ABSOLUTE BEGINNERS

This article featured in the January 2002 Beginners Magazine

DARK MATTER IN THE UNIVERSE

When we look up into the night sky we can see thousands of stars. With a telescope we can look out into space and see millions of stars. We can see stars in other galaxies and the most powerful telescopes on Earth and in space show us that there are billions of galaxies containing billions of stars. Our minds come near to bursting point trying to come to terms with the numbers but recent studies are beginning to reveal that all we see is not all there is. There appears to be much more matter in the universe than we can see so what is it?

When there is matter in the universe that is not producing or reflecting light then we call it dark matter and there are a number of types of dark matter. There is one form of dark matter that we can in fact see or rather we can see that it exists. Nebulae are clouds of gas and dust that we can see throughout our galaxy and indeed can be seen in other galaxies. Where stars are imbedded in the cloud or are close by, the light of the star will illuminate the cloud. Where stars do not illuminate the clouds they do not shine and we cannot see them, these are called Dark Nebula. We can however see some of these dark nebulae because they block out the stars or bright parts of the nebula behind them and appear as dark patches, among the famous dark nebulae are the Coal Sack and the Horse Head.

 

THE HORSE HEAD NEBULA IN ORION

There are other forms of dark matter, the most sinister are of course the Black Holes. These are massive stars which have collapsed and all the material they were comprised off has been compressed into tiny volume known as a singularity. The mass of the star is still there but it cannot be seen. Giant black holes are also known to exist at the centre of many galaxies, these may contain the mass of millions of stars and are known as Super Massive Black Holes.

There are other stars which have collapsed and formed dense massive but very small objects called Neutron Stars. These form from dying stars two to three times the mass of our Sun. They are not big enough to form a Black Hole but collapse under the force of their own gravity to form a small but very dense remnant of the former star. Initially they are very hot but over many millions of years cool down to become a super dense cinder. Stars like our own Sun will also form a type of dense dark cinder. As this type of star dies it first expands into a Red Giant then contracts into a small hot White Dwarf star. Eventually this white dwarf will cool down and become a small dense dark cinder too.

Recently objects have been identified which are too small to be a true star, these are called Brown Dwarfs. These can be thought of as being failed stars. When these objects formed they did not have enough material to produce the pressure and heat required to start the nuclear fusion process to enable stars to produce their energy and light. Through the compression of the gas and the radioactive decay of materials they do have, they produce a lot of heat and glow red like an electric fire. There are thought to be many millions of these objects in every galaxy.

Another form of dark matter may exist in the form of wandering planets. Theories predict that stars like our Sun may form with many more planets than they end up with. Our Sun may have had as many as 50 or even 100. In the early formation period there would have been many close encounters where planets may have disturbed their orbits or even collided. Some of these encounters may have thrown a planet into the Sun or thrown it out of the Solar System into space. Those thrown out will be destined to wander forever in the cold dark empty space between the stars.

So far we have considered the matter we can see or know as ordinary material known as Baryonic Matter. This is the normal stuff made of atoms and the atomic particles such as Protons, Neutrons, Electrons and the known sub-atomic particles. There is however a mystery and that is the Baryonic material appears to make up only a small proportion of the total mass of the universe, there appears to be a lot of stuff out there we can't see. If we can't see this missing mass a good question would be 'how do we know it is there ? The answer is we know it's there because it exerts a gravitational pull on the matter we can see. Spiral galaxies for example rotate but the problem is they do not rotate according to the normal laws of physics if we assume the only material in the galaxies is what we can account for. According to the laws of physics and motion the inner parts of a spiral galaxy should rotate faster than the outer parts but this is not what is seen. The whole galactic disc appears to be rotating at nearly the same speed. The only answer appears to be that there is a huge amount of unseen matter on the outer edges of the disc. It seems that more than 80% of the mass of these galaxies is in this unseen stuff.

So far the nature of this material is very uncertain. There are a number of theories and ideas as to what it might be. One suggestion is that the some mass could be accounted for if the sub atomic particles known as Neutrinos had mass. An unbelievable number of neutrinos pervade space, some originating in normal stars, some from exploding stars and other perhaps from the Big Bang itself. These particles are thought to have no mass and very rarely interact with normal matter. In fact most neutrinos will pass clean through the Earth with no effect at all. Recent studies have shown that neutrinos may have some mass but it is very, very small and does not help to solve the problem of the missing mass.

Another idea is that this invisible material is completely different to the baryonic materials and does not interact with it in any way. We are made of baryonic material and so are all our detectors so we cannot detect its presence. One suggestion is that the basic units of this non-baryonic material may have atoms the size of stars or even solar systems but they have no effect other than through gravity. Another enigma has come to light recently in the wider field of cosmology. This is in the calculations to determine the fate of the Universe. The question cosmologists are trying to answer is 'will the universe continue to expand forever or collapse back on itself into a Big Crunch ?' The answer hangs on how much material there is in the universe. If there is more than a certain figure it will collapse if there is less it will expand forever. Calculations have shown that the mass is somewhere around the mid point but we can only account for about 10% of the mass. It may be that space is not so empty after all.

 

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