IT'S not just the nature of dark matter that's a mystery - even its abundance is inexplicable. But if our universe is just one of many possible universes, at least this conundrum can be explained.
The total amount of dark matter - the unseen stuff thought to make up most of the mass of the universe - is five to six times that of normal matter. This difference sounds pretty significant, but it could have been much greater, because the two types of matter probably formed via radically different processes shortly after the big bang. The fact that the ratio is so conducive to a life-bearing universe "looks like a tremendous coincidence", says Raphael Bousso at the University of California, Berkeley.
Ben Freivogel, also at UCB, wondered if the ratio can be explained using the anthropic principle which, loosely stated, says that the properties of the universe must be suitable for the emergence of life, otherwise we wouldn't be here asking questions about it. In order to avoid questions about how these properties became so finely tuned, the anthropic principle is combined with the idea that our universe is part of a multiverse, in which each universe has randomly determined properties.
Freivogel focused on one of the favoured candidate-particles for dark matter, the axion. Axions have the right characteristics to be dark matter, but for one problem: a certain property called its "misalignment angle", which would have affected the amount of dark matter produced in the early universe. If this property is randomly determined, in most cases it would result in a severe overabundance of dark matter, leading to a universe without the large-scale structure of clusters of galaxies. To result in our universe, it has to be just the right value.
In a multiverse, each universe will have a random value for the axion's misalignment angle, giving some universes the right amount of dark matter needed to give rise to galaxies, stars, planets and life as we know it.
Freivogel combined the cosmological models of large-scale structure formation with the physics of axions to predict the most likely value for the ratio of dark matter to normal matter that would allow observers like us to emerge. He assumed that the number of observers in a universe is proportional to the number of galaxies within it.
In Freivogel's model, changing the ratio of matter type impacts the formation of galaxies, and hence observers; for example, too little dark matter would prevent the formation of galaxies and stars. His calculations show that of all the observers that might exist across the many universes, most would live in a universe with the dark matter abundance found in ours. In other words, we would be less likely to be here if our abundance of dark matter were different (www.arxiv.org/abs/0810.0703).