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Saturday Feature
The missing and dark constituent of cosmos
Syed Fattahul Alim

          Matter is still the ultimate mystery that man is yet to crack. What is then all this progress of science about, if the essence of the basic building block of the universe is still beyond his grasp? To the layman, the scientists, or more specifically, the physicists, whose job is to discover the dynamic properties of matter as well as to go to its heart to know how it came to be in the first place, have by now come to learn everything about it. There is, however, no reason why the man in the street will not consider the men of science with such trust and even awe after all the dazzling progresses made in the field of technology. The physicists, they know, have unlocked the secret of power that keeps the sun and the stars burning. They also know that the universe in which they live was not like the way it is now since eternity. Rather it had a very explosive beginning in time called Big Bang, which is already a household word, and it may also going to have an antithesis in the form of a similar Big Crunch. The scientists have even calculated the time the universe has already traversed to reach its present stage since that Big Bang. So, are there any more things yet to be known about the universe and the basic substance of which it is made?
But the scientists are still not that sure about the whole matter of matter. All the many forms of matter they have already come across cannot still give a foolproof picture of the universe. So, they think, there must be other forms of matter that gives the universe its present shape.
Cosmologists, who are in the habit of peering into the heart as well as the edges of the universe, know that the visible mass of the universe cannot fully account for its present structure and answer the questions regarding the formation of the galaxies. There must be some amount of matter that is always missing the eye of the astronomers and cosmologists. The astrophysicists and the cosmologists have even given a name to this matter-the missing matter. But what is this missing matter made of? This is still an unsolved riddle. However, scientists are claiming that they have been able to make a major breakthrough in their search for discovering the characteristics and properties of missing matter. The method the cosmologists now using to know the dark matter that still the eludes the eyes of the astronomers is one of as if retracing the evolutionary path of the universe on computer. In fact the scientists are using the simulation technique to achieve this extraordinary feat. And they are very hopeful, too in this regard. Carlos Frenk, the Mexican-born astrophysicist of Durham University, England, who is also the principal investigator of the "Virgo Consortium" (the team of astrophysicists that uses supercomputer to simulate the universe) expresses his optimism like this:
"Computer-simulated universes are a very powerful tool because they allow you to produce material evidence for what various assumptions about the universe translate into, and then you can take this material evidence and compare it against reality. Because the universe is so complex, most mathematical treatments require many approximations and simplifications, so they are of limited applicability. Yet with a computer simulation you don't need to make any of those approximations. You solve the equations in the full generality, so it's a very appealing activity for theoreticians to do".
According to Einstein's model the universe is smooth through to the end. It has really no scope of being inhomogeneous according to his view of the structure of space-time. But in reality it is hardly a homogeneous pace. The universe has galaxies and clusters of galaxies strewn all over. Moreover, a smooth universe a la General Theory of Relativity as expounded by Einstein would be a rather sterile one. In that case, there would at least be no humans to discuss its structure at the moment. But thank God, His whole scheme of the universe was not a barren exercise because it had its kinks and knots. In fact, what the astrophysicists think there had been a small departure from the assumed early condition of the universe at its beginning. Due to this initial imperfection, the matter, of which the early cosmos was constituted, could not be distributed homogeneously over the space being created along with the evolution of the universe. So, there developed regions of space where matter was densely distributed, while at others it was thin. But the departure we are talking about was very insignificant. It was so small that to start with it did hardly affect the basic assumption of a rather homogeneous universe. But the tiny difference as it was made all the difference. And thank our stars that the universe failed to be impeccably homogeneous at the time of its birth. And so humans are here to think and retrace the evolutionary cycle of the universe.
It would be interesting to again hear from professor Carlos Frenk:
"In April 1992, there was a very important discovery in cosmology that made the headline news all over the world-the discovery of ripples in the structure of the microwave background radiation. These ripples are nothing other than these little inhomogeneities we are talking about.
In the real universe, the whole evolutionary process is driven by gravity and gravity is produced by mass, so in order to create a simulated universe, we need to know what sort of mass our universe has. One of the critical discoveries of astronomers in the last 25 or 30 years is the realization that there must be more mass in the universe than is accounted for by what we can see.
That means most of the mass in the universe is made up of what we call dark matter, which simply describes matter that doesn't shine. To perform a successful computer simulation one needs to specify: what is the dark matter? What is it made of and how much is there?
The amazing thing is that if you make different assumptions you end up with different universes. So what many of us have been working on for the last 20 years is exploring various possibilities, evolving them in the computer to the present, and picking out those that look more like the real universe than others. Each mock universe that's made up in the computer can be compared with the real universe in a variety of ways. You could look at different properties of the real universe and you could ask, "How many lumps are there?" or "How big are the lumps?" or "How are the lumps distributed?" You then can ask corresponding questions in the real universe and compare the two.
There are various candidates for the dark matter, but today one of the most popular is a very exotic elementary particle we call a WIMP, or weakly interacting massive particle. The WIMPs are just elementary, subatomic particles-fundamental constituents of matter. Tiny little individual things, they themselves come basically in two types: the so-called hot dark matter and cold dark matter. Hot dark matter consists of quickly moving small particles such as neutrinos, a particle which may or may not have a mass and therefore may or may not contribute to the shape of the universe. Cold dark matter is made up of particles that are sluggish-they move more slowly and are therefore cold. Predicted in a certain class of theories of fundamental interactions called supersymmetric theories, they have yet to be discovered experimentally.
Still, there are experiments to detect even these very weakly interacting particles by side effects. If you have a semiconductor, occasionally one of the WIMPs could have a head-on collision with a silicon atom and cause the atom to recoil. Now these hits are very, very rare so you have to have several kilograms of semiconductor and you are trying to find one atom moving just because it gets hit by a WIMP. Until these experimental searches succeed we cannot be certain that the theories are correct. But the exciting part is that the experiments are in place and the particles are detectable. If they exist, we will know about them in a few years".
So, the missing dark matter whether cold or hot holds the key to the present structure of the cosmos and to even our existence. Meanwhile, scientists will be waiting with their fingers crossed for the success of their experiments with the missing matter.


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