Most articles on the Anthropic principle tend to focus on the value of the cosmological constant (or dark energy) which has to be exact to within one part in 10120 in order for the universe to conceivably give rise to any life at all, let alone complex carbon based organisms that can examine the stars and marvel at their good fortune. Now dark matter appears to be getting a look in. According to the New Scientist:
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. …..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.
You probably know where this is going…
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
Just when I had stopped worrying about the nefarious activities of my multiverse clones, another anthropic coincidence turns up!. I recommend Max Tegmark’s guide for the perplexed, as well as Jim's series for misanthropes (part one and part two). In this piece Amanda Gefter wonders whether there might not be another option:
Physicist John Wheeler once offered a suggestion: maybe we should approach cosmic fine-tuning not as a problem but as a clue. Perhaps it is evidence that we somehow endow the universe with certain features by the mere act of observation... If we in some sense create the universe, it is not surprising that the universe is well suited to us.
This is not entirely implausible. Referring to Genesis I see that:
'In the beginning God decided to create the heavens and the earth
And lo, God decided to hire Consultants for advice on 'change management'
And they did say to God, 'we advise that you outsource the process of creation'
This can be done by subcontracting to quantum observers thus achieving time and energy savings
So God created quantum observers, and the consultants said that it was good
But lo, the observers were incompetent and ended up creating a universe containing haemorrhoids, muzak, traffic wardens and A.C Grayling'
Perhaps global warming is just a quantum projection of our own self loathing. Its an intriguing thought.
EDIT : Other articles on the subject of multiverses have appeared in Discover magazine here and Guardian Comment here.
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