A hypothetical multiverse seems less likely after modelling by researchers in Australia and the UK threw one of its key assumptions into doubt.
The multiverse concept suggests that our universe is but one of many. It finds support among some of the world’s most accomplished physicists, including Brian Greene, Max Tegmark, Neil deGrasse Tyson and the late Stephen Hawking.
One of the prime attractions of the idea is that it potentially accounts for an anomaly in calculations for dark energy.
The mysterious force is thought to be responsible for the accelerating expansion of our own universe. Current theories, however, predict there should be rather more of it around than there appears to be. This throws up another set of problems: if the amount of dark energy around was as much as equations require – and that is many trillions of times the level that seems to exist – the universe would expand so rapidly that stars and planets would not form – and life, thus, would not be possible.
The multiverse idea to an extent accounts for and accommodates this oddly small – but life-permitting – dark energy quotient. Essentially it permits a curiously self-serving explanation: there are a vast number of universes all with differing amounts of dark energy. We exist in one that has an amount low enough to permit stars and so on to form, and thus life to exist. (And we find ourselves here, runs the logic, because we couldn’t find ourselves anywhere else.)
So far, so anthropic. But now a group of astronomers, including Luke Barnes from the University of Sydney in Australia and Jaime Salcido from Durham University in the UK, has published two papers in the journal Monthly Notices of the Royal Astronomical Society that show the dark energy and star formation balance isn’t quite as fine as previous estimates have suggested.
The team created simulations of the universe using the supercomputer architecture contained within the Evolution and Assembly of GaLaxies and their Environments (EAGLE) project. This is a UK-based collaboration that models some 10,000 galaxies over a distance of 300 million-light years, and compares the results with actual observations from the Hubble Telescope and other observatories.
The simulations allowed the researchers to adjust the amount of dark energy in the universe and watch what happened.
The results were a surprise. The research revealed that the amount of dark energy could be increased a couple of hundred times – or reduced equally drastically – without substantially affecting anything else.
“For many physicists, the unexplained but seemingly special amount of dark energy in our universe is a frustrating puzzle,” says Salcido.
“Our simulations show that even if there was much more dark energy or even very little in the universe then it would only have a minimal effect on star and planet formation.”
And this, he suggests, implies that life could potentially exist in many multiverse universes – ironically enough, an uncomfortable conclusion.
“The multiverse was previously thought to explain the observed value of dark energy as a lottery – we have a lucky ticket and live in the universe that forms beautiful galaxies which permit life as we know it,” says Barnes.
“Our work shows that our ticket seems a little too lucky, so to speak. It’s more special than it needs to be for life. This is a problem for the multiverse; a puzzle remains.”
It is a puzzle that goes right to the heart of the matter: if the dark energy assumptions are flawed, does a multiverse even exist? The researchers acknowledge that their results do not preclude it – but they do diminish the likelihood.
“The formation of stars in a universe is a battle between the attraction of gravity, and the repulsion of dark energy,” says co-author Richard Bower, also from Durham University.
“We have found in our simulations that universes with much more dark energy than ours can happily form stars. So why such a paltry amount of dark energy in our universe?
“I think we should be looking for a new law of physics to explain this strange property of our universe, and the multiverse theory does little to rescue physicists’ discomfort.”
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