Friday, July 13, 2007
Dark Energy, Hidden Dimensions
No progress today, since I was kicked out of the house for the monthly cleaning this afternoon, and spent all morning researching Aztec culture (good luck finding consensus amongst historians about Aztec educational practices). So, in lieu of anything I've actually done today, here's a nifty article from New Scientist about the possibility of dark energy being found in hidden spatial dimenions.
The mysterious cosmic presence called dark energy, which is accelerating the expansion of the universe, might be lurking in hidden dimensions of space. The idea would explain how these dimensions remain stable – a big problem for the unified scheme of physics called string theory.More stuff of interest in the link.
Ever since astronomers discovered in the mid-1990s that other galaxies are accelerating away from us, physicists have struggled to explain why. Their favourite suggestion is quantum vibrations in the vacuum of space (called vacuum energy or the cosmological constant) that could produce repulsive gravity.
According to the calculations, however, these vibrations should either possess a ridiculously high energy density – 122 orders of magnitude larger than are observed – or cancel out to exactly zero. To make them almost-but-not-quite cancel, in agreement with astronomical observations, means fudging the quantum field equations.
Unless, that is, the quantum vibrations are stuck in a small space. Brian Greene and Janna Levin of Columbia University in New York, US, realised that in a confined space, natural resonant frequencies will stand out, preventing the vibrations from cancelling entirely. It's a little like the resonant notes produced by a musical instrument – except that instead of sound waves, the vibrations are fluctuating quantum force fields, and the instrument is a set of dimensions at right angles to familiar reality.
Even though the vibration is imprisoned in these other dimensions, it can extend its gravitational influence into our space. Its gravity is also repulsive in our space, just like the "ordinary" cosmological constant, so it would cause cosmic acceleration. To get the same amount of acceleration seen by astronomers, Greene and Levin calculate that the extra dimensions should have a scale of about 0.01 millimetre. Dark energy would be hiding less than a hair's breadth away.