What is the Universe made of? Most people could come up with a simple answer to that : atoms. Our May speaker, Professor Phil Walker Head of the Centre for Nuclear and Radiation Physics at Surrey University, put us straight on that; the answer is anything but simple.
Let’s though stay with the basic ideas for a moment. GCSE science students would agree that the Universe is made of atoms. They could name the three particles making up atoms : electrons, protons and neutrons. Some of them would be able to describe a bit about the quarks in protons and neutrons and might even mention positrons. They should be able to describe how atoms are made within stars by successive nuclear fusion events and how supernova explosions are responsible for dispersing those atoms which eventually find their way into you and me. It is always a bit of a shock, to put it mildly, when they realise that all the atoms from which they are made have come from a supernova; even more of a shock when I used to suggest to my classes that perhaps the atoms which make me might have come from the same supernova as their atoms. Not a happy thought for them.
So why is the simple idea about atoms as the building blocks of the Universe wrong? It isn’t so much that it is wrong but it is incomplete, very incomplete. Atoms are thought to make up a mere 5% of the Universe. 25% of the Universe is now thought to be dark matter with the remaining 70% being dark energy. As with all our speakers, Professor Walker covered a wide range of ideas, indeed a mind-blowingly wide range of ideas. Here is a very brief introduction to just one of those ideas – dark matter.
A key piece of evidence for dark matter comes from observations about the speed at which stars within galaxies orbit. The results are often presented as a rotation curve for stars which shows their speed of rotation v distance from the centre of the galaxy. You measure the speed of rotation of stars within a galaxy seen edge-on by looking at their red shift . We are generally familiar with the idea that whole galaxies display a red shift because they are moving away from us. Given the rotation of the galaxy, some stars within it will be spinning away from us and will have an additional component to this red-shift whereas those stars on the opposite side of the galaxy will be spinning towards us partly cancelling the red-shift out with a bit of blue-shift.
Simple gravitational theory predicts that stars in the outermost parts of the galaxy should be moving more slowly than those closer to the centre, in much the same way as the outer planets in our Solar System are orbiting more slowly than the inner planets. The startling result is that this is not the case. The outermost stars orbit at a very similar speed to the inner ones; indeed the rotation curve for M33 ( speed v distance from the galaxy centre) shows the speed increasing with distance.
The rotation curves show that the outer stars orbit more quickly than expected if you just apply classical gravitational ideas allowing for the amount of observable mass in the rotating system and its distribution across the galaxy. This faster-than-expected rotation implies a greater mass, particularly in the outer regions of the galaxy. We do not see this – hence the idea of dark matter.
Wikipedia has a very full and seemingly authoritative description of this topic which you might like to look at as it gives a wider range of experimental evidence about dark matter – far more detail that I could ever get my head around.