So what is Torsion?, and should we believe in it?. Torsion is spin-spin coupling in one of the simplest extensions to General Relativity, called Einstein-Cartan Theory.

And it turns out that once we try to couple Fermions (half integral spin particles) to generally relativity, we quickly find we need Torsion. Here is an example construction, that might explain inflation. In fact both string theory and loop quantum gravity seem actually to be theories of Einstein-Cartan gravity, its just difficult to have fermions in a quantum gravity theory without Torsion.

Torsion is a very small effect, its generally a four-fermion interaction scaled by the gravitional constant G. So is too weak to be measured in normal circumstances in the universe. However, if the above paper by Mahato is true, then existance of torsion, leaves open the existance of axial forces which may much stronger.

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