Scalar field solution
In general relativity, a scalar field solution is an exact solution of the Einstein field equation in which the gravitational field is due entirely to the field energy and momentum of a scalar field. Such a field may or may not be "massless", and it may be taken to have "minimal curvature coupling", or some other choice, such as "conformal coupling". Mathematical definition. In general relativity, the geometric setting for physical phenomena is a Lorentzian manifold, which is physically interpreted as a curved spacetime, and which is mathematically specified by defining a metric tensor formula_1 (or by  defining a frame field). The curvature tensor formula_2 of this manifold and associated quantities such as the Einstein tensor formula_3, are well-defined even in the absence of any physical theory, but in general relativity they acquire a physical interpretation as geometric manifestations of the gravitational field. In addition, we must specify a scalar field by giving a function formula_4. This function is required to satisfy two following conditions: formula_6. Both conditions follow from varying the Lagrangian density for the scalar field, which in the case of a minimally coupled massless scalar field is Here, gives the wave equation, while  gives the Einstein equation (in the case where the field energy of the scalar field is the only source of the gravitational field). Physical interpretation. Scalar fields are often interpreted as classical approximations, in the sense of effective field theory, to some quantum field. In general relativity, the speculative quintessence field can appear as a scalar field. For example, a flux of neutral pions can in principle be modeled as a minimally coupled massless scalar field. Einstein tensor. The components of a tensor computed with respect to a frame field rather than the coordinate basis are often called "physical components , because these are the components which can (in principle) be measured by an observer. In the special case of a minimally coupled massless scalar field", an "adapted frame" can always be found in which the Einstein tensor takes the simple form formula_11 where formula_12 is the "energy density" of the scalar field. Eigenvalues. The characteristic polynomial of the Einstein tensor in a minimally coupled massless scalar field solution must have the form In other words, we have a simple eigenvalue and a triple eigenvalue, each being the negative of the other. Multiply out and using Gröbner basis methods, we find  that the following three invariants must vanish identically: Using Newton's identities, we can rewrite these in terms of the traces of the powers. We find that We can rewrite this in terms of index gymnastics as the manifestly invariant criteria: Examples. Notable individual scalar field solutions include