A UNIVERSE riddled with holes would do away with the need for dark energy. Unfortunately that idea itself now seems to have a few holes.
In 1998, astronomers found that distant supernovae were dimmer, and thus farther away, than expected. According to the standard model of cosmology, which assumes that the universe is homogenous on large scales, this suggested that the expansion of the universe was speeding up. That acceleration was attributed to a mysterious 鈥渄ark energy鈥� inherent in the fabric of space-time.
Then, last year, Edward Kolb of the Fermi National Accelerator Laboratory in Batavia, Illinois, and colleagues re-examined the supernovae data in the context of the 鈥淪wiss-cheese鈥� model, which assumes the universe is full of holes or voids with less matter than other regions. It was inspired by the accidental discovery of a giant void about 1 billion light years across.
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The astronomers who discovered the supernovae anomaly in the late 1990s had assumed that matter was distributed more or less evenly. But in the Swiss-cheese model, less dense regions would act as a concave lens, bending part of the light from a distant object away from us. Therefore supernova light that travelled through a series of such voids would appear dimmer than expected without the need to invoke dark energy.
Yet it seems cosmologists should be taking their cues from the holes in real Swiss cheese. Kolb鈥檚 model 鈥渁ligned all the spherical voids up in a nice line,鈥� says 脡anna Flanagan of Cornell University in Ithaca, New York. 鈥淭he real universe is much more complicated, more random.鈥�
Flanagan and his colleagues developed a more realistic model in which the voids, each with a radius of about 1 billion light years, are distributed randomly throughout the cosmos. Since the mass in the universe has to be conserved, voids in some regions would lead to denser areas elsewhere.
The light from supernovae passes through both types of regions. Their model shows that some supernovae would appear dimmer because their light would pass predominantly through voids, but others would appear brighter because the light was passing mainly through denser regions and bending towards us. However, this contradicts observations which show that the more distant supernovae appear dimmer regardless of where they are in the sky. 鈥淲e are arguing that [the Swiss-cheese] effect cannot explain the supernova data and that we do need dark energy,鈥� says Flanagan (Physical Review D, vol 78, p 083511).
鈥淲e are arguing that the Swiss-cheese effect cannot explain observations and that we do need dark energy鈥�
鈥淚t is an important thing to clear up, because voids are more natural than dark energy,鈥� says Pedro Gil Ferreira of the University of Oxford. However, Ferreira鈥檚 team reckons that the supernovae data can be explained if we live in a giant void a few billion light years across (Physical Review Letters, vol 101, p 131302).
Flanagan says that while this idea is possible, it contradicts the Copernican principle. 鈥淭his [principle] says that we are not in a special place in the universe. We would have to be right at the centre of this spherical under-dense region, and it seems unlikely.鈥�
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