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A boost-Killing congruence in flat space: acceleration source and conservation of the redshift-weighted energy current
Example
Let carry the identity chart, the flat metric and the time orientation , with the standard coordinate orientation , and metric-selected connection (Smooth time-oriented Lorentzian four-manifolds: the geometric objects of the model, 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). Let
let on , let , and let with constant (the tensor of the dust model , The dust matter model: rest density, four-velocity, stress-energy and dust system). Then:
- is open, is a Killing field on , and is a smooth future-directed observer congruence on with and acceleration and expansion
- satisfies the dust system and on .
- The measured fields of The observer split of a symmetric two-tensor: energy density, energy-momentum current, flux and stress for the congruence are
- and : the balance of Divergence of the measured energy-momentum current and the finite-chain balance identity clause (b) holds with a nonvanishing acceleration source, so the measured energy current is not conserved.
- satisfies and : the redshift-weighted current is conserved, in accordance with Kinematics of a timelike Killing congruence and conservation of the redshift-weighted energy current clause (d). Consequently, for every finite smooth singular -chain with , the flux satisfies ; in particular whenever the velocity vectors of every simplex of span a hyperplane containing .
Facts & Assumptions
Given: The flat Lorentzian four-manifold with identity chart, , metric-selected connection , time orientation and standard coordinate orientation; the open set ; the vector field , the positive function and the congruence ; the constant and the tensor .
On the identity chart the Christoffel symbols vanish and is the coordinate connection, so for smooth fields the covariant derivatives are the coordinate derivatives; in particular the divergence of a vector field is the coordinate divergence (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, The inverse metric, musical isomorphisms and index raising).
For a smooth vector field and a covariant -tensor field with constant chart components, ; is Killing exactly when (The coordinate formula for the Lie derivative of a covariant tensor, Killing fields of a Lorentzian metric).
is open and on both and is timelike and future-directed: and ; hence satisfies , and is a smooth observer congruence (Observer congruences, their observer curves and adapted charts, Observers, rest spaces and the spatial projection at an event, Timelike, null, spacelike and causal vectors of a Lorentz vector space, Real powers for positive bases, with the zero-base positive-exponent convention, Continuity and derivatives of positive-base real powers).
For such a timelike Killing field one has , and (Kinematics of a timelike Killing congruence and conservation of the redshift-weighted energy current clauses (a) and (b)).
is a dust model with tensor ; the dust system holds for constant and constant , and it is equivalent to (The dust matter model: rest density, four-velocity, stress-energy and dust system, Divergence-free dust is exactly the dust system, and divergence-free perfect fluid is exactly the fluid equations clause (a)).
The measured fields satisfy and the decomposition of The orthogonal observer decomposition of a symmetric two-tensor and its uniqueness; with the identities and hold (The observer split of a symmetric two-tensor: energy density, energy-momentum current, flux and stress, Divergence of the measured energy-momentum current and the finite-chain balance identity clauses (a) and (b)).
For a timelike Killing field with and : , , , and the flux balance holds on chains contained in (Kinematics of a timelike Killing congruence and conservation of the redshift-weighted energy current clauses (c) and (d)).
Verification
(Killing field.) In the identity chart the components of are constant, so [L2] gives , , , and the remaining components vanish because and the metric is diagonal with , . Hence and is Killing on , hence on .
(The congruence and its kinematics.) is open as the preimage of under the continuous function . By [L3], is a smooth future-directed observer congruence on , and [L4] gives , and ; since and , the covector has components and the vector .
(The dust model.) is constant and smooth, and is a smooth future-directed unit timelike field with constant components; the dust system is and by [L1], so [L5] gives on .
(Measured fields.) By [L1], and . Since and , we get ; further , so and has components and , using . This is clause 3, and the decomposition of [L6] holds by [L6].
(The balance and its source.) By [L1] and step 2.1, , using . On the other side, , so , consistent with [L7]. Moreover and , so with the balance of [L6] holds and its source does not vanish.
(The weighted current and the flux balance.) , so by [L1], ; and . Applying the flux balance of [L7] on with its induced standard orientation to gives for every finite smooth singular -chain with . If the velocity vectors of every simplex of span a hyperplane containing , then vanishes on those velocity vectors, because for linearly dependent arguments and is a function multiple of ; hence and . This completes the verification.
Remarks
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Contrast with the unweighted current. The measured energy current is not divergence-free: the boost congruence is accelerated, and clause 4 exhibits the source . The redshift weight converts into the divergence-free current , which is the content of Kinematics of a timelike Killing congruence and conservation of the redshift-weighted energy current in this explicit model.
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Interpretation of the weight. On the observers of the congruence move with rapidity relative to the static dust; is the corresponding normalization, and the measured energy density is the -enhanced density of the static dust seen by them.
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No measurement. The displayed values are exact identities of the flat model; the example asserts no measured value and no empirical claim.
Depends on
The observer split of a symmetric two-tensor: energy density, energy-momentum current, flux and stress
The orthogonal observer decomposition of a symmetric two-tensor and its uniqueness
Acceleration, expansion and the along-congruence derivative of an observer congruence
Spatiality and curve identity of the acceleration, the divergence form of the expansion, and the along-congruence derivative
Divergence of the measured energy-momentum current and the finite-chain balance identity
Kinematics of a timelike Killing congruence and conservation of the redshift-weighted energy current
Killing fields of a Lorentzian metric
Killing fields and conserved currents, and on-shell conservation of the Hilbert stress-energy tensor
The dust matter model: rest density, four-velocity, stress-energy and dust system
Divergence-free dust is exactly the dust system, and divergence-free perfect fluid is exactly the fluid equations
Smooth time-oriented Lorentzian four-manifolds: the geometric objects of the model
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
The inverse metric, musical isomorphisms and index raising
Smooth covariant 2-tensor fields on a smooth manifold
Tensor fields of type (r,s) on a Lorentzian four-manifold and their components
Observer congruences, their observer curves and adapted charts
Observers, rest spaces and the spatial projection at an event
Timelike, null, spacelike and causal vectors of a Lorentz vector space
The coordinate formula for the Lie derivative of a covariant tensor
Real powers for positive bases, with the zero-base positive-exponent convention
Continuity and derivatives of positive-base real powers
and smooth maps between smooth manifolds
A smooth vector field is a smooth section of the tangent bundle
Dependency tree · 2 levels
Sources
- H. S. Reall, Part III General Relativity (Cambridge, October 12, 2022 revision), section 5.3: Killing vectors, the Killing equation and conserved quantities along geodesics, printed pp. 53-56 (PDF pp. 54-57)
- S. M. Carroll, Lecture Notes on General Relativity, arXiv:gr-qc/9712019v1 (1997), Ch. 5: Killing vectors and the boost symmetry of Minkowski space, printed pp. 140-142 (PDF pp. 147-149)
- E. Gourgoulhon, 3+1 Formalism and Bases of Numerical Relativity, arXiv:gr-qc/0703035v1 (2007), sections 3.3.3-3.3.4: the acceleration of an Eulerian congruence as the gradient of the logarithm of the lapse, printed pp. 43-44 (PDF pp. 43-44)