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A boost-Killing congruence in flat space: acceleration source and conservation of the redshift-weighted energy current

Example

Let M=R4 carry the identity chart, the flat metric g=diag⁡(−1,1,1,1) and the time orientation N=∂0, with the standard coordinate orientation dx0∧dx1∧dx2∧dx3, 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

U:={p∈M: x1(p)>∣x0(p)∣},ξ:=x1∂0+x0∂1,

let X:=(x1)2−(x0)2>0 on U, let n:=ξ/X, and let Tab:=ρ ∂0a∂0b with ρ>0 constant (the tensor of the dust model (ρ,∂0), The dust matter model: rest density, four-velocity, stress-energy and dust system). Then:

  1. U is open, ξ is a Killing field on U, and n is a smooth future-directed observer congruence on U with na∇aX=0 and acceleration and expansion

a=∇ln⁡X=x0X2 ∂0+x1X2 ∂1,θ=0.

  1. (ρ,∂0) satisfies the dust system and ∇aTab=0 on M.
  2. The measured fields of The observer split of a symmetric two-tensor: energy density, energy-momentum current, flux and stress for the congruence n are

μ=ρ(x1)2X2,J=ρx1X ∂0,Φ=−ρx1(x0)2X3 ∂0−ρ(x1)2x0X3 ∂1.

  1. ∇aJa=−Φaaa=ρx0x1/X3 and Dμ+μθ+∇aΦa=ρx0x1/X3: 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.
  2. XJ=ρx1∂0 satisfies ∇a(XJa)=0 and Tabξb=−XJa: 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 4-chain C⊆U with ∂C=Σ2−Σ1+B, the flux ΦXJ satisfies ΦXJ(Σ2)−ΦXJ(Σ1)=−ΦXJ(B); in particular ΦXJ(Σ2)=ΦXJ(Σ1) whenever the velocity vectors of every simplex of B span a hyperplane containing ∂0.

Facts & Assumptions

Given: The flat Lorentzian four-manifold (R4,g) with identity chart, g=diag⁡(−1,1,1,1), metric-selected connection ∇, time orientation N=∂0 and standard coordinate orientation; the open set U; the vector field ξ=x1∂0+x0∂1, the positive function X=(x1)2−(x0)2 and the congruence n=ξ/X; the constant ρ>0 and the tensor Tab=ρ ∂0a∂0b.

[L1]

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).

[L2]

For a smooth vector field ξ and a covariant 2-tensor field with constant chart components, (Lξg)ab=gcb∂aξc+gac∂bξc; ξ is Killing exactly when Lξg=0 (The coordinate formula for the Lie derivative of a covariant tensor, Killing fields of a Lorentzian metric).

[L3]

U is open and on U both X>0 and ξ is timelike and future-directed: g(ξ,ξ)=−(x1)2+(x0)2=−X2 and g(ξ,N)=−x1<0; hence n=ξ/X satisfies g(n,n)=−1, g(n,N)=−x1/X<0 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).

[L4]

For such a timelike Killing field one has na∇aX=0, a=∇ln⁡X and θ=0 (Kinematics of a timelike Killing congruence and conservation of the redshift-weighted energy current clauses (a) and (b)).

[L5]

(ρ,∂0) is a dust model with tensor Tab=ρ ∂0a∂0b; the dust system holds for constant ρ and constant ∂0, and it is equivalent to ∇aTab=0 (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)).

[L6]

The measured fields satisfy J=μn+Φ and the decomposition of The orthogonal observer decomposition of a symmetric two-tensor and its uniqueness; with ∇aTab=0 the identities ∇aJa=−Tab∇anb and Dμ+μθ+∇aΦa=∇aJa 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)).

[L7]

For a timelike Killing field ξ with X=−g(ξ,ξ) and n=ξ/X: Tab∇anb=Φaaa, ∇a(XJa)=0, Tabξb=−XJa, and the flux balance ΦXJ(Σ2)−ΦXJ(Σ1)=−ΦXJ(B) holds on chains contained in U (Kinematics of a timelike Killing congruence and conservation of the redshift-weighted energy current clauses (c) and (d)).

Verification

technique · direct; check the Killing equation in the identity chart, verify the unit and future conditions of $n$, compute the measured fields and the balance explicitly, and identify the divergence-free weighted current
1.1L2algebra

(Killing field.) In the identity chart the components of g are constant, so [L2] gives (Lξg)00=2g00∂0ξ0=2(−1)(0)=0, (Lξg)11=2g11∂1ξ1=2(1)(0)=0, (Lξg)01=g11∂0ξ1+g00∂1ξ0=1⋅1+(−1)⋅1=0, and the remaining components vanish because ξ2=ξ3=0 and the metric is diagonal with ∂aξ0=δa1, ∂aξ1=δa0. Hence Lξg=0 and ξ is Killing on M, hence on U.

1.2L3L4algebra

(The congruence and its kinematics.) U is open as the preimage of (0,∞) under the continuous function x1−∣x0∣. By [L3], n=ξ/X is a smooth future-directed observer congruence on U, and [L4] gives na∇aX=0, θ=0 and a=∇ln⁡X; since ∂0X=−x0/X and ∂1X=x1/X, the covector aa=∂aln⁡X has components (−x0/X2, x1/X2) and the vector a=(x0/X2)∂0+(x1/X2)∂1.

1.3L1L5algebra

(The dust model.) ρ>0 is constant and smooth, and ∂0 is a smooth future-directed unit timelike field with constant components; the dust system is ∇a(ρ∂0a)=ρ ∂0(1)=0 and ∂0a∇a∂0b=0 by [L1], so [L5] gives ∇aTab=0 on M.

2.1L1step 1.2L6algebra

(Measured fields.) By [L1], na=(x1/X)δ0a+(x0/X)δ1a and na=(−x1/X)δa0+(x0/X)δa1. Since Tab=ρ ∂a0∂b0 and ∂0ana=n0=−x1/X, we get μ=Tabnanb=ρ(x1)2/X2; further Tabnb=ρ ∂0an0=−ρ(x1/X)∂0a, so Ja=−Tabnb=ρ(x1/X)∂0a and Φ=J−μn has components Φ0=ρx1/X−ρ(x1)2/X2⋅x1/X=−ρx1(x0)2/X3 and Φ1=0−ρ(x1)2/X2⋅x0/X=−ρ(x1)2x0/X3, using (x1)2−X2=(x0)2. This is clause 3, and the decomposition of [L6] holds by [L6].

3.1step 1.2step 2.1L1L6L7algebra

(The balance and its source.) By [L1] and step 2.1, ∇aJa=∂0 ⁣(ρx1/X)=ρx1∂0(X−1)=−ρx1X−2∂0X=ρx0x1/X3, using ∂0X=−x0/X. On the other side, Φaaa=Φ0(−x0/X2)+Φ1(x1/X2)=ρx1(x0)3/X5−ρ(x1)3x0/X5=ρx0x1((x0)2−(x1)2)/X5=−ρx0x1/X3, so ∇aJa=−Φaaa, consistent with [L7]. Moreover Dμ=na∂aμ=ρ[(x1/X)(2(x1)2x0/X4)+(x0/X)(2x1/X2−2(x1)3/X4)]=2ρx0x1/X3 and ∇aΦa=∂0Φ0+∂1Φ1=−ρx0x1/X3, so with θ=0 the balance Dμ+μθ+∇aΦa=ρx0x1/X3=−Φaaa of [L6] holds and its source does not vanish.

4.1step 1.2step 3.1L1L7algebra∎

(The weighted current and the flux balance.) XJ=ρx1∂0, so by [L1], ∇a(XJa)=∂0(ρx1)=0; and Tabξb=ρ ∂0a∂0bξb=ρ ∂0aξ0=−ρx1∂0a=−XJa. Applying the flux balance of [L7] on U with its induced standard orientation to K=XJ gives ΦXJ(Σ2)−ΦXJ(Σ1)=−ΦXJ(B) for every finite smooth singular 4-chain C⊆U with ∂C=Σ2−Σ1+B. If the velocity vectors of every simplex of B span a hyperplane containing ∂0, then ιXJvolg vanishes on those velocity vectors, because volg(∂0,v1,v2,v3)=0 for linearly dependent arguments and XJ is a function multiple of ∂0; hence ΦXJ(B)=0 and ΦXJ(Σ2)=ΦXJ(Σ1). This completes the verification.

Remarks

  • Contrast with the unweighted current. The measured energy current J is not divergence-free: the boost congruence is accelerated, and clause 4 exhibits the source −Φaaa. The redshift weight X converts J into the divergence-free current −T(⋅,ξ), which is the content of Kinematics of a timelike Killing congruence and conservation of the redshift-weighted energy current in this explicit model.

  • Interpretation of the weight. On U the observers of the congruence move with rapidity artanh⁡(x0/x1) relative to the static dust; X=(x1)2−(x0)2 is the corresponding normalization, and the measured energy density μ=ρ(x1)2/X2 is the γ2-enhanced density of the static dust seen by them.

  • No measurement. The displayed values are exact identities of the flat model; the example asserts no measured value and no empirical claim.

Depends on

Dependency tree · 2 levels

Sources