Dark Matter

Dark Matter


At 0915hrs in Chelsea & Westminster Hospital lying on my left side, I found myself having an in-depth conversation with my consultant as to the nature of dark matter and dark energy. Not a subject that I would necessarily advise
during a colonoscopy, still, at least he was talking about what’s out there, as opposed to what’s in there!

I’d decided against sedation, so I could read my astronomy magazine while he carried out the procedure. He asked me did I know anything about dark matter and energy and could it actually be in the room. Before I could answer (No) he said “I want you to fart.” Well I think that that was as near an explanation of dark energy as we were going to get at that point.

As yet no-one knows what it is and given that he ordered me not to speak again I was left to dwell on dark matter of the cosmic kind. Astronomers might be more confident about their picture of the Universe were it not for dark matter. Observations show that the Universe is populated with some unseen form of matter; and plenty of it.

Dark matter is so named because it doesn’t interact at all with light, and is therefore invisible to us. The only way that dark matter influences the Universe around it, is by its mass exerting a gravitational pull on visible matter or on light, and it’s this influence that allows us to say that it must exist, and in fact to measure how much dark matter there is. By studying the rotation curves of galaxies, astronomers can glimpse how some dark matter is distributed.

Newton’s Law of Gravity says that stars revolving about the centre of a galaxy should slow dramatically the further away they are from the galactic centre. But the rotation curves of galaxies are ‘flat’, meaning the stars in an individual galaxy orbit at a steady velocity, independent of how far from the galactic centre they are. The most logical explanation for this is that massive spherical halos of dark matter surround the visible matter in galaxies.

In the 1930’s American astronomer Fritz Zwicky found ten times the mass of visible light in the Coma cluster of galaxies about 300million light-years from Earth. Adding up all this mass, and calculating the total gravitational pull of all the visible galaxies, he was able to deduce “the escape velocity” of the cluster. But when he then analysed the velocities of the galaxies in the cluster he discovered they were moving far faster than the escape velocity; so quickly in fact they should have been flung apart, thus confirming in his mind that there must be some invisible matter within the cluster that we couldn’t see, holding them together.

Every model of the cosmos is constantly being refined, and the current model known as the ‘standard cosmological model’ suggests that the Universe is composed of 5% baryonic matter, 27% dark matter and 68% dark energy. Baryons are a class of subatomic particle consisting of three quarks, and are the building blocks of atoms.
The most well-known baryons are protons and neutrons. Combine these in various quantities and you get the nuclei of all chemical elements. These baryons make up most of the mass of the ordinary matter in the Universe (you, me, trees, plants, buildings, planes, moons, planets and stars etc) and so we call this ordinary matter ‘baryonic matter’.

So, do we have any idea what dark matter is? Ideas abound, MACHO’s (Massive Astrophysical Compact Halo Objects), such as gas and dust, black holes, neutron stars or brown dwarfs, may account for some of it, but there just aren’t enough of them to account for the required additional mass. Of non-baryonic dark matter, two basic types exist; hot dark matter (HDM) and cold dark matter (CDM). The ‘temperature’ in each model refers to the particles’ velocities. Neutrinos represent the likeliest HDM candidate but recent observations have largely discredited the HDM model.

The current best guess for most of the cold dark matter are WIMPS (Weakly Interacting Massive Particles) such as axions, massive neutrinos, and photinos. Because they interact so weakly, and not at all with radiation, detecting WIMPS is incredibly difficult. There is a long way to go before we know the exact identity of dark matter, one of
the century’s greatest astronomical mysteries. One thing I do know I won’t be taking articles on Uranus to discuss with my Consultant next time.

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