Comic book writers love multiverses. It’s such an easy way to explain continuity problems; just say it happened in a different universe and move on.
But a number of very serious scientists want to take the concept out of comic books and into the real world.
They believe the universe we call home may be but one of an infinite number of universes, each slightly different.
It might be, they say, the best explanation we have for our own existence.
A new piece of Australian-led research, however, throws some serious cold water on the idea.
What is a multiverse?
“The idea is there is a lot of things about our universe which seem ‘fine-tuned’,” says Luke Barnes, a cosmologist with Western Sydney University.
“There are a whole lot of properties – like say how heavy an electron is – we don’t know why they are that weight. And if you change them, even slightly, the universe becomes too simple to make life.”
Imagine gravity is on a dial that ranges from one to 100. It needs to be tuned right to make atoms compress into stars. Without stars, there are no planets, and without planets there is no life.
The odds of our universe being so perfect are too low, believers in the theory argue.
One way our universe’s existence becomes likely is if we exist in a multiverse containing all possible universes. With enough possible universes, one is bound to be just right for life, they argue.
How to find a multiverse
Of course, scientists have no way of detecting other universes – which naturally makes the multiverse theory controversial.
“We’ve got some people, including some very senior cosmologists, basically saying the multiverse is not science,” says Dr Barnes.
“We’re supposed to follow the evidence. But we have no hope of ever observing another universe.”
Faced with this, scientists like Dr Barnes are trying to find ways of estimating the probability of the existence of a multiverse. This is rather complicated.
First, they construct mathematical models of what a multiverse might look like using supercomputers.
Return to the idea of gravity as a dial that can range between one and 100. According to the mathematical model, multiverses with one “setting” will be more likely than those with another setting.
Then you can check the strength of gravity in our universe. If the model deems it likely, it adds weight to the possibility of the existence of a multiverse. If the model deems it unlikely, it suggests something else altogether is going on.
Dark energy
Sadly for comic book fans, a pair of papers published on Monday in the Monthly Notices of the Royal Astronomical Society found some very unlikely numbers indeed.
The team was looking at a substance called dark energy. Our universe has an unusually small amount of it.
It was thought you needed a small amount of dark energy in the universe to allow stars to form. The fact we had about the right amount was considered good evidence for the multiverse.
But after running thousands of hours of supercomputer simulations at the University of Sydney, where he worked until recently, Dr Barnes’ team found that wasn’t the case at all.
“We turned it up to 300 and still did not kill the universe. We tried, but we still did not see that star obliterating universe.”
The team found nearly every setting from 1 to 300 could support life.
That’s bad news for the multiverse, because it means there’s nothing particularly special about our universe – and no need for a multiverse to explain our existence.