According to a straightforward interpretation of general relativity, the Big Bang wasn't the start of 'everything'.
Taking Einstein's famous equations at face value and making as few assumptions as possible, a team of researchers has rewound the clock on our Universe to find it wouldn't lead to a stopping point at all, but would take us through a different kind of beginning into a flipped space.
To understand what all the fuss over the Big Bang is, we need to rewind a bit to understand why physicists think it may not have been the start of everything.
Around 90 years ago, a Belgian astronomer named Georges Lemaître proposed that observed changes in the shifting of light from distant galaxies implied the Universe is expanding. If it's getting bigger, it stands that it used to be smaller.
Keep rewinding the clock – by around 13.8 billion years – and we get to a point where space has to be confined to an incredibly tiny volume, also known as a singularity.
"At this time, the Big Bang, all the matter in the universe, would have been on top of itself. The density would have been infinite," Stephen Hawking once explained in his lecture on The Beginning of Time.
There are a number of models physicists use to describe the nothingness of empty space. Einstein's general relativity is one - it describes gravity as it relates to the geometry of the Universe's underlying fabric.
Theorems proposed by Hawking and mathematician Roger Penrose claim that solutions to general relativity's equations on an infinitely constrained scale – like the one inside a singularity – are incomplete.
In everyday terms, it's often said physics breaks down at the singularity, leading to a mix of speculations on what little we can tease out of the physics that still makes sense.
Hawking only recently gave his own take in an interview with Neil deGrasse Tyson, where he likened the space-time dimensions of the Big Bang to the South Pole. "There is nothing south of the South Pole, so there was nothing around before the Big Bang," he said.
But other physicists have argued there's something beyond the Big Bang. Some propose that there is a mirror Universe on the other side, where time moves backwards. Others argue in favour of a rebounding Universe.
Taking a slightly different approach, physicists Tim A. Koslowski, Flavio Mercati, and David Sloan have come up with a new model, pointing out that the breakdown arises from a contradiction in properties at a particular point in time as defined by general relativity.
What the theorem doesn't imply is how the Universe as we observe it necessarily gets to that point in the first place.
Stepping back from the whole singularity issue, the researchers reinterpreted the existing model of shrinking space by distinguishing the map of space-time itself from the 'stuff' in it.
"All the terms that are problematic turn out to be irrelevant when working out the behaviour of quantities that determine how the Universe appears from the inside," says Sloan, a physicist from the University of Oxford.
What this essentially adds up to is a description of the Big Bang where physics remains intact as the stage it acts upon reorientates.
Rather than a singularity, the team call this a Janus Point, named after the Roman god with two faces.
The relative positions and scales of the stuff that makes up the Universe effectively flatten into a two-dimensional pancake as we rewind time. Passing through the Janus Point, that pancake turns 3D again, only back-to-front.
What that means in physical terms is hard to say, but the researchers believe it could have profound implications on symmetry in particle physics, maybe even producing a Universe based primarily on antimatter.
While the idea of a flipped Universe is old news, the approach of working around the singularity problem in this particular way is novel.
"We introduce no new principles, and make no modifications to Einstein's theory of general relativity – only of the interpretation that is put upon objects," says Sloan.
No doubt this debate will rage on well into the future. Who knows? Maybe there's a similar argument happening in the mirror Universe sometime on the other side of the Janus Point.
This research was published in Physics Letters B.