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WE COULD be living inside a black hole. This head-spinning idea is one cosmologist's conclusion based on a modification of Einstein's equations of general relativity that changes our picture of what happens at the core of a black hole.

In an analysis of the motion of particles entering a black hole, published in March, Nikodem Poplawski of Indiana University in Bloomington showed that inside each black hole there could exist another universe (Physics Letters B, DOI: 10.1016/j.physletb.2010.03.029). "Maybe the huge black holes at the centre of the Milky Way and other galaxies are bridges to different universes," Poplawski says. If that is correct - and it's a big "if" - there is nothing to rule out our universe itself being inside a black hole.

In Einstein's general relativity (GR), the insides of black holes are "singularities" - regions where the density of matter reaches infinity. Whether the singularity is an actual point of infinite density or just a mathematical inadequacy of GR is unclear, as the equations of GR break down inside black holes. Either way, the modified version of Einstein's equations used by Poplawski does away with the singularity altogether.

For his analysis, Poplawski turned to a variant of GR called the Einstein-Cartan-Kibble-Sciama (ECKS) theory of gravity. Unlike Einstein's equations, ECKS gravity takes account of the spin or angular momentum of elementary particles. Including the spin of matter makes it possible to calculate a property of the geometry of space-time called torsion.

When the density of matter reaches gargantuan proportions (more than about 1050 kilograms per cubic metre) inside a black hole, torsion manifests itself as a force that counters gravity. This prevents matter compressing indefinitely to reach infinite density, so there is no singularity. Instead, says Poplawski, matter rebounds and starts expanding again.

Now, in what is sure to be a controversial study, Poplawski has applied these ideas to model the behaviour of space-time inside a black hole the instant it starts rebounding (arxiv.org/abs/1007.0587). The scenario resembles what happens when you compress a spring: Poplawski has calculated that gravity initially overcomes torsion's repulsive force and keeps compressing matter, but eventually the repulsive force gets so strong that the matter stops collapsing and rebounds. Poplawski's calculations show that space-time inside the black hole expands to about 1.4 times its smallest size in as little as 10-46 seconds.

This staggeringly fast bounce-back, says Poplawski, could have been what led to the expanding universe we observe today.

How would we know if we are living inside a black hole? Well, a spinning black hole would have imparted some spin to the space-time inside it, and this should show up as a "preferred direction" in our universe, says Poplawski. Such a preferred direction would result in the violation of a property of space-time called Lorentz symmetry, which links space and time. It has been suggested that such a violation could be responsible for the observed oscillations of neutrinos from one type to another (Physical Review D, DOI: 10.1103/PhysRevD.74.105009).

If we are living inside a black hole, it would have imparted a 'special direction' to our universe

Sadly, there is no point in us looking for other universes inside black holes. As you approach a black hole, the increasing gravitational field makes time tick slower and slower. So, for an external observer, any new universe inside would form only after an infinite amount of time had elapsed.

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