Dark Matter - A Taste






Given all the information about the workings of our planets and the solar system, and even how the universe and life as we know it formed, one would assume that scientists seemed to understand exactly how the universe worked. Everything – that is us, the earth, the stars, the supernovas, are all made up of atoms, which were all created at time zero – the Big Bang. In between these atoms was literally nothing, a void, a space. However recent discoveries have led to an unraveling (or further complication) of the mystery – there is a lot more to the universe than meets the eye. According to the best estimates of scientists these days we only know what about 4% of the universe is made up of...but if this 4% is made up of only atoms what is the rest made of? Well 'the rest' is made of mysterious entities about which very little is understood – dark matter and dark energy.

In 1974 the astronomer, Vera Rubin, was working on a project involving the investigation of stars on the outer edges of galaxies – and what she actually discovered came as a great surprise to the scientific community. Before we discuss what she found let us remember Newton's discovery, that is gravity is universal. An apple falling on his head on earth obeys the same mathematical laws as an apple falling on the other side of the universe. In the same way that the sun controls the orbiting planets by exerting gravity on them, a spiral galaxy must be controlled by the gravity giving black hole at it centre. It has long been known that Pluto, which is at the edge of our solar system, travels slower then Mercury, which is closest to the sun. Essentially it was observations like these which allowed Newton to finalise his laws in the 17th century. When Rubin carried out her work on galaxies she expected to find that as you reach the edge of the galaxy the stars would be moving much slower than those close to the centre – but in actual fact it didn't work out like that at all.

She found that almost all of the stars in spiral galaxies are racing around the centre at approximately the same speed. This finding was to say the least a contradiction of the laws proposed by Newton – laws that had come to be universally accepted by many. Questioning Newton seemed unthinkable so scientists went down a different route altogether – rather than variable gravity they argued that there had to be something else in galaxies, something that was providing extra gravity. With extra gravity, the stars would be pulled harder and would travel faster, as Rubin's observations suggested. So the name they gave to this extra something – Dark Matter.

Two professors at Princeton University – Peebles and Ostriker – looked further into the issue of dark matter, suggesting that there was at least ten times more of it than there was ordinary matter. Yet despite this growing acceptance as to the existence of Dark Matter, its real identity remained completely unknown. Nothing that particle physics came up with appeared to fit the bill, and even the discovery of the neutrino could not put together the missing pieces of the jigsaw. What was needed was something with mass but also something which does not interact with ordinary matter. Professor Sumner proposed a possible answer – a new hypothetical particle called the neutralino. It is thought to have the right mass and exist in suitably vast quantities – but has never been detected.

If dark matter in our galaxy is everywhere in our galaxy, then it must be present here on our earth, in fact thousands of tonnes of the stuff must pass through the earth everyday. As it doesn't interact with ordinary matter it can pass straight through us, the earth, everything we are familiar with.

Of course not everyone was as keen to pursue the ideas underpinning dark matter. In 1974 an Israeli astrophysicist, Professor Milgrom, tried something even more audacious by attempting to re–write Newton's laws of gravity. Of course knowing such a move would not be wholly welcomed by the scientific community he worked in private until he was ready to reveal his ideas in 1981. He called his ideas the Modified Newtonian Dynamics (MOND), and used it to show how gravity could be a little stronger than previously thought, across the huge distances that galaxies cover.

In 1997 another can of worms was opened when Professor Perlmutter discovered that not only were stars and galaxies moving away from each other, but they were doing so at greater and greater speeds. This could mean that one day in the future we could look up to the sky and see no stars as they are all too far away from us. However it also meant that something was pushing the stars apart. This anti–gravity force was completely new to science, but again what it actually was remained a mystery. It was given a name though–Dark Energy.

When we bring these things together it appears that the universe comprises of 4% ordinary matter, 21% dark matter and 75% dark energy. Whilst there are skeptics who ask where the evidence exists for such views, supporters (including myself)  believe that dark matter and dark energy have been vital to the development of the universe. The results of the WMAP satellite survey appear to confirm the quantity of each of the dark components.

So where are we at now? Well astronomers cannot detect dark matter directly because it emits no light or radiation, yet its presence can be inferred from the way galaxies rotate: their stars move so fast they would fly apart if they were not being held together by the gravitational attraction or some unseen material. However the institute of astronomy at Cambridge university have provided further information with a study of 12 dwarf galaxies that skirt the edge of our own milky way. Using some of the most technical and largest telescopes available the team made detailed 3D maps of the galaxies, using the movement of their stars to trace the impression of the dark matter among them and weigh it very precisely. With the aid of 7,000 separate measurements the researchers have been able to establish that the galaxies contain about 400 times the amount of dark matter as they do normal matter. It also comes in a 'magic volume' which happens to correspond to an amount which is 30 million times the mass of the sun. According to Professor Nichol you cannot 'ever pack it smaller than about 300 parsecs–1000 light years, this stuff will not let you. That tells you a speed at which dark matter particles are moving (Approx. 9km/s).' These are the first properties other than existence that the scientific community have been able to determine.

The issue of speed is a huge surprise – with current theory predicting dark matter particles being extremely cold and moving at a few millimetres per second. However these observations prove the particles must actually be quite warm at around 10,000 degrees. The most likely candidate for dark matter is the so called weakly interacting massive particle



Copyright(C) 2007 - 2020. All rights reserved.