Dark matter: the superglue of the universe?
Tom Borrett
Everything we see around us – the Earth, planets, the sun and stars, even us humans – all comprise the same basic building blocks. Quarks form baryons which together with electrons form atoms. These are responsible for our everyday world and the visible universe. This world that we can see and touch we have studied since the Greek philosophers Leucippus and Democritus suggested the concept of the atom right up to Murray Gell-Mann and George Zweig first proposed in 1964 the existence of quarks. Yet, after studying other galaxies, scientists have concluded that this observable matter only accounts for around fifteen percent of what should exist. To explain this perplexing puzzle scientists developed the proposition of ‘ darkmatter ’ , so called due to its lack of inter-activity with light, themethod by which we observe the universe. In this essay I will examine what dark matter could possibly be.
In 1933, Fritz Zwicky first proposed dark matter, 1 while studying the Coma cluster. He calculated that the visible mass accounted for one percent of the mass required to keep the cluster together. He concluded that most of the matter in the cluster was ‘ dunkle materie ’ (dark matter). Like much of his work on neutron stars, gravitational lensing and supernovae, his observations did not initially gain wider attention. However, by the 1970s the search for dark matter began in earnest after Rubin and Ford’s work on galaxy rotation speeds. They observed that, rather than the velocity of
Figure 5: The velocity versus distance from the centre. A is predicted and B is observed
objects decreasing as the distance to the centre of the galaxy (or cluster) increases, velocity remains flat after a certain distance (see figure 1), suggesting the force of gravity to be stronger that it would be if there were just visible matter or it is not concentrated at the centre, rather is in a halo (figure 2).
Further proof of the existence of dark matter is provided by general relativity. An object lying between a source of light and an observer causes the light to be bent like a lens. The more massive the object, the greater the bending. However, there is not enough visible matter to cause the extreme lensing we can see. Therefore, the mass we can see through our telescopes cannot be all there is. By measuring the distortion geometry, we can calculate the mass of the object distorting the light. Lensing can also create multiple copies of an image. 2 By analysing the distribution of these copies, we can deduce a map of the distribution of dark matter (see figure 2).
Figure 6: Map of dark matter of MACS J0416.1-2403 based on distribution of multiple image copies
But what is dark matter? Unfortunately, it is much easier to rule out possibilities than it is to answer the question. There are three distinct categories of dark matter theories: cold dark matter, warm dark matter and hot dark matter. Each category refers to its relative speed as compared to the speed of light:
1 Bergh (1999). 2 Strong gravitational lensing can cause multiple images of the same background object around the cluster.
154
Made with FlippingBook Digital Publishing Software