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Physics[ThreePlusOne][ADMEquations] - The ADM Equations as the 3+1 decomposition of the Einstein equations

Calling Sequence

ADMEquations()

ADMEquations(keyword = ...)

Parameters

keyword = ...

-

optional, the left-hand side can be inert and the right-hand side can be true (default) or false; or the left-hand side can be output and the right hand side can be 4D (default) or 3D, to return with the free indices of spacetime or space kind.

Description

• 

The ADMEquations command returns the ADM equations in inert form, that is, the 3+1 decomposition of Einstein's equations expressed in terms of the Lapse, Shift, the 3D metric gamma3_, the ExtrinsicCurvature and the EnergyMomentum tensor.

• 

You can set the values of the Lapse and Shift using Setup and its lapseandshift keyword. There are three possible values for lapseandshift: standard, arbitrary, or a list of algebraic expressions representing the Lapse and Shift. The value chosen determines the values of the components of all the ThreePlusOne tensors and so of tensorial expressions involving them (e.g. the ExtrinsicCurvature and ADMEquations). Those components are always computed first in terms of the Lapse and Shift and the space part gi,j of the 4D metric, then in a second step, if lapseandshift = standard, the Lapse and the Shift are replaced by their expressions in terms of the g0,μ part of the 4D metric, according to α2=g0,0-1 and βj=g0,j, or if lapseandshift was set passing a list to Setup then in terms of the values indicated in that list (these values are not used to set the g0,μ part of the 4D metric). When lapseandshift = arbitrary, the second step is not performed and the Lapse and Shift evaluate to themselves, representing an arbitrary value for them. In the three cases, standard, arbitrary, or a list of algebraic expressions, the components of the ThreePlusOne tensors are computed in terms of a metric with line element ds2=α2dt2+γi,jdxi+βiidtdxj+βjjdt, where α and βi have the Lapse and Shift values mentioned, and γi,j=gi,j is the ThreePlusOne:-gamma3_ metric of the 3D hypersurface. This design permits working with any 4D metric set and, without changing its value, experimenting with different values of the Lapse and Shift (different values of g0,μ) for the 3+1 decomposition of Einstein's equations. See also LapseAndShiftConditions.

• 

The output of ADMEquations consists of a column vector with four equations: the Hamiltonian and momentum constraints, and the evolution equations for the ExtrinsicCurvature and for the 3D gamma3_ metric. These equations are expressed using the inert form of the tensors involved in order to allow for different kinds of manipulations. Once the system has the desired form, the inert representations can be transformed into active ones and the computations represented be performed using the value command, possibly followed by convert to g_, with or without the option only = {Lapse, Shift}, to express the Lapse and Shift in terms of the spacetime metric. Before or after that you can also use SumOverRepeatedIndices, TensorArray or Decompose to do the corresponding manipulations on these ADM equations. These different steps can be performed in any preferred order.

• 

Alternatively, by passing the option inert = false, first all occurrences of the Lapse and Shift get rewritten in terms of the spacetime metric g_, and then the inert representations are transformed into active using value.

• 

The free indices in the equations returned are 4D spacetime indices. To request these equations with 3D space indices use the option output = 3D (on the worksheet, when you input 3D it will get converted to the product 3*D, which for the purpose of requesting 3D indices works fine, the same way as the symbol `3D`)

• 

To define the EnergyMomentum tensor that enters the equations returned by ADMEquations, use the Define command.

Examples

with(Physics): with(ThreePlusOne):

_______________________________________________________

Setting lowercaselatin_is letters to represent space indices

Defined as 4D spacetime tensors see ?Physics,ThreePlusOne,γμ,ν,μ,Γμ,ν,α,Rμ,ν,Rμ,ν,α,β,βμ,nμ,tμ,Κμ,ν

Changing the signature of spacetime from - - - + to + + + - in order to match the signature customarily used in the ADM formalism

Systems of spacetime coordinates are:X=x,y,z,t

_______________________________________________________

(1)

Setup(mathematicalnotation = true);

mathematicalnotation=true

(2)

Set the Schwarzschild metric in spherical coordinates (see g_)

g_[sc];

_______________________________________________________

Systems of spacetime coordinates are:X=r,θ,φ,t

Default differentiation variables for d_, D_ and dAlembertian are:X=r,θ,φ,t

The Schwarzschild metric in coordinates r,θ,φ,t

Parameters: m

Signature: + + + -

_______________________________________________________

gμ,ν=r2mr0000r20000r2sinθ200002mrr

(3)

Check the current (default) value of the EnergyMomentum tensor

EnergyMomentum[];

Τμ,ν=0000000000000000

(4)

This value coincides with the value of this tensor for a Schwarzschild solution in vacuum. Check the definition of the EnergyMomentum tensor, in a curved spacetime, it is the source of the gravitational field

EnergyMomentum[definition];

Τμ,ν=Gμ,ν8π

(5)

The above are actually Einstein's equations with the Einstein tensor on the right-hand side. Compute a tensor array with the equation components of this tensorial equation

TensorArray((5));

0=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=0

(6)

The ADM equations are the 3+1 split of Einstein's equations, presented by ADMEquations as a column vector of four equations: first the two constraints then the two evolution equations

ADMEquations();

Κα,βΚα,βα,β+ΚυυυυΚχχχχ+Rυυυυ=16πnαnβΤα,βα,ββΚμβμβμΚυυυυ=8πγμβμβnττΤβ,τtΚμ,ν=μνα+αΚμ,νΚυυυυ2Κμ,τΚτντν+Rμ,ν8παγμκμκγνλνλΤκ,λγμ,νγκλκλγκ,σκ,σΤλ,σΤα,βα,βnαnβ2+βΚμ,νtγμ,ν=2αΚμ,ν+βγμ,ν

(7)

Where  represents the LieDerivative and t and β indexing  respectively represent the TimeVector and the Shift. Note the display of tensors in gray; they are all inert representations - to activate them use value.

Having set the Schwarzschild metric, if we now compute the components of each of these equations we should obtain 0=0 for all of them. For that purpose, first compute these equations in active form (not inert as in the above), then compute a tensor array for them

ADMEquations(inert=false);

Κα,βΚα,βα,β=16πnαnβΤα,βα,ββΚμβμβ=8πγμβμβnττΤβ,τtτττΚμ,ν+Κτ,νμtττ+Κμ,τνtττ=3γντνττr2mrrτr2mr2γμυμυυr2mrrγμυμυυr2mr24r2mr52+γντντμτr2mr2τrγμυμυυr2mr22rτrγμυμυυr2mr3rμτr2mr22r2mr32+2Κμ,τΚτντν+Rμ,νr2mr8πγμκμκγνλνλΤκ,λγμ,νγκλκλγκ,σκ,σΤλ,σΤα,βα,βnαnβ2r2mr+βτττΚμ,ν+Κτ,νΓτμ,υτμ,υβυυ+Κμ,τΓτν,υτν,υβυυtτττγμ,ν+γτ,νμtττ+γμ,τνtττ=2Κμ,νr2mr+βτττγμ,ν+γτ,νΓτμ,υτμ,υβυυ+γμ,τΓτν,υτν,υβυυ

(8)

TensorArray((8), simplifier=simplify);

0=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=0

(9)

To obtain the 3D version of these equations, pass the optional argument 3D that you can input as the product 3*D or as the symbol `3D`

ADMEquations(inert = false, output = 3*D);

Κμ,νΚμ,νμ,ν=16πnμnνΤμ,νμ,ννΚiνiν=8πγiνiνnααΤα,νtτττΚi,j+Κτ,jitττ+Κi,τjtττ=3γjτjττr2mrrτr2mr2γiυiυυr2mrrγiυiυυr2mr24r2mr52+γjτjτiτr2mr2τrγiυiυυr2mr22rτrγiυiυυr2mr3riτr2mr22r2mr32+2Κα,iΚαjαj+Ri,jr2mr8πγiκiκγjλjλΤκ,λγi,jγκλκλγκ,σκ,σΤλ,σΤμ,νμ,νnμnν2r2mr+βτττΚi,j+Κτ,jΓτi,υτi,υβυυ+Κi,τΓτj,υτj,υβυυtτττγi,j+γτ,jitττ+γi,τjtττ=2Κi,jr2mr+βτττγi,j+γτ,jΓτi,υτi,υβυυ+γi,τΓτj,υτj,υβυυ

(10)

Check the repeated and free indices of each of these four equations

map(Check, (10), all);

The repeated indices per term are: ...,...,..., the free indices are: ...

The products in the given expression check ok.

The repeated indices per term are: ...,...,..., the free indices are: ...

The products in the given expression check ok.

The repeated indices per term are: ...,...,..., the free indices are: ...

The repeated indices per term are: ...,...,..., the free indices are: ...

μ,ν,=μ,ν,ν,i=α,ν,iτ,τ,τ,i,j=τ,υ,τ,υ,α,κ,λ,μ,ν,σ,τ,τ,υ,τ,υ,i,jτ,τ,τ,i,j=,τ,τ,υ,τ,υ,i,j

(11)

In this result we see the free indices i,j are 3D space indices, and all the repeated ones are 4D spacetime indices. For this 3D version of the ADM equations we also expect all of them identically satisfied and equal to 0

TensorArray((10), simplifier=simplify);

0=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=00=0

(12)

Consider now the Lemaitre-Tolman-Bondi metric, that in the Maple database of solutions to Einstein's equations can be retrieved directly using a portion of the word Tolman as an index to the metric g_

g_[tol];

_______________________________________________________

The Tolman metric in coordinates r,θ,φ,t

Parameters: Rt,r,Er

Signature: + + + -

_______________________________________________________

gμ,ν=rRt,r21+2Er0000Rt,r20000Rt,r2sinθ20000−1

(13)

CompactDisplay((13));

Erwill now be displayed asE

Rt,rwill now be displayed asR

(14)

The EnergyMomentum tensor for this metric is given by

EnergyMomentum[~mu, nu] = -rho__M(t, r)*g_[`~mu`, 0]*g_[nu, `~0`]-rho__Lambda*g_[`~mu`, nu];

Τμνμν=ρ__Mt,rδ4μ4μδν4ν4ρ__Λδνμνμ

(15)

where ρMt,r is the matter density, uμ=δ0μ is the 4-velocity of the matter that is comoving and we keep the vacuum energy ρΛ=constant for illustration purposes only. Define this tensor and use a CompactDisplay for ρMt,r

Define((15));

Defined objects with tensor properties

μ,μ,γμ,σμ,Rμ,ν,Rμ,ν,Rμ,ν,α,β,βμ,Cμ,ν,α,β,μ,gμ,ν,γμ,ν,γi,j,Γμ,ν,α,Γμ,ν,α,Gμ,ν,Τμ,ν,εα,β,μ,ν,Rμ,ν,α,β,tμ,Κμ,ν,nμ,Xμ

(16)

CompactDisplay((15));

ρ__Mt,rwill now be displayed asρ__M

(17)

Take now the 4D form of Einstein's equations and derive an expression for ρMt,r as a function of R,E

isolate((5), Einstein[mu, nu]);

Gμ,ν=8Τμ,νπ

(18)

EQ4 := TensorArray((5));

EQ4ρ__ΛRr21+2E=Rr22RR..+R.22E8R21+2Eπ0=00=00=00=0ρ__ΛR2=R3Rt2rR+R..Rr+R.2RtrEr8Rrπ0=00=00=00=0ρ__ΛR2sinθ2=Rsinθ23Rt2rR+R..Rr+R.2RtrEr8Rrπ0=00=00=00=0ρ__M+ρ__Λ=R.2Rr2R.2RtrR+2ErR+2RrE8R2Rrπ

(19)

The relationship we are looking for is in EQ44,4 and can be simplified further if we introduce Mr, the gravitational mass within the comoving sphere at radius r

M(r) = -1/2*(-(diff(R(t, r), t))^2+2*E(r))*R(t, r);

Mr=R.2+2ER2

(20)

So simplify the expression obtained for ρM+ρΛ introducing Mr and eliminating E (see simplify, siderelations)

simplify(EQ4[4,4], {(20)}, {E(r)});

ρ__M+ρ__Λ=Mr4R2Rrπ

(21)

Consider now the 3 + 1 ADM equations equivalent to the 4D Einstein equations EQ4, and show that it results in the same expression for ρM+ρΛ and that both systems of equations are one and the same. Start from the active form of the ADM equations

eq := ADMEquations(inert = false);

eq2R.Rr+2RtrR2Rr2R2Κα,βΚα,βα,β4RrE+ErRRrR2=16πnαnβΤα,βα,ββΚμβμβγμτμτ22Rtrτr+R..τtRr+2R.Rr,rτr+2Rtrτt+3Rtr2τr+3Rt2rτtR+2RtrτRRrR+2R.Rr+2RtrRRr,rτr+2RtrτtRr2R+2R.Rr+2RtrRτRRrR2=8πγμβμβnττΤβ,τtτττΚμ,ν+Κτ,νΓτμ,υτμ,υtυυ+Κμ,τΓτν,υτν,υtυυ=2R.Rr+2RtrRΚμ,νRrR2Κμ,τΚτντν+Rμ,ν8πγμκμκγνλνλΤκ,λγμ,νγκλκλγκ,σκ,σΤλ,σΤα,βα,βnαnβ2+βτττΚμ,ν+Κτ,νΓτμ,υτμ,υβυυ+Κμ,τΓτν,υτν,υβυυtτττγμ,ν+γτ,νΓτμ,υτμ,υtυυ+γμ,τΓτν,υτν,υtυυ=2Κμ,ν+βτττγμ,ν+γτ,νΓτμ,υτμ,υβυυ+γμ,τΓτν,υτν,υβυυ

(22)

The expression for ρM+ρΛ in terms of Mr is obtained now from eq1

eq[1];

2R.Rr+2RtrR2Rr2R2Κα,βΚα,βα,β4RrE+ErRRrR2=16πnαnβΤα,βα,β

(23)

SumOverRepeatedIndices(eq[1]);

R.2Rrsinθ2cscθ23R.2Rr4R.2RtrR+4RrE+4ErRRrR2=16πρ__M+ρ__Λ

(24)

simplify(isolate((24), rho__M(t, r) + rho__Lambda));

ρ__M+ρ__Λ=2R.2RtrR+R.22ERr2ErR8R2Rrπ

(25)

simplify((25), {(20)}, {E(r)});

ρ__M+ρ__Λ=Mr4R2Rrπ

(26)

The second equation, eq2, is identically satisfied

eq[2];

βΚμβμβγμτμτ22Rtrτr+R..τtRr+2R.Rr,rτr+2Rtrτt+3Rtr2τr+3Rt2rτtR+2RtrτRRrR+2R.Rr+2RtrRRr,rτr+2RtrτtRr2R+2R.Rr+2RtrRτRRrR2=8πγμβμβnττΤβ,τ

(27)

TensorArray(eq[2], simplifier = simplify);

0=00=00=00=0

(28)

The fourth equation, eq4, is also identically satisfied (basically, this is the definition of the ExtrinsicCurvature)

eq[4];

tτττγμ,ν+γν,τΓτμ,υτμ,υtυυ+γμ,τΓτν,υτν,υtυυ=2Κμ,ν+βτττγμ,ν+γν,τΓτμ,υτμ,υβυυ+γμ,τΓτν,υτν,υβυυ

(29)

TensorArray(eq[2], simplifier = simplify);

0=00=00=00=0

(30)

So it is in eq3 where the evolution of the system is encoded, in terms of the functions ρM,E,R and their derivatives

eq[3];

(31)

EQ3 := TensorArray(eq[3], simplifier = simplify);

EQ32Rtr2Rr3Rt2r1+2E=2Rtr2R+22RtrRrR.8RrπRρ__Λ+ρ__M2Rr+Er4R1+2E0=00=00=00=0R.2RR..=R.2RtrR+8ρ__Λ+ρ__M2πR22ERrErRRr0=00=00=00=0sinθ2R.2RR..=R.2RtrR+8ρ__Λ+ρ__M2πR2+2ERr+ErRsinθ2Rr0=00=00=00=00=0

(32)

To demonstrate that this system of equations EQ3 together with the constraint eq1 (23), that is the ADMEquations decomposition, is equivalent to the 4D system of equations EQ4 obtained directly from Einstein's equations in (19), it suffices to show that each of these two systems entirely reduces the other one. For this purpose, convert these arrays of equations to sets of equations

EQ4 := convert(EQ4, setofequations);

EQ40=0,ρ__ΛR2=R3Rt2rR+R..Rr+R.2RtrEr8Rrπ,ρ__ΛR2sinθ2=Rsinθ23Rt2rR+R..Rr+R.2RtrEr8Rrπ,ρ__ΛRr21+2E=Rr22RR..+R.22E8R21+2Eπ,ρ__M+ρ__Λ=R.2Rr2R.2RtrR+2ErR+2RrE8R2Rrπ

(33)

EQ3 := convert(EQ3, setofequations) union {(25)};

EQ30=0,2Rtr2Rr3Rt2r1+2E=2Rtr2R+22RtrRrR.8RrπRρ__Λ+ρ__M2Rr+Er4R1+2E,sinθ2R.2RR..=R.2RtrR+8ρ__Λ+ρ__M2πR2+2ERr+ErRsinθ2Rr,R.2RR..=R.2RtrR+8ρ__Λ+ρ__M2πR22ERrErRRr,ρ__M+ρ__Λ=2R.2RtrR+R.22ERr2ErR8R2Rrπ

(34)

The differential reductions can now be performed using PDEtools:-ReducedForm

simplify(PDEtools:-ReducedForm(EQ4, EQ3));

0,0,0,0,0where

(35)

The reduction the other way around

simplify(PDEtools:-ReducedForm(EQ3, EQ4));

0,0,0,0,0where

(36)

See Also

Check, Decompose, Einstein, EnergyMomentum, ExtrinsicCurvature, Lapse, LapseAndShiftConditions, LieDerivative, PDEtools:-ReducedForm, Physics, Physics conventions, Physics examples, Physics Updates, Tensors - a complete guide, Mini-Course Computer Algebra for Physicists, Ricci3, Riemann3, Setup, Shift, SumOverRepeatedIndices, TensorArray, ThreePlusOne, TimeVector, UnitNormalVector, value

References

  

[1] Landau, L.D., and Lifshitz, E.M. The Classical Theory of Fields, Course of Theoretical Physics Volume 2, fourth revised English edition. Elsevier, 1975.

  

[2] Alcubierre, M., Introduction to 3+1 Numerical Relativity, International Series of Monographs on Physics 140, Oxford University Press, 2008.

  

[3] Baumgarte, T.W., Shapiro, S.L., Numerical Relativity, Solving Einstein's Equations on a Computer, Cambridge University Press, 2010.

  

[4] Gourgoulhon, E., 3+1 Formalism and Bases of Numerical Relativity, Lecture notes, 2007, https://arxiv.org/pdf/gr-qc/0703035v1.pdf.