Dark Matter

Early in the 20th centuary, astronomers discovered that many of the patches of light they saw in the sky, were actually huge collections of stars, like the Milky Way, seen from a great distance. These are, of course, what we now call galaxies.

M31 - The Andromeda Galaxy

When astronomers started measuring the velocities of stars in external galaxies, like Andromeda, they found that they were moving far too fast, given the amount of matter they could see in the form of stars, gas and dust.

Similarly, when it was realised that galaxies were grouped together in clusters, there didn't seem to be enough matter in these cluster to hold them together.  Galaxies and clusters of galaxies should be flying apart!

The answer is, of course, that there is much more matter present than we can see - dark matter. The existence of dark matter has been confirmed using weak gravitational lensing.  The following 3D map of dark matter was created using from a Hubble Space Telecope survey called COSMOS using gravitational lensing techniques.

3D Map of Dark Matter

One of the big question in modern astronomy is what form does this dark matter take?  One idea is that it's made up of some as yet undiscovered sub-atomic particle that interacts weakly, if at all, with normal matter.  These are termed WIMPs (Weakly Interacting Massive Particles).

The other alternative is called MACHOs (Massive Compact Halo Objects). The most likely candidates for MACHOs are brown dwarfs.  Brown dwarfs are failed stars - object that are  not quite massive enough for nuclear fusion to take place.  The mass of a brown dwarf is between thirteen and seventy-five times the mass of Jupiter.

Since their only source of energy is their own gravitational contaction, brown dwarfs may glow a dull brown (hence the name) and may be visible under certain circumstances.

And now I'm beginning to get to the point.  In my previous post I mentioned that astronomers had discovered a stream of stars between the Large and Small Magallenic Clouds (the LMC and SMC).

What they were actually looking for was MACHOS.  They were hoping to detect MACHOS using microlensing.  During a microlensing event, a nearby object passes in front of a more distant star. The gravity of the closer object bends light from the star like a lens, magnifying it and causing it to brighten.

What they hoped to see was MACHOS within the Milky Way microlensing stars in the LMC.  The number of microlensing events seen, however, was not enough to account for dark matter but was higher than expected.

Computer simulations showed that the most likely explanation for the observed microlensing events was an unseen population of stars removed by the LMC from its companion, the SMC. Foreground stars in the LMC are gravitationally lensing the trail of removed stars located behind the LMC from our point of view.

Supersymmetry

Since MACHOs have been rules out, what about WIMPS?  The Large Hadron Collider wasn't just built to find the Higgs Boson.  (Before I carry on, the Higgs boson is not a likely candidate for dark matter - it disintegrates into lots of smaller particles almost instantly.)

One of the other purposes of the LHC was to investigate the theory of Supersymmetry (SUSY).  According to Supersymmetry, for every known particle there is much heavier superpartner.  A stable supersummetry particle (e.g. the superpartner of the neutron is called the neutralino) would be a good candidate for a WIMP.

Unfortunately, researchers at the LHC have dealt a blow to the theory of supersymmetry.  They have been looking for the decay of a particle called the Bs meson into two muons.  However, this particular decay was seen only three times out of a billion Bs meson decays.  If supersymmetry was correct, this decay would bee seen many more times.

There are of course other candidates for WIMP particles, e.g. axions.  More exitingly however, this could be a clue point us towards to a new theory (or theories) that might one day supplant both quantum mechanics and general relativity.