The thesis is structured as follows: in chapter 2 I will review the derivation of the 3+1 decomposed formulation of the Einstein equations including the divergent terms at null infinity, then transform them into their Generalized BSSN (GBSSN) and Z4 conformal (Z4c) formulations, which are commonly used in current numerical simulations, and finally present the spherically symmetric reduction that will be implemented. Chapter 3 is devoted to describing the calculation of appropriate initial data, as well as the study of hyperboloidal foliations in spacetimes including a spherically symmetric BH. A very important ingredient are the gauge conditions, which play a critical role in the simulations and require special adjustment and tuning to obtain well-behaved numerical evolutions;

they are discussed in chapter 4. In chapter 5 I present the conditions that the equations have to satisfy at the continuum level to result in a well-behaved evolution, as well as the regularity conditions that have to hold at null infinity. Chapter 6 describes the numerical implementation in the code. The main experiments performed are explained in chapter 7 and the results obtained are presented in chapter 8. A discussion of the achieved goals and future prospects of this work follow in chapter 9. The expressions used to construct the Penrose diagrams are included in appendix A.

## Chapter 2

## Initial value formulation

We will adopt the abstract index notation for the derivations in this chapter. Abstract ten-sor indices will be denoted by a, b, c, ..., four-dimensional tensor components byµ, ν, σ, ...

and three-dimensional tensor components by i, j, k, ... . Most of the algebraic derivations were performed using the MathematicapackagexAct [113].

### 2.1 Conformally rescaled equations

The Einstein equations written in terms of the rescaled metric ¯g_{ab} = Ω^{2}g˜_{ab} (1.3) have
already been presented in section 1.2.3 as (1.13). In this work we will restrict to the
case of a vanishing cosmological constant Λ = 0. We will derive the equations for our
initial value problem within the Z4 formalism [35, 36]. More specifically we will derive
the conformally rescaled equations starting from the Einstein equations for the physical
metric ˜g_{ab}:

G[˜g]_{ab}+ 2 ˜∇_{(a}Z¯_{b)}−g˜_{ab}∇˜^{c}Z¯_{c}−κ_{1} 2 ˜n_{(a}Z¯_{b)}+κ_{2}g˜_{ab}n˜^{c}Z¯_{c}

= 8πT[˜g]_{ab}. (2.1)
Here againG[˜g]ab =R[˜g]ab−^{1}_{2}˜gabR[˜g] is the Einstein tensor constructed from the physical
metric and T[˜g]_{ab} is the stress-energy-momentum tensor. The extra dynamical quantity
Z¯_{a} introduced in the Z4 formalism appears in the constraint propagation terms of the
Z4 formulation (second and third terms in (2.1)’s left-hand-side (LHS)) and its damping
terms [83] proportional to the timelike normal vector ˜n^{a} (with the parameters κ_{1} and κ_{2}
chosen empirically). The Einstein equations are satisfied when the field ¯Z_{a} vanishes.

The vector ˜n^{a}is defined as the future-directed normal to a three-dimensional spacelike
hypersurface ˜Σ_{t}labeled with a constant value of the parameter t(that will be interpreted
as the time). The normal vector ˜n^{a} is such that ˜n^{a}n˜_{a} = −1 is satisfied (it is a timelike
unit vector). Under the conformal rescaling for the metric (1.3) the unit normal vector
transforms as:

¯

n^{a}= n˜^{a}

Ω and n¯_{a} = Ω ˜n_{a}. (2.2)

A conformal transformation leaves the orientation of the objects (and thus the causal
structure of the spacetime) invariant, so that ¯n^{a}continues to be perpendicular to the now
transformed hypersurface ¯Σ_{t} and the transformations in (2.2) are set in such a way that

¯

n^{a}n¯_{a}=−1 is satisfied.

13

The transformation of the Ricci tensor due to the conformal rescaling of the metric is (see a standard textbook like [155])

R[˜g]_{ab} =R[¯g]_{ab}+ 1

Ω(2 ¯∇_{a}∇¯_{b}Ω + ¯g_{ab}¯Ω)− 3

Ω^{2}¯g_{ab}( ¯∇_{c}Ω)( ¯∇^{c}Ω). (2.3)
It is calculated by means of the transformation of the connection, which is also shown
here for completeness:

Γ˜^{c}_{ab} = ¯Γ^{c}_{ab}− 1

Ω δ^{c}_{a}∇¯bΩ +δ_{b}^{c}∇¯aΩ−g¯ab¯g^{cd}∇¯dΩ

. (2.4)

Under the conformal rescalings of the metric and the normal unit vector the Einstein equations become

G[˜g]_{ab}+ 2 ¯∇_{(a}Z¯_{b)}−g¯_{ab}∇¯^{c}Z¯_{c}+ 4

ΩZ¯_{(a}∇¯_{b)}Ω−κ_{1}

Ω 2 ¯n_{(a}Z¯_{b)}+κ_{2}g¯_{ab}n¯^{c}Z¯_{c}

= 8πT[_{Ω}^{g}^{¯}2]_{ab}, (2.5)
where the physical metric appearing in the stress-energy tensor is expressed in terms of
the rescaled one, ¯g_{ab}, and all indices are raised and lowered with the conformal metric ¯g_{ab},
whose covariant derivative is denoted by ¯∇ and ¯≡¯g^{ab}∇¯_{a}∇¯_{b}. The Einstein tensor of the
physical metric,G[˜g]_{ab}, is related to that of the conformal metric, G[¯g]_{ab}, as

G[˜g]_{ab} =G[¯g]_{ab}+ 2

Ω( ¯∇_{a}∇¯_{b}Ω−g¯_{ab}¯Ω) + 3

Ω^{2}¯g_{ab}( ¯∇_{c}Ω)( ¯∇^{c}Ω). (2.6)
By setting ¯Z_{a} to zero the two previous equations reduce to (1.13) with vanishing
cosmo-logical constant.

The Z4 quantities were introduced in the physical Einstein equations (2.1), but adding them at the level of the conformal metric equations is in principle also feasible. In this case, (2.5) would look like

G[˜g]ab+ 2 ¯∇(aZ¯b)−¯gab∇¯^{c}Z¯c−κ1 2 ¯n(aZ¯b)+κ2g¯abn¯^{c}Z¯c

= 8πT[_{Ω}^{¯}^{g}2]ab. (2.7)
There are no divergent conformal factor terms multiplying the Z4 variable. Although
this last expression could a priori be expected to present better stability properties than
(2.5), the divergent damping terms appearing in (2.5) actually play a decisive role in
controlling the continuum instabilities that arise in the equations. This will be explained
in subsection 7.3.3.

### 2.2 3+1 decomposition

### 2.2.1 3+1 foliations

We will now slice the conformally compactified spacetime into three-dimensional spacelike
hypersurfaces following the common procedure. The normal to the spacelike hypersurfaces
Σ¯_{t}, defined by a constant value of the parametert, was introduced in (2.2). It is expressed
in terms of the parametert as a future pointing vector:

¯

n_{a}=−α∇¯_{a}t or equivalently n¯^{a}=−α¯g^{ab}∇¯_{b}t. (2.8)

2.2. 3+1 decomposition 15
The scalar quantity αis called the lapse function and satisfies α= (−¯g^{ab}∇¯_{a}t∇¯_{b}t)^{−1/2}.
It can be interpreted as the proper time elapsed between the hypersurfaces as seen by an
observer moving along the normal direction (dτ =αdt).

The change of coordinates between two hypersurfaces ¯Σ_{t}and ¯Σ_{t+dt} can be expressed as
x^{i}_{t+dt}=x^{i}_{t}−β^{i}dt. The three components of a vectorβ^{i} control the change in coordinates
in the three spatial dimensions from one spacelike hypersurface to the next and belong to
the vectorβ^{µ} = (0, β^{i})^{T}, called the shift vector. The shift vector is spacelike, so that it is
orthogonal to the timelike normal: ¯n_{a}β^{a} = 0.

The time vector t^{a} is defined in terms

Σ_{t}
Σ_{t+dt}

A (x^{i})

B (x^{i}+dx^{i})
dτ =αdt

dx^{i} =β^{i}dt
αn^{a}

β^{a}

Figure 2.1: Two spacelike slices Σtand Σt+dt

and the change in the coordinates between the points A and B. Neither tildes nor over-bars are added to the symbols, as this de-composition is valid both in the physical noted by tildes) and in the conformal (de-noted by overbars) pictures.

of the previous quantities as:

t^{a}=α¯n^{a}+β^{a}. (2.9)
It is tangential to the time lines, the lines
with constant spatial coordinates. In
gen-eral t_{a} 6= ¯∇_{a}t, because the previous
rela-tion and (2.8) yieldt_{a}=−α^{2}∇¯_{a}t+β_{a}(with
βa= ¯gabβ^{b}). Using the definitions oft^{a}and

¯

n_{a}we obtain that t^{a}∇¯_{a}t= 1, which means
that t^{a} is a basis vector and ¯∇_{a}t is a basis
covector.

The interpretation of (2.9) is that the
evolution from one slice ¯Σ_{t}to the next one
is determined by the lapse and the shift:

The first determines the proper time along
the vector ¯n^{a}and the second regulates how
spatial coordinates are shifted with respect
to the normal vector.

### 2.2.2 3+1 decomposition of the variables

The tensors that appear in the equations are projected perpendicular to the spacelike surfaces or tangential to them. To project them perpendicular to ¯Σt (that is, parallel to

¯

n^{a}), the tensor has to be contracted with ¯n^{a}. The projection tangential to ¯Σ_{t} (normal to

¯

n^{a}) is performed using the projection operator

⊥¯^{a}_{b} ≡δ^{a}_{b} + ¯n^{a}n¯_{b} (2.10)
to contract the quantities. To illustrate the 3+1 projection procedure, the decomposition
of a tensorT_{a}^{b} is given by

T_{a}^{b} =δ_{a}^{c}δ^{b}_{d}T_{c}^{d} = ( ¯⊥^{c}_{a}−n¯_{a}n¯^{c})( ¯⊥^{b}_{d}−n¯_{d}n¯^{b})T_{c}^{d}

= ⊥¯^{c}_{a}⊥¯^{b}_{d}T_{c}^{d}−⊥¯^{c}_{a}n¯_{d}n¯^{b}T_{c}^{d}−⊥¯^{b}_{d}n¯_{a}n¯^{c}T_{c}^{d}+ ¯n_{a}¯n^{c}n¯_{d}n¯^{b}T_{c}^{d}. (2.11)
The first term in (2.11)’s right-hand-side (RHS) is the tangential term and the last one
is the normal term, while the other two terms are mixed. Each index of the tensor has to
be projected.

Metric

The result of applying the projection operator to the metric ¯⊥^{c}_{a}⊥¯^{d}_{b}g¯_{cd} ≡γ¯_{ab} gives a
space-like projection of the metric ¯γ_{ab}, induced on ¯Σ_{t}from the four-dimensional metric ¯g_{ab}. The
relation between the original metric and the spacelike projected one is

¯

γab≡g¯ab+ ¯nan¯b, (2.12)
where we used the relation ¯n^{a}¯γab = 0, which holds because ¯γab is spacelike.

The three dimensional space on the spacelike hypersurface ¯Σ_{t} can be described using
the three dimensional spatial part of ¯γ_{ab}, that will be denoted by ¯γ_{ab}:

dl^{2} = ¯γ_{ij}dx^{i}dx^{j}. (2.13)

Note that ¯γ^{ab} is not the inverse of the projected metric ¯γ_{ab}. Their relation is

¯

γ^{ac}γ¯_{cb} = ¯g^{ac}γ¯_{cb}= ¯γ_{b}^{a}≡⊥¯^{a}_{b} =δ_{b}^{a}+ ¯n^{a}n¯_{b}. (2.14)
The spacelike four-dimensional metric ¯γ^{ab} and ¯γ_{ab} can be considered as the projection
operator with indices raised or lowered with the four-dimensional metric ¯gab.

The components of ¯g_{ab} are expressed in terms of α, β^{i} and ¯γ_{ij} for its decomposition.

For this, each of its components has to be determined. They are given by ¯g_{µν} = ¯g_{ab}e^{a}_{µ}e^{b}_{ν} as
follows, where the relations ¯n^{a}n¯_{a}=−1,β^{a}n¯_{a}= 0, β^{µ}= (0, β^{i})^{T}, ¯n_{a}⊥¯^{a}_{i} = 0 andβ_{a}⊥¯^{a}_{i} =β_{i}
have been used:

¯

g_{00} = ¯g_{ab}e^{a}_{0}e^{b}_{0} = ¯g_{ab}t^{a}t^{b} =t^{a}t_{a} = (α¯n^{a}+β^{a})(α¯n_{a}+β_{a}) = −α^{2}+β^{k}β_{k}, (2.15a)

¯

g_{0j} = ¯g_{ab}e^{a}_{0}e^{b}_{j} = ¯g_{ab}t^{a}e^{b}_{j} =t_{b}e^{b}_{j} = (αn¯_{b}+β_{b}) ¯⊥^{b}_{j} =β_{b}⊥¯^{b}_{j} =β_{j}, (2.15b)

¯

g_{i0} = ¯g_{ab}e^{a}_{i}e^{b}_{0} =...=β_{i}, (2.15c)

¯

g_{ij} = ¯g_{ab}e^{a}_{i}e^{b}_{j} = ¯g_{ab}⊥¯^{a}_{i}⊥¯^{b}_{j} = ¯γ_{ij}. (2.15d)
The four dimensional metric ¯g_{ab} and its inverse can be written as

¯
g_{µν} =

−α^{2}+β_{k}β^{k} β_{j}
β_{i} γ¯_{ij}

and ¯g^{µν} = 1
α^{2}

−1 β^{j}
β^{i} α^{2}γ¯^{ij} −β^{i}β^{j}

. (2.16)
Using (2.12) and the fact that now the normal timelike vector in components takes the
form ¯nµ = (−α,0) and ¯n^{µ}= _{α}^{1}(1,−β^{i})^{T}, the spacelike projection ¯γab and inverse are given
by

¯
γ_{µν} =

β_{k}β^{k} β_{j}
β_{i} γ¯_{ij}

and γ¯^{µν} =

0 0
0 ¯γ^{ij}

. (2.17)

The line element now takes the form
d¯s^{2} = −α^{2}+β_{i}β^{i}

dt^{2}+ 2β_{i}dtdx^{i} + ¯γ_{ij}dx^{i}dx^{j}. (2.18)
The index of the three-dimensional shift vector has to be lowered with the three-dimensional
spatial metric β_{i} ≡¯γ_{ij}β^{j}.

2.2. 3+1 decomposition 17 Extrinsic curvature

The Einstein equations contain second derivatives of the metric, so that second order
derivatives in time will appear in the decomposition. To express them as a first order in
time system a new variable has to be introduced: the extrinsic curvature ¯K_{ab} is defined
as

K¯_{ab} ≡ −⊥¯^{c}_{a}∇¯_{c}n_{b} =−( ¯∇_{a}n¯_{b} + ¯n_{a}n¯^{c}∇¯_{c}n¯_{b}). (2.19)
It is a purely spacelike and symmetric tensor that describes the curvature of the spacelike
hypersurface ¯Σ_{t} with respect to its embedding in the four-dimensional spacetime. The
sign convention is the common one in Numerical Relativity, opposite to the one used in
[155].

The extrinsic curvature ¯K_{ab} can also be expressed using the Lie derivative along the
normal direction L_{n}_{¯}:

L_{¯}_{n}γ¯_{ab} = ¯n^{c}∇¯_{c}¯γ_{ab}+ ¯γ_{ac}∇¯_{b}¯n^{c}+ ¯γ_{cb}∇¯_{a}n¯^{c}=...= ¯γ_{a}^{c}∇¯_{c}n¯_{b}+ ¯γ_{b}^{c}∇¯_{c}n¯_{a}=−2 ¯K_{ab}. (2.20)
Here the relations ¯n^{a}∇¯_{b}n¯_{a} = 0, ¯∇_{c}¯g_{ab} = 0 and the symmetry property of ¯K_{ab} have been
used. The extrinsic curvature can now be expressed in terms of the spacelike projection
of the metric:

K¯_{ab} =−1

2L_{¯}_{n}γ¯_{ab}. (2.21)

### 2.2.3 3+1 decomposition of the equations

The spacelike equivalent to the covariant derivative ¯∇_{a} that acts on spatial tensors is
the “projected” covariant derivative ¯D_{a} ≡ ⊥¯^{b}_{a}∇¯_{b}. The projection operator ¯⊥^{b}_{a} has to
be applied to all the indices in the expression, not just ¯∇_{a}. The new three-dimensional
spatial covariant derivative applied to the three-dimensional spacial metric vanishes:

D¯_{k}¯γ_{ij} = 0. (2.22)

Decomposition of the Riemann tensor

An intermediate step to decomposing the Einstein equations in (2.5) is the
decompo-sition of the four-dimensional Riemann tensor R[¯g]^{a}_{bcd}. We perform three independent
projections:

• Full projection onto the spacelike hypersurface ¯Σt - Gauss-Codazzi equation:

⊥¯^{e}_{a}⊥¯^{f}_{b}⊥¯^{g}_{c}⊥¯^{h}_{d}R[¯g]_{ef gh} =R[¯γ]_{abcd}+ ¯K_{ac}K¯_{bd}− K¯_{ad}K¯_{bc}. (2.23)

• Projection onto ¯Σ_{t} of the Riemann tensor contracted once with the normal vector
-Codazzi-Mainardi equations:

⊥¯^{e}_{a}⊥¯^{f}_{b}⊥¯^{g}_{c}n¯^{d}R[¯g]_{ef gd} = ¯D_{b}K¯_{ac}−D¯_{a}K¯_{bc}. (2.24)

• Projection onto ¯Σ_{t} of the Riemann tensor contracted twice with the normal vector
- Ricci equations:

⊥¯^{e}_{a}⊥¯^{f}_{c}n¯^{b}n¯^{d}R[¯g]_{ebf d} =L_{¯}_{n}K¯_{ac}+ 1
α

D¯_{a}D¯_{c}α+ ¯K_{ad}K¯_{c}^{d}. (2.25)

The Riemann tensor R[¯g]_{abc}^{d} is a function of the spacetime metric ¯g_{ab}, whereas the

“induced” Riemann tensorR[¯γ]_{abc}^{d} that describes ¯Σ_{t}’s curvature is expressed in terms of
the projected metric ¯γ_{ij}.

Derivation of the 3+1 equations

The following relations will be used in the decomposition of the Einstein equations:

⊥¯^{ac}⊥¯^{bd}R[¯g]_{abcd} = R[¯g] + 2¯n^{a}n¯^{b}R[¯g]_{ab} = 2¯n^{a}n¯^{b}G[¯g]_{ab}, (2.26a)

⊥¯^{ab}n¯^{c}R[¯g]_{bc} = ⊥¯^{ab}n¯^{c}G[¯g]_{bc}, (2.26b)
where R[¯g]ab and R[¯g] are the four dimensional Ricci tensor and scalar and G[¯g]ab is the
four dimensional Einstein tensorG[¯g]_{ab} =R[¯g]_{ab}−^{1}_{2}g¯_{ab}R[¯g], all of them expressed in terms
of the conformal metric ¯g_{ab}.

Contracting the Gauss-Codazzi equation (2.23) twice (using ¯⊥^{ab}≡γ¯^{ab}) we find that

⊥¯^{ac}⊥¯^{bd}R[¯g]_{abcd} = R[¯γ] + ¯K^{2}−K¯_{ab}K¯^{ab}, (2.27)
where ¯K ≡K¯_{a}^{a} the trace of the extrinsic curvature tensor. Substituting (2.26a) gives

2¯n^{a}n¯^{b}G[¯g]_{ab} =R[¯γ] + ¯K^{2}−K¯_{ab}K¯^{ab}. (2.28)
Finally we use the Einstein equations for the conformal metric ((2.5) and (2.6))

R[¯γ] + ¯K^{2}−K¯_{ab}K¯^{ab}
+4 ¯γ^{ab}D¯_{a}D¯_{b}Ω + ¯KL_{¯}_{n}Ω

Ω +6

(L_{¯}_{n}Ω)^{2}−¯γ^{ab}( ¯D_{a}Ω)( ¯D_{b}Ω)
Ω^{2}

−2 ¯KΘ + 2 ¯D_{a}Z^{a}− 2Z^{a}D¯_{a}α

α −2κ_{1}(2 +κ_{2}) Θ

Ω −2L_{n}_{¯}Θ− 8ΘL_{¯}_{n}Ω

Ω = 16πρ.(2.29)
The scalar quantityρdenotes the local energy density as measured by observers following
the normal trajectories to the spacelike hypersurfaces and is defined as ρ ≡ n¯^{a}n¯^{b}T[¯g]_{ab}.
The variables Θ and Z_{a} are introduced as the projections of ¯Z_{a} along the normal and
tangential directions to ¯Σ_{t} respectively. The decomposition of the Z4 quantity is thus
Z¯a=Za+ ¯naΘ, where Θ =−¯n^{a}Z¯a and Za= ¯⊥^{b}_{a}Z¯b.

From the Codazzi-Mainardi equations (2.24) and using (2.26b) one obtains

⊥¯^{ab}n¯^{c}G[¯g]_{bc}= ¯⊥^{ab}n¯^{c}R[¯g]_{bc} = ¯D^{a}K¯ −D¯_{b}K¯^{ab}. (2.30)
Substituting the field equations and writingJ^{a}≡ −⊥¯^{ab}n¯^{c}T[¯g]_{bc}for the momentum density
measured along the normal direction gives

D¯_{b}K¯^{ab}−D¯^{a}K¯

−2K¯^{ab}D¯_{b}Ω + ¯γ^{ab}D¯_{b}(L_{¯}_{n}Ω)

Ω −2 ¯K^{ab}Zb− κ_{1}Z^{a}
Ω
+¯γ^{ab}D¯_{b}Θ− γ¯^{ab}Θ ¯D_{b}α

α +2¯γ^{ab}Θ ¯D_{b}Ω

Ω −¯γ^{ab}L_{n}_{¯}Z_{b}−2Z^{a}L_{n}_{¯}Ω

Ω = 8πJ^{a}. (2.31)

2.2. 3+1 decomposition 19 Contracting only once the Gauss-Codazzi equation yields the following relation:

⊥¯^{d}_{a}⊥¯^{f}_{b}(R[¯g]_{df}+ ¯n^{c}n¯^{e}R[¯g]_{cdef}) =R[¯γ]_{ab}+ ¯KK¯_{ab}−K¯_{ac}K¯_{b}^{c}. (2.32)

The second term in its LHS can be substituted by the Ricci equations (2.25), while the first one appears in the projection of the Einstein equations written in terms of the Ricci tensor, which is

⊥¯^{c}_{a}⊥¯^{d}_{b}R[¯g]_{cd} = ⊥¯^{c}_{a}⊥¯^{d}_{b}

8π

T[¯g]_{cd}−^{1}_{2}g¯_{cd}¯g^{ef}T[¯g]_{ef}

−

2 ¯∇_{a}∇¯_{b}Ω + ¯g_{ab}¯Ω

Ω −3¯g_{ab}( ¯∇_{c}Ω)( ¯∇^{c}Ω)
Ω^{2}

− 2 ¯∇_{(a}Z¯_{b)}+4 ¯Z_{(a}∇¯_{b)}Ω

Ω −2¯gabZ¯^{c}∇¯_{c}Ω

Ω −κ1 2 ¯n_{(a}Z¯_{b)}−(1 +κ2)¯g_{ab}n¯^{c}Z¯c

Ω

!#

.(2.33)

Performing these substitutions and introducing a new variable S_{ab} ≡ ⊥¯^{c}_{a}⊥¯^{d}_{b}T[¯g]_{cd} as
the spatial stress tensor (S ≡S_{a}^{a}), an evolution equation for the extrinsic curvature can
be derived:

L_{n}_{¯}K¯_{ab} = −1
α

D¯_{a}D¯_{b}α+R[¯γ]−2 ¯K_{ac}K¯_{b}^{c}+ ¯K_{ab}( ¯K−2Θ) + 2 ¯D_{(a}Z_{b)}− κ_{1}(1 +κ_{2})¯γ_{ab}Θ
Ω
+3¯γab

(∂⊥Ω)^{2}−α^{2}D¯^{c}Ω ¯DcΩ

α^{2}Ω^{2} +4Z_{(a}D¯_{b)}Ω

Ω +2 ¯D_{b}D¯_{a}Ω

Ω − 2¯γ_{ab}Z^{c}D¯_{c}Ω
Ω
+γ¯_{ab}D¯^{c}αD¯_{c}Ω

αΩ + ¯γ_{ab}4Ω¯

Ω + 2 ¯K_{ab}L_{¯}_{n}Ω

Ω + γ¯_{ab}( ¯K−2Θ)L_{¯}_{n}Ω

Ω − γ¯_{ab}L_{¯}_{n}L_{n}_{¯}Ω
Ω

+4π[¯γ_{ab}(S−ρ)−2S_{ab}]. (2.34)

Decomposed evolution and constraint equations

Even if the induced quantities on the spacelike hypersurface ¯Σ_{t} are four-dimensional,
the information is encoded exclusively in their spatial components. This means that in
the adapted coordinate system that has been chosen, we do not need to consider the
timelike components. From now on the indices in the equations will thus only cover the
three spatial coordinates. The Lie derivative along the normal direction L_{n} can also be
expressed in adapted coordinates asL_{n}_{¯} ≡ _{α}^{1}∂_{⊥}, with∂_{⊥} =∂_{t}− L_{β}. The three-dimensional
spatial metric ¯γ_{ab} is used to raise and lower indices, and we use the notation ¯4 ≡¯γ^{ab}D¯_{a}D¯_{b}.
Using (2.21) as evolution equation for the physical metric and solving (2.29), (2.31)

and (2.34) for∂⊥Θ, ∂⊥Za and ∂⊥K¯ab respectively, the equations of motion are finally while the Hamiltonian and momentum constraints are given by

H=R[¯γ]−K¯_{ab}K¯^{ab}+ ¯K^{2}+6
The evolution equations of the Z4 variables can also be expressed as

∂⊥Θ = α where the dependence on the constraints has been explicitly written.

Dropping the conformal factor terms and the Z4 terms, the equations (2.35a), (2.35b), (2.36a) and (2.36b) are the Arnowitt–Deser–Misner (ADM) equations [19], specifically in the form obtained by York [163]. In the original ADM formulation a multiple of the Hamiltonian constraint appears in (2.35b)’s RHS.

The coefficientCZ4cis introduced to label the Z4 non-damping non-principal-part that
are treated differently in variations of the Z4 formulation. ForC_{Z4c} = 0 those terms are
dropped in an equivalent way as done in the Z4 conformal (Z4c) formulation [28, 156],
while for CZ4c = 1 all Z4 terms are kept as in the conformal and covariant Z4 (CCZ4)
formulation [13].

### 2.2.4 Relation between physical and conformal quantities

The 3+1 decomposition just performed is formally the same in the conformal and in the
physical picture. The physical extrinsic curvature ˜K¯_{ab} can also be expressed in terms of

2.3. Generalized BSSN and conformal Z4 21