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Conformal Einstein tensor transformation
Statement
Let be the smooth oriented, time-oriented Lorentzian four-manifold of Oriented Lorentzian fields, observers and coordinate conventions, with the curvature convention and tensors of The Riemann curvature operator and the derived Ricci, scalar and Einstein tensors. Let be smooth and dimensionless, and put with the same orientation. Use the metric-selected connection and its tensor derivative of Metric compatibility, torsion freedom and the metric-selected connection class and Existence and uniqueness of the metric-selected connection of a nondegenerate metric, and its extension to tensor fields. Define the smooth one-form and its contractions by
Then and
All curvature conventions are those stated in The Riemann curvature operator and the derived Ricci, scalar and Einstein tensors. These are mathematical transformation identities, not an assertion that the Einstein equation is conformally invariant.
Facts & Assumptions
Given: A smooth oriented Lorentzian four-manifold, smooth positive , and the metric-selected connections of and . The inherited geometric setup carries as in [A1].
The geometry is smooth, oriented and time-oriented, with nondegenerate Lorentz metric of signature ; its smooth real forms and metric volume form have the definitions of Oriented Lorentzian fields, observers and coordinate conventions.
Curvature is , with , , , and . (The Riemann curvature operator and the derived Ricci, scalar and Einstein tensors)
For either smooth Lorentz metric or , the metric-selected connection is affine and torsion-free and has Christoffel formula . (Metric compatibility, torsion freedom and the metric-selected connection class, Existence and uniqueness of the metric-selected connection of a nondegenerate metric, and its extension to tensor fields)
For a type tensor , the tensor derivative has components ; it commutes with contractions and annihilates and . (Existence and uniqueness of the metric-selected connection of a nondegenerate metric, and its extension to tensor fields)
The exterior derivative sends a smooth -form to a smooth -form, has coordinate formula , and obeys and the graded product rule. (Lorentzian exterior calculus and Hodge star)
For and , ; the chain rule holds for total derivatives. (Continuity and derivatives of positive-base real powers, The chain rule for total derivatives: )
For each finite , finite componentwise sums and products of Euclidean maps and compositions of composable Euclidean maps are . Applying this at every finite in smooth manifold charts shows that finite sums, products and compositions of smooth maps are smooth. ( Euclidean maps are closed under componentwise algebra and composition)
For either metric-selected connection, with the curvature convention of [F2], the coordinate curvature formula is , with summation over . (Tensoriality of the curvature, its coordinate formula and its point-normal metric formula)
The inherited choice assumption is Countable Choice , not full Choice. (The Axiom of Countable Choice ())
Proof
In a chart, and . Substituting these into [F3] cancels the common and gives , where . Iterating the derivative formula in [F6] gives for every integer and ; all these derivatives are continuous, so is smooth on . Its composition with is smooth by [F7], and is a smooth one-form by [F5], so and the displayed components are smooth. Finally, define for smooth local vector fields. The affine-connection rules in [F3] give and for every smooth function , while its components are the smooth differences ; hence is a smooth tensor of type with those components.
By [F5], , so . Therefore . Torsion freedom in [F3] gives , hence .
Apply the coordinate curvature formula [F8] to and and subtract, using from step 1.1. Since is a tensor, [F4] gives ; the two covariant-derivative terms acting on the lower curvature-direction slots cancel because , while the lower -slot contributions are the last two displayed terms. These are exactly the partial-derivative and mixed terms in the subtraction, and the remaining terms are . Therefore .
Contracting with as in [F2], the derivative terms are . The definition of gives and, using from [F4], by step 2.1. Thus their difference is .
The first quadratic contraction is . Expanding the second contraction into its nine terms gives . Their difference is . Combining this with step 3.1 proves the Ricci transformation in the Statement.
Contracting the Ricci transformation with gives .
By definition, . Substituting the Ricci formula from step 4.1 and from step 5.1 cancels the scalar-curvature terms and leaves . This is the claimed Einstein-tensor formula.
Remarks
The curvature sign and Ricci contraction were taken from the inspected Lorentzian supplier The Riemann curvature operator and the derived Ricci, scalar and Einstein tensors. The conformal identities above were derived with that convention; no Riemannian curvature formula or sign convention is imported.
Depends on
The Axiom of Countable Choice ()
Oriented Lorentzian fields, observers and coordinate conventions
The Riemann curvature operator and the derived Ricci, scalar and Einstein tensors
Metric compatibility, torsion freedom and the metric-selected connection class
Smooth time-oriented Lorentzian four-manifolds: the geometric objects of the model
Lorentzian exterior calculus and Hodge star
The chain rule for total derivatives:
Tensoriality of the curvature, its coordinate formula and its point-normal metric formula
Existence and uniqueness of the metric-selected connection of a nondegenerate metric, and its extension to tensor fields
Continuity and derivatives of positive-base real powers
Euclidean maps are closed under componentwise algebra and composition
Dependency tree · 2 levels
Sources
- David Tong, General Relativity, Cambridge Part III lecture notes, §4, author-hosted full-text HTML, conformal metric and Einstein equations
- Sean M. Carroll, Lecture Notes on General Relativity, arXiv:gr-qc/9712019v1, §§3.3–3.5 and 7.2, curvature contractions, conformal compactification and Einstein–Maxwell equations