Our universe seems to contain a suspiciously perfect amount of dark energy to sustain life – any more and the fabric of reality would tear apart before life has a chance to take hold. But new research suggests that that might not be the case. By simulating how a universe would grow with different amounts of the stuff, scientists have found that life is still possible even with far more of it than we have, and the results have some big implications for the multiverse theory.
In the 1990s, it was found that the universe isn't expanding at a constant rate, it's accelerating, and this work earned its discoverers the 2011 Nobel Prize in Physics. Scientists dubbed the force behind this acceleration "dark energy," although that's more of a placeholder name than any actual description of it. To try to shed more light on dark energy, the Dark Energy Camera was launched in 2012 and a huge collection of data from the Survey was released earlier this year.
Dark energy has been calculated to account for more than 68 percent of the contents of the universe. But the question remains – why exactly that much? Our universe seems conveniently set up to support life, and it's assumed that if there was even a little bit more or less dark energy, then stars, planets and, by extension, life, wouldn't have been able to develop.
The multiverse theory helps explain this Goldilocks level of dark energy. According to this idea, our universe is just one of an infinite number of possible universes, so chances are that somewhere among them are universes where the conditions are just right. A selection bias known as the weak anthropic principle plays a part too – basically, the only way we're able to question how unlikely our existence is, is if we exist.
The new study, from researchers at the University of Sydney, Durham University, Western Sydney University and the University of Western Australia, looked at how different levels of dark energy might affect the development of life. The team produced simulations using the Evolution and Assembly of GaLaxies and their Environments (EAGLE) project, which is one of the most comprehensive simulations of the observed universe.
The study found that dark energy has a much smaller impact on the formation of stars and planets than was previously believed. Even when the simulated universe contained hundreds of times more dark energy than there is in our universe, or far less, life still found a way.
"We asked ourselves how much dark energy can there be before life is impossible?" says Pascal Elahi, an author of the study. "Our simulations showed that the accelerated expansion driven by dark energy has hardly any impact on the birth of stars, and hence places for life to arise. Even increasing dark energy many hundreds of times might not be enough to make a dead universe."
That has wide-reaching implications for life in the multiverse. Previously, it was thought that most parallel universes would have evolved with conditions hostile to life, but the new study suggests life might be far more common across the multiverse. Or, perhaps this could be a blow to the very idea of a multiverse. After all, the amount of dark energy in our universe is no longer a suspiciously perfect amount. To the contrary, the researchers say the multiverse theory suggests there should be 50 times more dark energy in our universe than there is.
"The formation of stars in a universe is a battle between the attraction of gravity, and the repulsion of dark energy," says Richard Bower, an author of the study. "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."
Interestingly, Stephen Hawking's final paper, posthumously published earlier this month, also steps back somewhat from the multiverse theory, of which he was once a huge proponent.
The research was published in the journal Monthly Notices of the Royal Astronomical Society.
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