Evidence of Dark Matter Found

"I don't know what I may seem to the world, but as to myself, I seem to have been only like a boy playing on the sea-shore and diverting myself in now and then finding a smoother pebble or a prettier shell than ordinary, whilst the great ocean of truth lay all undiscovered before me". -- Isaac Newton

This week it was announced that a group of astronomers from Johns Hopkins University have located what appears to be a ring of dark matter in the galaxy structure ZwC10024+1652 (also known by its nickname C10024+17) which is located 5 billion light years away from our solar system. They used the Hubble Space Telescope to make this discovery. Read about it HERE.

Dark matter is that mysterious substance that allegedly makes up 90% of our universe. In the 1930s, the eccentric Swiss astronomer Franz Zwicky found evidence of dark matter by using the Doppler shift to measure velocities of individual galaxies in the "Coma Cluster" (they just don't name galaxy clusters like they used to). Zwicky discovered that the galaxies were moving too rapidly relative to one another, to be held together only by the gravitational pull of all the stars in the cluster; the galaxies should have come apart and gone their own ways. He discovered the same thing in other clusters. He concluded that some unseen matter - and plenty of it - must be holding the clusters together: dark matter.
All the ordinary matter that we are familiar with (usually called baryonic matter) - stuff like trees, rocks, people, stars, galaxies, air, water etc. - is made of protons, neutrons and electrons. We know nothing about what constitutes dark matter - or non-baryonic matter.
If dark matter can't be seen and cannot interact with baryonic matter, how can we ever hope to detect it? Well, dark matter does interact with normal matter via gravity. Astronomers have been using a technique called "gravitational lensing" to get clues about the nature of dark matter, and here's how it works:
Suppose that the Earth, a massive galaxy and a background light source - say, a galaxy or a quasar - are in almost perfect alignment, as seen in the following diagram.



Since the space around the massive galaxy is warped, light is bent around it as it travels towards to the Earth and we end up seeing 2 images of the distant quasar. In this case, the massive galaxy acts as a gravitational lens. The following diagram shows what I mean by warping of space; it shows space as a 2 dimensional sheet, warped by mass.



If the Earth, the gravitational lens and the distant light source are not in perfect alignment, the second image received would be too faint to be visible, but we would get a distorted image of the distant light source - be it a galaxy or a quasar.

In the case were the gravitational lens is a cluster of galaxies (galaxies usually congregate in clusters), we may see multiple distorted images of the same distant galaxy which may lie billions of light-years beyond the cluster. The following following is a picture of ZwC10024+1652, which allegedly shows a ring of dark matter.



What we see in this picture are many galaxies, which are orange/tan in colour. Also, around the centre of the cluster, we can see some unusual blue-coloured galaxy shapes. these are multiple images of a distant galaxy. This unfortunately-named galaxy cluster is acting as a gravitational lens. The existence of the dark matter ring is indicated by the weak distortions of the many distant galaxies all over the image. Using computer modelling, the distorted light was mapped, in order to find out how the dark matter is distributed in the cluster, and the dark matter ring was detected. I'd really love to know how the astronomers managed to detect it. The computer-modelled dark matter ring is superimposed on the image in blue. The astronomers hypothesize that the dark matter ring, which is a mind-boggling 2.6 million light-years across - was formed 1-2 billion years ago when 2 galaxy clusters collided.

Why do we need to know the nature of dark matter? Well, according to Einstein's general relativity equations, the universe will either continue to expand forever (the universe will reach near absolute zero) or will contract into a fiery collapse - the so called "Big Crunch". Until we resolve the question of this dark matter, we cannot resolve the question of the ultimate fate of the universe. And besides, knowing the nature of this material can give us new perspectives about the origins of our universe and our place in it.

What could this non-baryonic matter be? A number of candidates have been proposed; some hypothetical particles with cool names like axions, gluinos, gravitinos, sneutrinos, magnetic monopoles, mirror matter, cosmic strings, and WIMPS (weakly interactive massive particles). It has been proposed that dark matter may be something banal such as faint low mass stars that are undetectable by astronomers (baryonic but undetectable), but current research seems to suggest that it must be non-baryonic. Other candidates are MACHOs - Massive Compact Halo Objects- such as black holes, brown dwarfs, white dwarfs, neutron stars. MACHOs can be detected only by gravitational effects, so we cannot determine what type of matter forms a MACHO. MACHOs may even be composed of baryonic matter. Maybe dark matter is made up of a number of different types of matter. It has even been suggested that Newton's law of gravity may need modification which may explain some of the alleged evidence for dark matter ... although this could be difficult to prove since normal matter and dark matter seem to be mixed together.

It is very humbling to think that much of the matter that makes up the universe is totally unrelated to the matter that we are made of. Just think: over 90% of everything in the universe has never been seen ... all we can see is just the tip of the iceberg. Nearing the end of his life, Isaac Newton is said to have remarked,

"I don't know what I may seem to the world, but as to myself, I seem to have been only like a boy playing on the sea-shore and diverting myself in now and then finding a smoother pebble or a prettier shell than ordinary, whilst the great ocean of truth lay all undiscovered before me".

Whatever this dark matter is, now is a great time to be an astrophysicist; what, with this great ocean of mysterious matter out there waiting to be discovered.
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