This theory stands in contrast to the Big Bang Theory, which claims the Universe rapidly expanded from a dense point of singularity. The Big Bang Theory fails to explain the uniformity in temperature across the Universe, according to co-theorist Niayesh Afshordi, an astrophysicist at the Perimeter Institute for Theoretical Physics. He says if everything originated from a single point, it is unlikely that temperature across the Universe could have evened out so quickly in the relatively short time it has existed (about 13.8 billion years).
The main proposal is our three-dimensional universe is a membrane, or “event horizon”, for an imperceptible four-dimensional universe, also called a “Bulk Universe.” When a 4D star dies, they explode as supernovae, like stars in out Universe. The inner layers of the star collapse and form a black hole, while the debris from the outer layers is sprayed forcefully outward, forming a 3D membrane, which we know as our Universe. This 3D membrane slowly expands outwards, which accounts for 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.
The new theory builds off an existing study, which took place in 2000 and was led by Gia Dvali of New York University, which initially proposed the idea that our universe is a 3D membrane floating through a 4D Universe. The basic reasoning rests behind the fact that stars in our Universe collapse into black holes as well. In our 3D Universe, an event horizon in a black hole can be formed by a 2D surface. Using this concept, Afshordi and his team modeled that the event horizon in a 4D black hole would be a 3D object, called a hypersphere.
The study also has an explanation for the uniformity of the Universe: Afshordi and his team theorize that the 4D Universe has existed for an infinitely long time, which means it would have enough time to reach a temperature equilibrium. When our 3D Universe was formed, it inherited the uniformity in from its parent Universe.
As is the case with many cosmological theories, there is no way to conclusively prove this theory, especially since it attempts to explain events which took place nearly 14 billion years ago. A great deal of additional study is still being done regarding this theory. Afshordi and his team continue to adjust their models and calculations in hopes of lending further credence to their argument. For now, it remains an interesting challenger to the Big Bang Theory.