Post
Big Bang
Burger Bar
24 September 2013
Lately there’s been news of a radical new theory proposing that the universe began from a hyperdimensional black hole. Most of the reports seem to stem from an article posted a while back on the Nature blog, which references the original paper.^{1} So let’s have a little reality check.
No one is abandoning the big bang model. The original paper hasn’t even been peer reviewed yet and the paper doesn’t present a radical new theory to overturn the big bang. What the paper is actually about is higherdimensional gravitational theory.
The standard theory of gravity (general relativity) describes our universe as a geometry of threedimensional space with one dimension of time. This is sometimes called 3 + 1 space, and it gives a very accurate description of the universe we observe. But theorists like to play around with alternative models to see how they differ from regular general relativity. They may look at 2 + 1 space, a kind of flatland with time, or 2 + 2, with two time dimensions. There isn’t necessarily anything “real” about these models, and there certainly isn’t any experimental evidence to support anything other than 3 + 1 gravity, but alternative models are useful because they help us gain a deeper understanding of general relativity. In this particular paper, the authors were exploring 4 + 1 gravity. That is, a fivedimensional universe with 4 spatial dimensions and 1 time.
Back in 2000, another team of authors proposed a model where our regular 3 + 1 gravity could be treated as a brane within a larger 4 + 1 universe. It is similar to the way a 2 + 1 universe could be imagined as a 2dimensional surface (the brane) within our 3dimensional space. In the 2000 paper, the authors showed that a particular 4 + 1 universe with a 3 + 1 brane could give rise to the type of gravity we actually see.^{2}
The new paper takes this model one step further. In it, the authors show that 4 + 1 gravity allows for the existence of black holes. So if a 4 + 1 universe had large stars, some of those stars could collapse into a 4dimensional “hyper black hole”. Like black holes in regular general relativity, these hyper black holes would have a central “singularity” of extremely dense and hot matter/energy. The authors then went on to show that a hyper black hole with the right conditions could not only create a threedimensional brane, but the new brane would look very similar to the early universe we actually observe.
In other words, if we imagine a fivedimensional 4 + 1 universe, and if such a universe could create stars that collapse into hyper black holes, and if a particular hyper black hole had the right energy, then it might be possible for for such a hyper black hole to produce a 3 + 1 braneuniverse with a beginning that looks like a big bang. That’s a lot of ifs.
Just to be clear, this is good theoretical work. The model is interesting, and it shows a curious connection between the universe we observe and higherdimensional gravity. It could also address some of the issues in cosmology, but it also predicts the universe is flat, which as I mentioned yesterday may not be the case. The authors note this problem, and are careful not to make broad claims. They also outline possible ways that such a model could be tested. This is what good theoreticians do.
But currently there is no experimental evidence to support higherdimensions, much less hyper black holes. So don’t toss the big bang just yet.

Pourhasan, Razieh, Niayesh Afshordi, and Robert B. Mann. “Out of the white hole: a holographic origin for the Big Bang.” Journal of Cosmology and Astroparticle Physics 2014.04 (2014): 005. ↩︎

Dvali, Gia, Gregory Gabadadze, and Massimo Porrati. “4D gravity on a brane in 5D Minkowski space.” Physics Letters B 485.13 (2000): 208214. ↩︎