When astronomers study distant galaxies, they see only a small fraction of the mass needed to hold these clumps of stars together. Without some kind of extra hidden mass, galaxies ought to fly apart.
Astronomers call this hidden mass ‘dark matter’ and physicists around the world are engaged in an increasingly desperate race to find evidence of it here on Earth. That’s why there are more than 30 experiments in various parts of the planet looking for the stuff.
The consensus is that, despite this global effort, dark matter remains well hidden. Nobody has had a whiff of the stuff.
That is nobody except an Italian group which has spent the last ten years or so watching a giant lump of sodium iodide. Their thinking is that any dark matter hitting the sodium iodide should generate a photon. And that as Earth moves around the Sun, they should see more photons when heading into the background sea of dark matter than when moving away from it.
Sure enough, this seasonal signal is exactly what this team says it sees. They claim that it’s experiment called DAMA/LIBRA is the first direct evidence of dark matter.
The trouble is that nobody else believes them, mainly because so many other experiments have seen nothing. The critics says something else must be responsible for these seasonal signals, perhaps some kind of environmental change like a variation in temperature.
Then, about a month ago, everything changed when an experiment called CoGent based in the US reported that it too had found a hint of dark matter. CoGent looks for evidence that dark matter particles have bumped into a crystal of germanium and sure enough, the CoGent team say that the experiment is producing abundant evidence of these kinds of collisions.
Curiously, while most experiments are looking for relatively heavy dark matter particles which should produce higher energy collisions, CoGent looks for much lighter particles.
The interesting thing is that the DAMA\LIBRA evidence is from a similar mass range.
Now the theoreticians are attempting to reconcile the DAMA and CoGent results by finding a dark matter model that can explain them both. Last month, Liam Fitzpatrick at Boston University and a couple of mates suggested that a light, weakly interacting dark matter particle could explain both results.
And today, Robert Foot from the University of Melbourne has an even more interesting solution. He says that mirror matter could explain both. “This result adds weight to the mirror dark matter interpretation of the direct detection experiments,” he says.
The theory behind mirror matter suggests that every particle in the standard model has a mirror equivalent that interacts with ordinary matter only very weakly.
However, mirror particles interact with each other in exactly the same way as ordinary particles. So in this scenario, the Universe is filled with mirror planets, stars and galaxies. That’s a mind blowing idea.
Foot is one of the leading proponents of mirror matter and says other observations also point to its existence.
Perhaps the new evidence will tempt astronomers to look harder. If it exists, mirror matter ought to be observable in other ways. For example, its gravity should bend light causing microlensing events although distinguishing mirror matter microlensing events from the same thing caused by ordinary but dim matter will be hard.
Still, an interesting avenue to pursue.
arxiv.org/abs/1004.1424 : A CoGeNT Confirmation Of The DAMA Signal
arxiv.org/abs/1003.0014: Implications Of CoGeNT And DAMA For Light WIMP Dark Matter