Nonmetricity tensor

In mathematics, the nonmetricity tensor in differential geometry is the covariant derivative of the metric tensor. It is therefore a tensor field of order three. It vanishes for the case of Riemannian geometry and can be used to study non-Riemannian spacetimes.

Definition
By components, it is defined as follows.


 * $$ Q_{\mu\alpha\beta}=\nabla_{\mu}g_{\alpha\beta} $$

It measures the rate of change of the components of the metric tensor along the flow of a given vector field, since


 * $$\nabla_{\mu}\equiv\nabla_{\partial_{\mu}} $$

where $$\{\partial_{\mu}\}_{\mu=0,1,2,3}$$ is the coordinate basis of vector fields of the tangent bundle, in the case of having a 4-dimensional manifold.

Relation to connection
We say that a connection $$\Gamma$$ is compatible with the metric when its associated covariant derivative of the metric tensor (call it $$\nabla^{\Gamma}$$, for example) is zero, i.e.


 * $$ \nabla^{\Gamma}_{\mu}g_{\alpha\beta}=0 .$$

If the connection is also torsion-free (i.e. totally symmetric) then it is known as the Levi-Civita connection, which is the only one without torsion and compatible with the metric tensor. If we see it from a geometrical point of view, a non-vanishing nonmetricity tensor for a metric tensor $$g$$ implies that the modulus of a vector defined on the tangent bundle to a certain point $$p$$ of the manifold, changes when it is evaluated along the direction (flow) of another arbitrary vector.