Is dark energy lurking in hidden spatial dimensions?
来源：未知 作者：祁鲒缜 时间：2019-03-14 10:11:12
By Stephen Battersby 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. 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. So far, the two physicists have only sketched out the idea. “We want to throw it out there so people can contribute their ideas,” Levin told New Scientist. But they can already use it to solve another, more abstruse problem. One of the most popular candidates for a theory that unites gravity with the other forces, string theory, requires there to be seven extra dimensions all curled up small. Unfortunately, they turn out to be unstable, liable to blow up. In the new theory, these extra dimensions can be forced to behave. The right balance of quantum force fields would act like a stiff spring within the extra dimensions, fixing them at a certain size. “It seems well worth exploring further to see where the idea leads,” says Glenn Starkman, a theoretical cosmologist at Case Western Reserve University in Cleveland, Ohio, US. Kimball Milton of the University of Oklahoma in Norman, US, agrees. “The model they present seems highly implausible,” Milton told New Scientist. “However, we might take this as an existence proof of a stable solution, so I hope and expect that it will spark renewed efforts in this direction.” So if the dimensions are indeed 10 microns across, why can’t we see them? At the moment, Greene and Levin’s theory only works within the “braneworld” picture of cosmology, which descibes our 3D universe as a membrane floating in higher-dimensional space. Most particles and fields are firmly fixed to the brane, which is why we can’t see the extra dimensions or step sideways into them. In the basic version of braneworld, the only force that reaches out beyond the brane is gravity. The gravitational field alone would not have vibrations with the right properties for Greene and Levin’s theory, so they had to add another field ad hoc and fix its strength to produce the right amount of repulsion. “We’ve made a toy model with pretend fields,” Levin says. One cause for optimism, however, is that their pretend field has almost exactly the same strength as the field associated with neutrinos – the lightweight and slippery particles that exert weak nuclear forces. Greene and Levin’s field cannot be coming from ordinary neutrinos, because like other particles, they are pinned to our brane. But they suggest that a “sterile neutrino” could exist in the extra dimensions, making its presence felt only as the source of dark energy. Greene and Levin’s theory implies that gravity should become stronger on short ranges, around the scale of the extra dimensions. Eric Adelberger and his team at the University of Washington in Seattle, US, have run a series of experiments using a twisting pendulum to measure the short-range strength of gravity, and they have already ruled out extra dimensions larger than a 0.1 millimetre. They are planning a new experiment to probe shorter distances still. If Adelberger’s pendulum does start to see gravity grow below 0.01 millimetre, it could be a sign that Greene and Levin are right,