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What’s the big deal about dark matter?

What’s the big deal about dark matter?

The big deal is that if the dark matter theory is right then we’re only aware of about four per cent of the universe. That includes all the stars, planets, gas and dust that make up all the trillions of galaxies.

Dark Matter Graph

Why is this so important?

This means we can only see less than 1/20th of what’s out there. The rest we cannot ‘see’. About 22 per cent is thought to be dark matter, and about 74 per cent is thought to be made of dark energy.

What is dark matter, and why can’t we see it?

This is a really good question. We could put the question better by asking what we mean by ‘seeing’? So, how do we know there is anything out there at all? To answer this we need to go back to basics.

OK, we know things exist because of the forces they exert on each other and on us. We think we know what these forces are. There are only four of them:

  • The strong nuclear force – this sticks atomic nuclei together
  • The weak nuclear force – helps quarks do quarky things
  • The electromagnetic force – this sticks atoms and molecules together
  • Gravity - this sticks planets, stars, solar systems and galaxies together

Dark matter, whatever it is, doesn’t seem to be affected by any of these forces, except gravity. This means that it cannot be made of protons or neutrons as they feel the strong nuclear force. If dark matter did feel this force, atoms, molecules and stars would form, but they don’t.

We think they may experience the weak nuclear force (hence weakly interacting), but this is very, very weak! If they could experience the electromagnetic force we would see photons of electromagnetic radiation. We don’t. In fact, the only way we know it is there at all is because it does experience gravity. We can work out how much is there by looking at its gravitational effects.

Gravity and orbits

Mercury! The fastest planet in the Solar System!

An orbit is a balance between gravity and speed. Think about the Solar System. Mercury, the planet closest to the Sun, experiences the biggest gravitational force. To balance this huge pull it must whizz around the sun very quickly. In fact, Mercury orbits the Sun every 88 days. If it travelled any slower the Sun’s gravity would pull it in, and it would be destroyed. If it moved any faster it would be able to pull away from the Sun and move to an orbit further out.

Mercury around the sun in 88 days

The planets further out experience a weaker pull from the Sun, and so need to travel slower. So, if you can see how fast an object moves in its orbit, we can tell how strong the gravity is that is pulling it. If you know how strong the gravity is you can work out how massive the object is. This is how we know the Sun weighs 2 x 1027 tonnes without building a very large weighing scales.

Sun Weighing Scales

What does all this tell us about dark matter?

Everything in the universe is attracted by gravity to everything else. A spiral galaxy is a huge cluster of stars. From the top its looks like this, a big spiral disc. From the side it looks like this, two fried eggs placed back to back. And it spins. This means that the stars towards the outer parts of the galaxy are orbiting the massive central bulge. What we should see is that the stars closest to inner bulge orbit faster, like Mercury orbiting the Sun. The outer stars should orbit more slowly, like Pluto orbits the Sun. By looking at the speed of these stars in orbit we should be able to work out the mass of the central bulge and get a good idea of how massive the Milky Way is.

The milky way

Unfortunately, this just doesn’t work. Almost everywhere you are in the disc you will orbit the central bulge at very nearly the same speed. Just like the Sun, all the stars take about 220 million years to go around once. What this means is that the outer stars are not just orbiting the central bulge, but they are actually part of it. This bulge in the amount of matter must extend out to, and beyond, the edge of the visible bits of the galaxy. It’s just that we can’t see all this matter. It is dark!

What are Einstein arcs?

Gravitational lensing also shows that Galaxy clusters are much more massive than the matter we can see would suggest. The picture shows a galaxy cluster with strange arc-like streaks crossing it. These are ‘Einstein arcs’. This is where an enormous gravitational field has distorted the light from a distant object and twisted it into these strange shapes. By looking at the shapes it is possible to work out the mass of the galaxy clusters. The mass is enormous. Over twenty times more than all the matter we can see. Again, most of the stuff it is made of is dark.

Einstein Arcs

But what is it?

The real WIMPS and MACHOS

WIMPS

WIMPS are Weakly Interacting Massive Particles. They are just about impossible to detect. There may be billions of them passing through your body at this very moment and you would be completely unaware of it.

This is where we started, if these things only interact or ‘see’ gravity and the weak force, but neither of the other forces. How do we detect them on the earth? The only other way of detecting them, apart from the gravity, is to look for the weak interaction. This happens very rarely and is incredibly hard to find. There are experiments designed to do this.

A technique used by the Cryogenic Dark Matter Search (CDMS) detector relies on very cold germanium and silicon crystals. The crystals, each about the size of a tennis ball, are cooled to just above absolute zero, less than -273°C (459.4F). A layer of metal at the surfaces is used to detect a WIMP passing through the crystal. This design hopes to detect vibrations in the crystal matrix generated by an atom being bumped by a WIMP. The tungsten metal sensors are held at such a low temperature so that they are superconductors. Large crystal vibrations will generate heat in the metal and are detectable because of a change in resistance.

CDMS Graph

No one has so far detected a WIMP using the weak interaction. One day they may be detected. If they are not, then we may be barking up completely the wrong tree and will have to look for another explanation.

CDMS

MACHOS

Another explanation is that there may be a huge amounts of matter locked up in MACHOs, MAssive Compact Halo Objects. A macho may be composed of ordinary matter (protons, neutrons and electrons) or be a black hole. An ordinary matter MACHO should emit some light, probably in the infra red part of the spectrum. They may be failed stars with a mass only a few times bigger than the planet Jupiter. There seem to be lots of them out there, but still nowhere near enough to account for all the missing mass.

So what about black holes?

Black holes can be truly black. They produce no light and they reflect no light. This makes them rather hard to see. Cosmologists don’t think this is likely because black holes tend to get very large and be few in number. There is a monster black hole at the centre of most galaxies. The one at the centre of the Milky Way is about as massive as about 100,000 suns.

Black Hole and sequence star

Where do you think all this stuff is hiding?

The other possibility is that it is not there at all. Now this could be really interesting because it would mean we have got something really stupendously wrong with our understanding of gravity and that could be very interesting!

The big questions:

What is the universe made of?

How big is the universe?

What is dark matter?

How do we know dark matter exists?

What is meant by ‘dark’?