Cosmology was always going to be trouble for particle physics. Traditional quantum theory predicts that the vacuum of space should bubble over with short-lived "virtual" particles, whose combined energy, represented by the so-called cosmological constant, would have long ago blown galaxies far and wide like grease on water at the touch of detergent. Lately, trying to make sense of why the cosmological constant is tiny, physicists have toyed with a concept based on string theory. Called the landscape model, it supposes that many universes with different cosmological constants are realized in a larger multiverse. Despite long-standing hopes to the contrary, landscapers now have found that singling out a universe from this array is mathematically nigh impossible.

In 1998 astronomers discovered that the universe's expansion is accelerating at a rate consistent with a cosmological constant 10-120 times the value predicted by quantum theory. String theory, which unites gravity with quantum mechanics, offered the hope of explaining the attenuated cosmological constant. It recasts particles as one-dimensional strings, or filaments of energy, which play around in tiny tangles of extra spatial dimensions. The shape of the tangles influences the properties of strings and therefore the vacuum's energy. But no mathematical principle forces the extra dimensions to fold in a unique way.

This article was originally published with the title Hard Landscape.

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