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How ditching the assumptions of uniform mass distribution led to better predictions of cosmological expansion
July 6, 2016 | Rachel Gaal
While much is known about the expansion of the universe, until recently the basis of our understanding has relied on approximations: In order to solve the equations of Einstein’s general theory of relativity, researchers have had to assume that matter is uniformly distributed across space. In reality, the universe’s clusters of galaxies and stars form pools of densely packed matter, creating inhomogeneous clumps.
The reason for the approximation has been the extreme difficulty of doing the full calculations, even with the best supercomputers. Recently, two independent groups decided to break this tradition — modeling the inhomogeneous universe in extraordinary detail.
James Mertens and Glenn Starkman from Case Western Reserve University, and Tom Giblin of Kenyon College in the U.S., published their work in Physical Review Letters and Physical Review D. Another team, Eloisa Bentivegna from the University of Catania in Italy and Marco Bruni from the University of Portsmouth in the U.K., published in Physical Review Letters. The results provide new evidence that the expansion rate of the Universe could be more accurately predicted when the uneven matter distribution across the cosmic scale is included.
And the results are robust. The two teams started with different initial conditions, and focused on different aspects of the early evolution of the universe: Starkman and his team concentrated on how the overall expansion was affected by the pooling of dense matter — while Bruni and Bentivegna studied the local expansion rates around under- and over-dense areas of the simulations.
Both teams found localized differences in evolution and expansion rates compared to conventional general relativistic simulations, but it remains to be seen whether the teams’ techniques will be adopted over the conventional approach. In future work, the teams hope to produce quantifiable results that astronomers can apply in their own work — for example, on how photons and the propagation of light respond to curvature of an inhomogeneous universe.
While the current model of cosmology still holds, the bigger question is to see if other groups can determine whether the pooling of dense matter, and the consequential forming galaxies and stars, have a larger effect on the expansion of the universe.
Solving the equations of general relativity assuming a clumpy universe is hard, but may give more insight into the expansion of the universe.