In Einstein gravity ( and ) the field equations (9.2) and (9.4) are identical to the equations (2.7) and (2.4), respectively. However, the difference appears for the f (R) models which include non-linear terms in . While the kinetic term is present in Eq. (2.7), such a term is absent in Palatini f (R) gravity. This has the important consequence that the oscillatory mode, which appears in the metric formalism, does not exist in the Palatini formalism. As we will see later on, Palatini f (R) theory corresponds to Brans–Dicke (BD) theory  with a parameter in the presence of a field potential. Such a theory should be treated separately, compared to BD theory with in which the field kinetic term is present.
As we have derived the action (2.21) from (2.18), the action in Palatini f (R) gravity is equivalent to
Using the relation (9.3), the action (9.5) can be written asf (R) gravity is equivalent to BD theory with the parameter [262, 470, 551]. As we will see in Section 10.1, this equivalence can be also seen by comparing Eqs. (9.1) and (9.4) with those obtained by varying the action (2.23) in BD theory. In the above discussion we have implicitly assumed that does not explicitly depend on the Christoffel connections . This is true for a scalar field or a perfect fluid, but it is not necessarily so for other matter Lagrangians such as those describing vector fields.
There is another way for taking the variation of the action, known as the metric-affine formalism [299, 558, 557, 121]. In this formalism the matter action depends not only on the metric but also on the connection . Since the connection is independent of the metric in this approach, one can define the quantity called hypermomentum , as . The usual assumption that the connection is symmetric is also dropped, so that the antisymmetric quantity called the Cartan torsion tensor, , is defined. The non-vanishing property of allows the presence of torsion in this theory. If the condition holds, it follows that the Cartan torsion tensor vanishes () . Hence the torsion is induced by matter fields with the anti-symmetric hypermomentum. The f (R) Palatini gravity belongs to f (R) theories in the metric-affine formalism with . In the following we do not discuss further f (R) theory in the metric-affine formalism. Readers who are interested in those theories may refer to the papers [557, 556].
This work is licensed under a Creative Commons License.