杏吧原创

The expansion of the universe is accelerating, and we鈥檙e not sure why. It may have to do with quantum fluctuations that make every point in space constantly grow and shrink, producing a roiling foam that, on the whole, is always expanding.

This is a complex solution to the 鈥渧acuum catastrophe鈥�, which concerns one of the most hotly-contested numbers in physics, the cosmological constant. The constant describes the energy stored in the vacuum of space and most theories predict a value that is up to 120 orders of magnitude higher than that given by actual observations of the universe鈥檚 expansion. The calculations are off by so much that it has been called 鈥渢he worst prediction in the history of physics鈥�.

Qingdi Wang and William Unruh at the University of British Columbia in Canada have come up with a hypothesis that could repair this catastrophe 鈥� a way to make the cosmological constant as low in theory as it seems to be in practice.

Calculations of the cosmological constant are partially based on one big assumption: that the universe is, on large enough scales, the same everywhere you look. Wang and Unruh鈥檚 hypothesis is based on the idea that even if that鈥檚 true, bits of the universe can look different at the quantum scale.

鈥淧icture a sheet of paper, crumple it up, and then flatten it out again. If you go down to small scales, you can see the creases where you crumpled it,鈥� says Steve Carlip at the University of California, Davis. 鈥淏ut if you look at the average over the whole sheet of paper it looks very nearly flat.鈥�

In Wang and Unruh鈥檚 model, the vacuum energy is constantly fluctuating. Each point in space is expanding and contracting, acting like what they call a 鈥渕icro-cyclic universe鈥�. A cubic centimetre would contain 10¹⁰⁰ of these universes, which are brought into existence with their own big bang, then snuffed out after about 10^-40 seconds with a reverse big bang, or 鈥渂ig crunch鈥�.

They鈥檇 be so small and so fast that we will likely never be able to observe them directly, Carlip says. Unruh imagines shrinking yourself down very small, and standing next to an equally small chair. 鈥淎t one point the chair would be 100 steps away from you and an incredibly short time thereafter you would be standing right next to it,鈥� he says. 鈥淎nd your body would be constantly growing and shrinking as well.鈥�

Overall, though, as time passed you would get farther and farther away from your tiny chair. The researchers calculated that, because of the 鈥渂ounces鈥� that occur when points in space switch from contracting to expanding, expansion wins out just a little bit.

It鈥檚 like a child on a swing 鈥� if they pump their legs just right, their swings will get higher and higher. In this case, the vacuum energy acts as the pump, and the height of the swing is the size of the 鈥榤icro-universe鈥�, which grows slightly with each oscillation between small and large.

The result is that as the 鈥渕icro-universes鈥� expand and contract, the universe as a whole only expands. On a human scale, we average out the fluctuations and end up with the universe as we see it, uniform and accelerating outward. The fluctuations roil beneath the surface unseen, and the vacuum catastrophe is prevented.

There are problems with this model, Unruh admits. One is the moment between the tiny expansions and contractions 鈥� it鈥檚 not clear what happens when you squeeze a small volume of space into a singularity. And we also don鈥檛 yet know how it would square with general relativity, which it must if it鈥檚 correct.

At this stage, the idea is fairly speculative, based on layers of assumptions and mathematics rather than observations. A number of physicists contacted by New 杏吧原创 declined to comment on the work. But Unruh thinks we have to start tackling the vacuum catastrophe somehow.

鈥淏efore what one had was a really hard problem with no idea how to really solve it,鈥� says Unruh. 鈥淗ere it鈥檚 explained by this mechanism, and we now have a bunch of mathematical difficulties that we have to solve, but at least we have well-defined mathematical problems to look at.鈥�

鈥淚 think that whether it actually solves the cosmological constant problem or not is at this point still a hope rather than something that鈥檚 been demonstrated,鈥� says Carlip. 鈥淏ut the standard arguments lead to something that鈥檚 obviously wrong.鈥� It鈥檚 time to take a serious look at nonstandard arguments like this one, he says.

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