Saturday, April 3, 2010

Multiplying universes: How many is the multiverse?

Multiplying universes: How many is the multiverse?

by Amanda Gefter

HOW many universes are there? Cosmologists Andrei Linde and Vitaly Vanchurin at Stanford University in California calculate that the number dwarfs the 10500 universes postulated in string theory, and raise the provocative notion that the answer may depend on the human brain.

The idea that there is more than one universe, each with its own laws of physics, arises out of several different theories, including string theory and cosmic inflation. This concept of a "multiverse" could explain a puzzling mystery - why dark energy, the furtive force that is accelerating the expansion of space, appears improbably fine-tuned for life. With a large number of universes, there is bound to be one that has a dark energy value like ours.

Calculating the probability of observing this value - and other features of the cosmos - depends on how many universes of various kinds populate the multiverse. String theory describes 10500 universes, but that just counts different vacuum states, which are like the blank canvases upon which universes are painted. The features of each canvas determine what the overall painting will look like - such as the laws of physics in that universe - but not the details.

Thanks to the randomness of quantum mechanics, two identical vacuum states can end up as very different universes. Small quantum fluctuations in the very early universe are stretched to astronomical scales by inflation, the period of faster-than-light expansion just after the big bang. These fluctuations lay down a gravitational blueprint that eventually determines the placement of stars and galaxies across the sky. Small differences in the form of these fluctuations can produce a universe in which the Milky Way is slightly bigger, or closer to its neighbours.

So just how many of these different universes can inflation's quantum fluctuations produce? According to Linde and Vanchurin, the total is about 101010,000,000 - that's a 10 raised to a number ending with 10 million zeros ( Suddenly string theory's multiverse of 10500 universes is looking rather claustrophobic.

It might be, however, that this number is irrelevant, and that in a world ruled by quantum physics what matters is how many universes a single observer can distinguish. "Before quantum mechanics," says Linde, "we thought that 'reality' was a well-defined word." In classical physics, observers are irrelevant - we simply want to know how many universes exist.
It may not matter how many universes exist - just how many a single observer can tell apart

According to quantum physics, observers affect the systems they measure (see "Restricted view"). If observers are an integral part of the cosmic formula, then it may not matter how many universes exist - just how many a single observer can tell apart. If the observer is a person, that depends on how many bits of information the brain can process. "Based on the number of synapses in a typical brain, a human observer can register 1016," says Linde. That means humans can differentiate 101016 universes, which is much more manageable than the 101010,000,000 Linde and Vanchurin found to start with.

But does the human brain really play a role in making predictions in the multiverse? "This goes deep into philosophy," Linde says. "It's a slippery slope."

Cosmologist Alex Vilenkin of Tufts University in Boston is equally ambivalent. "It could be right that what is important is what an observer sees," he says. "But there might be things an observer doesn't see that are still there."

Restricted view

Quantum theory splits the world into two parts: the system under study and the rest of the world, which contains the observer. The system hovers in a ghostly state of near-existence made up of a host of possibilities until the observer makes a measurement - and so reduces this to a single reality.

Cosmology suffers from the paradox that no observer can be outside the universe - so the universe is doomed to spend eternity as nothing more than a vague possibility. The lesson of quantum cosmology is that we can't talk about the universe as a whole, but only what a given observer inside it might measure. Applying that lesson to the multiverse, Andrei Linde and Vitaly Vanchurin suggest that what matters is not the total number of possible universes, but the number of universes a single observer could distinguish.

If that observer is a human, the brain limits the amount of information they can register. But any observer - even an inanimate one such as a galaxy - is limited in the information it can store. These limitations in what observers can measure whittle down the number of universes that come into play in cosmological predictions. That means an observer might make a difference in explaining the value of things like dark energy.

World Science Festival 2009: Infinite Worlds, Part 3 of 6 from World Science Festival on Vimeo.

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