive. For example, if you fill a balloon with some gas, the latter can make the balloon expand by doing work on it. In the process, the energy content of the gas will decrease. On the other hand, if the gas has negative pressure, then it can make the balloon expand and at the same time keep its energy content constant! This is precisely what is happening in the universe. The dark energy with negative pressure is driving the expansion of the universe today but without any decrease in its own energy content. This is in sharp contrast to normal matter and radiation, the energy densities of which will decrease as the universe expands.

One immediate consequence of this strange feature is that the expansion of the universe will be accelerated in the presence of dark energy. So, if we can measure the rate at which the universe was expanding in the past, and compare it with the current rate of expansion, we can directly verify the existence and nature of dark energy. It is possible to do this using the distant supernovas as powerful beacons of light. Such measurements performed over the last decade or so have dramatically confirmed the existence of dark energy as the dominant source of expansion of the universe.

So what is this dark energy? Nobody knows, and hence this constitutes the most perplexing problem in theoretical physics today. One of the simplest possibilities is the addition of a particular term (that is, the cosmological constant) to the equations in Einstein’s general theory of relativity. This term was originally introduced — and later abandoned — by Einstein himself, both for wrong reasons! The cosmological constant can behave like a fluid with negative pressure and can completely account for all observations. However, to do this, the value of this constant has to be adjusted to enormous precision, to one in 10123 parts. It is not clear at present why such a fine-tuning is required.

T. Padmanabhan is an astrophysicist at the InterUniversity Centre for Astronomy and Astrophysics, Pune