(Picture: Large scale distribution of matter in the Universe, which can be seen from this picture to be close to, but not exactly even. Is it this unevenness, rather than dark energy, driving the accelerated expansion of the Universe? Source: Durham University)
Earlier this month, Hofstra University Professor Bret Bochner presented a new cosmological model at the 13th Marcel Grossmann Meeting on General Relativity which he says recreates cosmological observations, but crucially does not feature the mysterious dark energy. Dark energy was proposed in 1998 after the observation that the expansion of the Universe was accelerating as such expansion could not be explained by the matter/energy that was known to exist in the Universe. This led to the idea that there must exist a previously unknown and exotic form of energy called dark energy which permeates the Universe causing repulsion between matter on very large scales.
Approximations are inevitable when modelling something as large and complex as the Universe and Bochner’s ideas comes from re-examining these approximations. One of the main approximations of cosmology is to assume that the matter-energy is completely evenly distributed throughout space, which is not a bad approximation as observation tells us that matter-energy IS evenly distributed to within a few per cent. However it is also fairly obvious that the matter-energy distribution is not COMPLETELY even and this becomes more evident on smaller scales: for example our own solar system is very unevenly distributed, with 99.85% of its mass being concentrated in a small volume at its centre (the Sun). More detailed models of the Universe have been created (which are often so complex that they require supercomputers to run) and they tend to suggest that the effects on small-scale unevenness cancels out, but the precise relationship between the Universe at a smaller scales and the Universe at a large scale is still not known.
Bochner says that the accelerated expansion of the Universe can in fact be explained by the physics of previously-known forms of matter and energy, without the need to resort to elusive dark energy. His idea is that the small deviations from evenness in the matter distribution which causes matter to cluster together to form stars, galaxies, galactic clusters, etc, have a cumulative effect throughout space which creates the apparent accelerated expansion of the Universe. Whilst it was previously thought the effect of what Bochner terms ‘causal backreaction’ was not strong enough to explain the observed acceleration of the Universe, he says his more detailed model combined with strong clustering can recreate the current standard ‘concordance’ model of big bang cosmology.
Whilst it’s unlikely that Bochner’s ideas will replace the dark energy paradigm, as his model makes its own simplifications (and it is questionable whether these simplifications are better than the standard simplifications of cosmology), it shows that, despite being widely accepted, the nature of dark energy is still almost a complete mystery to cosmologists and even its existence is not beyond question.-John Davis
Original paper: http://arxiv.org/abs/1207.6119