The idea that the explosion of a singularity was responsible for the birth of our Universe has been the dominant scientific creation story for decades. We call it the Big Bang.
But what caused the Big Bang?
Physicists usually dismiss the question as nonsense, since time did not exist before the Universe began expanding. Everything before t=0 is unknowable.
But what if the Big Bang was "just a mirage"? What if our Universe was really created by the death of a four-dimensional black hole?
That's the startling theory proposed in a new paper by Canadian Niayesh Afshordi, an astrophysicist at the Perimeter Institute in Waterloo, Ontario.
The idea is in the domain of "brane cosmology," which posits that our Universe is a "brane" contained within a higher-dimensional "bulk" universe.
Think of a 2-dimensional object like a sheet of paper. That object is contained within our 3-dimensional world. Now imagine a stack of paper. In the brane theory, each piece of paper is its own 2-dimensional universe contained within a 3-dimensional bulk universe.
Here's Afshordi's theory as summarized by Nature's Zeeya Merali:
Ashfordi's team realized that if the bulk universe contained its own four-dimensional (4D) stars, some of them could collapse, forming 4D black holes in the same way that massive stars in our Universe do: they explode as supernovae, violently ejecting their outer layers, while their inner layers collapse into a black hole.
In our Universe, a black hole is bounded by a spherical surface called an event horizon. Whereas in ordinary three-dimensional space it takes a two-dimensional object (a surface) to create a boundary inside a black hole, in the bulk universe the event horizon of a 4D black hole would be a 3D object — a shape called a hypersphere. When Afshordi’s team modelled the death of a 4D star, they found that the ejected material would form a 3D brane surrounding that 3D event horizon, and slowly expand.
The authors postulate that the 3D Universe we live in might be just such a brane — and that we detect the brane’s growth as cosmic expansion. “Astronomers measured that expansion and extrapolated back that the Universe must have begun with a Big Bang — but that is just a mirage,” says Afshordi.
Let's just say the math is, well, complex.
The theory has some advantages, though. Not only does it tell us what may have created our Universe, it also explains why it is expanding and why it is so uniform.
The question of why the universe is expanding when gravity should pull matter together is one of the central questions of physics today. It is the question that has propelled the search for dark energy, which is thought to balance the attractive force of gravity.
There is just one major problem with Ashfordi's otherwise neat theory: it doesn't align with the most recent observations of the cosmic background radiation -- the picture we have of what the Universe looked like in the early moments of creation.
Ashfordi told Nature he is refining the theory so it will more closely fit the new observations.
Until then, we can all keep busy by trying to picture four dimensions.
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