5.1 topology of the conformal boundary Σ
5.1.5. Focal Points on Σ
Remark. By restrictingσto the open set ˚Mi∩ U, we are left with a smooth functionσ : ˚Mi∪ U → R. σ−1( [α0,α1] ) are compact sets for 0 < αi < e if σ is positive on ˚Mi and −e < αi < 0 else. Hence by Theorem 1.5.1 the sets σ−1(∞,α0) and σ−1(∞,α1) are diffeomorphic and the former set is a deformation retract of the latter. Sincedσ is non-singular and closed in the constructed neighbourhood of∂M˚ i, the level sets ofσrepresent a1-dimensional foliation of ˚Mi in a neighbourhood of its boundary.
5.1.5. Focal Points on Σ
The vertices ofΣare special in the sense that they preventΣfrom being a smooth null hypersur-face. They actually have more interesting properties. The content of the next section will be to show that the only focal points along null geodesics inΣwith respect to any(n−2)-dimensional Riemannian submanifoldN⊂Σare exactly the vertices. This result would be a direct corollary of Lemma5.1.6 if only focal points inΣ were considered. But a priori we will allow variations of the geodesic that do not belong toΣ.
Lemma5.1.24. Let (M,g,σ) be an almost Einstein structure, γ : I → Σ a null geodesic on Σ and f :I →Ra smooth map such that f(t)γ˙(t) =gradσ(t). Moreover, let J∈X(γ)be a vector field along γ. Then
f ·Rg(J, ˙γ)γ˙ =g(γ, grad˙ ρ)J−g(γ,˙ J)gradρ−g(J, gradρ)γ.˙ Proof: Using Hessσ]=−σP]−ρid on almost Einstein structures we get atγ(t)
f ·Rg(J, ˙γ)γ˙ = Rg(J, ˙γ)gradσ
= ∇J∇γ˙gradσ− ∇γ˙∇Jgradσ− ∇[J, ˙γ]gradσ
(1.5)
= ∇JHessσ](γ˙)− ∇γ˙Hessσ](J)−Hessσ]([J, ˙γ])
= ∇γ˙σP](J) +ρJ
− ∇JσP](γ˙) +ργ˙
+σP]([J, ˙γ]) +ρ[J, ˙γ]
= (∇γ˙σ)P](J) + (∇γ˙ρ)J− ∇Jσ
P](γ˙)− ∇Jρ
˙ γ +σ
∇γ˙P](J)− ∇JP](γ˙)+P]([J, ˙γ]) +ρ ∇γ˙J− ∇Jγ˙+ [J, ˙γ]
= g(γ, grad˙ σ)P](J) +g(γ, grad˙ ρ)J−g(J, ˙γ)P](gradσ)−g(J, gradρ)γ˙ +σ
∇γ˙P]
(J)−∇JP] (γ˙).
For the transformation leading to the last line we used g(X, gradσ)T(γ˙) = g(X, ˙γ)T(gradσ), due to the requirements on ˙γ. Now taking into account that for almost Einstein structures dρ=P](gradσ)and that onΣit holdsg(gradσ, ˙γ) =0, we get the claimed result.
Corollary5.1.25. With the requirements of the last lemma J is a Jacobi field alongγif and only if f · ∇γ˙∇γ˙J=g(γ, grad˙ ρ)J−g(γ,˙ J)gradρ−g(J, gradρ)γ.˙ (5.10) Proof: First consider J to be a Jacobi field, i.e. ∇γ˙∇γ˙J =Rg(J, ˙γ)γ, then the claim follows from˙ the last lemma. Conversely consider J to solve (5.10). By the last lemma we have f· ∇γ˙∇γ˙J = f·Rg(J, ˙γ)γ˙ and hence the Jacobi equation is fulfilled where f 6=0. By Proposition5.1.1gradσ vanishes only on Σat isolated points p ∈ Σd. Since ˙γ is nowhere vanishing, we conclude from f(t)γ˙(t) =gradσγ(t) alongΣ that f may only be zero for isolated t ∈ I. The Jacobi equation then holds on a dense subset ofIand hence by smoothness of Jandγall over the interval.
5.1 topology of the conformal boundary Σ 99
Lemma5.1.26. Let(M,g,σ)be an almost Einstein structure,γ:I→Σca null geodesic and J an Jacobi field onγsuch that for some t0∈I it satisfies
J(t0) =0 ∇γ˙J(t0) =αγ˙(t0) withα∈R. Then J is given explicitly by
J(t) = (t−t0)αγ˙(t) for all t∈ I
Proof: The proof can be carried out by direct calculation. Consider ˜J= (t−t0)αγ˙(t). Sinceγis a geodesic, this yields∇γ˙J˜=αγ˙ and∇γ˙∇γ˙J˜≡0≡Rg(J, ˙˜ γ)γ. Hence ˜˙ Jis a Jacobi field with initial data ˜J(t0) =0 and ˙˜J(t0) =αγ˙(t0). By uniqueness of Jacobi fields for given initial data, Jand ˜J
coincide.
Lemma5.1.27. Let(M,g,σ)be an almost Einstein structure, N ⊂Σan(n−2)-dimensional spacelike submanifold ofΣandγ:I →Σa null geodesic starting at N, i.e. γ(0)∈ N. Let J now be an N-Jacobi field onγwith J(t0) =0for some t06=0, then J and its covariant derivative∇γ˙J alongγare tangent to Σfor all t∈I, i.e. J(t),∇γ˙J∈Tγ(t)Σfor allγ(t)∈Σc.
In particular, since a null vector tangent toΣis perpendicular to any spacelike submanifold of Σ,γis a null geodesic normal toNand therefore the requirements of the lemma implyγ(t0)to be a focal point ofN. The proof is based on an idea of the proof for [O’N83, Lemma8.7], namely considering the mapgγ(Jγ, ˙γ):I →R.
Proof: First we observe that for each p ∈ N we have the direct sum decomposition TpΣ = TpN⊕ hgradσpi. Since gradσis normal to the tangent space ofΣcit is normal toTpN.
Now let J ∈ X(γ) be a N-Jacobi field on the null geodesic γ with focal point at γ(t0), i.e.
J(t0) =0. This gives
d2
dt2g(J, ˙γ) =∇γ˙∇γ˙g(J, ˙γ)
=g(∇γ˙∇γ˙J, ˙γ)
=g(Rg(J, ˙γ)γ, ˙˙ γ)
≡0
As a resultg(J, ˙γ)is a linear functionκ(t) =α+β·talongγ. Atγ(0),Jis tangent toNwhile ˙γ is normal toNand we findκ(0) =α=0. On the other hand atγ(t0)the requirementJ(t0) =0 givesκ(t0) = β·t0 = 0 and hence β = 0. Hence g(J, ˙γ) and dtdg(J, ˙γ) = g(∇γ˙J, ˙γ) are trivial maps alongγ, which is equivalent toJ,∇γ˙J∈Tγ(t)Σfor allt∈ I.
Corollary5.1.28.Let(M,g,σ)be an almost Einstein structure, N ⊂Σan(n−2)-dimensional spacelike submanifold ofΣand γ : I → Σa null geodesic normal to N, in particularγ(0) ∈ N. Ifγ(t0)is an element ofΣcand a focal point of N and if J is the related N-Jacobi field with J(t0) =0, then ∇dtJ(t0)is tangent toγin t0, i.e.
(∇γ˙J) (t0) =αγ˙(t0) for someα∈R.
Proof: Let 0 ∈ H ⊂ R be a small interval. First we will construct a geodesic variation δγ : I×H→Mnormal to Nsuch that Jis its variation vector field, i.e.
δγ(t, 0) =γ(t) δγ(0,H)⊂N
(∂sδγ) (t, 0) =J(t) (∂tδγ) (0,s)∈Tδγ(0,s)N⊥. Throughout the proof we will use the following short notation
∂s∂tδγ:= ∇
ds∂tγ αt(s):=δγ(t,s)
∂t∂sδγ:= ∇
dt∂sγ α(s):=α0(s) J˙:=∇γ˙J = ∇
dtJ.
100 Chapter 5: almost einstein structures with vanishing almost scalar curvature
Derivatives with respect to t will be denoted with a dot, while derivatives with respect to s will be denoted with a prime. From Lemma 5.1.27 we get ˙J(t) ∈ Tγ(t)Σ for all t ∈ I, where J˙(0) =πN(J˙(0)) +κgradσγ(0)for someκ∈R, in particularπ⊥(J˙(0)) =κgradσγ(0). In addition, since J is an N-Jacobi field, we have J(0) ∈ Tγ(0)N. Now assume without loss of generality
˙
γ(0) =gradσγ(0)and considerα:H→Nto be an arbitrary curve such that α(0) =γ(0) α0(0) = J(0) ∈ Tγ(0)N. We now define a vector fieldZ onαwith values in the normal bundle overNby
Z : H → TN⊥
s 7 → (1+κs)gradσα(s)
If the intervalHis sufficiently small and ifαis well behaved, we will use the equivalent notation Z(s) = Zα(s). By Lemma5.1.5we have ∇α0gradσ = −ρ α0 on Σand hence the projection to the normal bundle givesπ⊥(∇α0gradσ) ≡0 along αsuch that
Z0(0) = (∇α0Z) (0)
= (∇α0gradσ) (0)−π⊥( (∇α0gradσ) (0) ) +κgradσα(0)
= ˜II(α0(0), gradσα(0)) +π⊥(J˙(0) )
= ˜II(J(0), ˙γ(0) ) +π⊥(˙J(0) )
= πN(˙J(0) ) +π⊥(˙J(0) )
= ˙J(0).
See Equation (1.80) for the definition of ˜II. Now we can define the desired geodesic variationδ γ via the normal exponential map exp : TN⊥ → M on the normal bundle ofNby:
δ γ: I×H → M
(t,s) 7 → expα(s)(t·Z(s) ).
Since we don’t requireM to be complete we may have to choose Hsufficiently small such that t·Z(s) is in the domain of exp. By definition δ γ(·,s) are geodesics for fixed s and since
∂tδ γ(0,s) = Z(s) ∝ gradσα(s), they are normal to N such that δ γ clearly is a geodesic variation of null geodesics normal to N. Moreover, we have
∂t∂sδ γ(0, 0) =∂s∂tδ γ(0, 0)
= ∂sZ(0)
= J˙(0).
Therefore the Jacobi field defined by ∂sδ γ(·, 0) coincides with J at t = 0 and hence on the whole interval I due to uniqueness of Jacobi fields.
The second step is to derive ˙Jatγ(t0) = δ γ(t0, 0). We recall that at the focal pointδ γ(t0, 0) we have
α0t0(0) = ∂sδ γ(t0, 0) = J(t0) =0. (5.11) We observe that for fixed s ∈ H the mapδ γ(·,s) : I → Σc is a null geodesic. Now consider t ∈ (t0−e,t0] to be close enough to t0 such that gradσγ(t) 6= 0. If necessary shrink the interval H such that gradσ does not vanish along the variation δ γ, i.e. gradσδ γ(t,s) 6= 0 for all(t,s) ∈ (t0−e]×H. Then the tangent vector ∂tδ γ(t,s) is collinear to gradσ and hence there is a smooth mapη :(t0−e,t0]×H →Rsuch that
∂tδ γ(t0,s) =η(t0,s)gradσδ γ(t0,s). Now we derive ˙J(t0)as follows
˙J(t0) = ∂t∂sδ γ(t0, 0)
5.1 topology of the conformal boundary Σ 101
= ∂s∂tδ γ(t0, 0)
= ∇α0
tη(t0,·)gradσ
(0)
(5.1)
= η0(t0, 0)gradσδ γ(t,0)−ρ η(t0, 0)α0t0(0)
(5.11)
= η0(t0, 0)gradσδ γ(t0,0),
which proves the proposition since gradσδ γ(t0,0) = gradσγ(t0) ∝ γ˙(t0) Proposition5.1.29. Let(M,g,σ) be an almost Einstein structure, N ⊂ Σcan (n−2)-dimensional spacelike submanifold andγ : [0,t0] → Σa geodesic normal to N. Ifγ(t0)is a focal point of N with respect toγthen it is a vertex ofΣ, i.e.
γ(t0)focal point of N ⇒ γ(t0)∈Σd
Proof: We will show that a point inΣccannot be a focal point of Nwith respect toγ. Consider γ(t0) ∈ Σc to be a focal point and J an N-Jacobi field on γ. Then by Corollary5.1.28, ˙J(t0) = αγ˙(t0). Lemma5.1.26then implies Jto be of the formJ(t) = (t−t0)αγ˙(t). Since J(0)∈Tγ(0)N, it must vanish att=0 such thatα=0. But thenJ≡0 contradictsJto be a non-vanishing vector
field. Henceγ(t0)∈Σd.
Let p ∈ Σd be a vertex of Σ. We will now construct coordinates for a neighbourhood of a segment of a radial null geodesicη :t7→expp(tu) ∈Σ. Proposition1.2.8 guaranteesu ∈ CpM in the first place. The geodesic does not have to be complete but we will consider it to be well defined fort∈ (−e, 1+e) =:I for somee> 0. Moreover, we requireuto be sufficiently close to the origin ofTpMsuch that expp(tu)∈Σcfor allt∈ I\ {0}. In particularpis the only vertex along the null geodesic segmentγ.
First we will fix some notation. We define
U⊂TpM
to be a convex neighbourhood of the origin inTpM. It is mapped by the exponential map to U :=expp(U).
Moreover, we will requireUto be small enough such that pis the only critical point of gradσin U, i.e.
gradexp
p(X)=0 ⇔ X=0 forX∈U1. Now let
u∈CpM\U
be a null vector outsideU. The domain of expp is star-shaped with respect to the origin, hence there is ans>1 and ˜u∈Usuch that
u=su.˜
We now choose a neighbourhood Uu˜ ⊂ U\ {0} of ˜u not containing the origin and define the following objects
p= expp(0) eU:=Uu˜∩CpM x:= expp(u˜) Ue:= expp
eU q:= expp(u).
Since eU does not contain the origin, it is an (n−1)-dimensional submanifold of TpM and a neighbourhood of ˜u within the null cone CpM. The restriction expp : eU → U ⊂e Σc then is a diffeomorphism.
1 This choice is possible due to Proposition5.1.1
102 Chapter 5: almost einstein structures with vanishing almost scalar curvature
We assert that gradση(t) is non-vanishing and tangent toη(t) along the radial null geodesic η: I→Σc∪ {p}fort 6=0. Moreover, we haveη s−1
=expp(u˜) = xand η(1) =expp(u) =q.
Henceη:[s−1, 1]→Σcis a null curve fromxtoqwith non-vanishing tangent vector. By Lemma 5.1.6(iiv) it can be reparametrised to an integral curve of gradσ. Expressing the integral curve in xvia the flow of gradσ, namelyΦ(·,x)by the previous considerations there has to be at0∈R such thatΦ(t0,x) =q. Without loss of generality we assume it to be positive, which is equivalent to assume pto be a repeller of gradσ. We will point out the changes that will have to be made to adapt the following construction to a negativet0.
We will now construct and define a special orthonormal frame of the tangent space in TpM adapted to the null coneCpM. Consider an orthonormal frame{ei}ofTpMsuch thatg(ei,ei) = 1 fori ∈ {1, . . . ,n−1}and g(e0,e0) = −1. Moreover, it can be chosen such that ˜u = e1+e02. The null cone in pcan be written as
CpM=nX=X0e0+X0e
as a necessary and sufficient condition for null vectors in an orthonormal frame. This definition implies that
gp(e0+e1,ei) =0 ∀i≥2, such that{e0+e1,ei}i≥2 generates the tangent space Tu˜ CpM
⊂ Tu˜ TpM
' TpM of the null cone in ˜u. Consequently, we can choose coordinates on the sphere and considereas coordinate function
e:V ⊂Rn−2→TpM (5.12)
with the identification just made. We will require the coordinates to be such that3 e(0) =e1 (∂ie)(0) =ei+1.
Figure5.:schematics of coordinates (red) in a neighbourhood of null geodesics onΣc
Altogether the last considerations can be summarised as follows
Lemma5.1.30. Using the notation above, let p ∈ Σd and η : (0−δ, 1+δ) → Σc∪ {p} a null geodesic of the neighbourhood of p in M. The construction is as follows.
ConsiderV ⊂Rn−2to be a neighbourhood of0such that
∀α∈ V : (e0+e(α))∈U˜. Now let f be the map defined by is a diffeomorphism onto its image and im ϕ−1
,ϕ desired property. Here we canonically identified the tangent spaceTpMwithRn.
5.1 topology of the conformal boundary Σ 103
Before going on with the proof, we will point out some important facts. First consider the case wheret0is negative. Then we have to interchange the boundaries of the interval in the definition ofϕ−1. The proof then essentially is the same. Next we observe that as result of the construction above the mapϕhas the following properties
ϕ−1(0, 0) =Φ(0,x) =x=expp(u˜) ϕ−1(t0, 0) =Φ(t0,x) =q=expp(su˜).
Henceϕ−1(t, 0) =Φ(t,x)witht∈[0,t0]is a pregeodesic fromxtoqand therefore the segment expp([1,s]u˜)is a subset of im ϕ−1
.
Proof: We defineS := f(V). ThenS is a smooth submanifold ofΣccontaining x = f(0). Now let α0 ∈ V. We will show the tangent space Tf(α0)Σc to be a direct sum Tf(α0)Σc = Tf(α0)S⊕ hgradσf(α0)i. For that consider a curveη(t) = f(α0+t·α)inSwithη(0) = f(α0). Then ˙η(0) = d
expp
e0+e(α0)(Y), where Y = ∂t|t=0e(α0+tα) by definition is a spacelike vector tangent to the null cone CpM. In particular Y is non-null and therefore transversal to the null direction of the cone. Sinceα0+tα ∈ eU, d
expp
e0+e is bijective and consequentlyd expp
e0+e(α0)(Y) is transversal to the null direction of Σc in f(α0). As stated before the null direction on Σc
is given by gradσf(α0). The function f itself is a diffeomorphism, since it is a composition of diffeomorphisms. Therefore we conclude that Tf(α0)S = d fα0(Rn−2), which is transversal to gradσf(α0). This gives the claim.
We will now show S∩Φ(t,S) =∅for all t6= 0. Lety1,y2∈ Sand assumey2 =Φ(t,y1) for somet, where without loss of generality t ≥ 0. If t < 0 interchange y1 and y2. By definition of S we have yi ∈ S ⊂ expp(U˜). Consequently with respect to the frame constructed above there are α1,α2 ∈ V such that yi = expp(e0+e(αi)) for i = 1, 2. Moreover, Φ(·,y2) is a null pregeodesic along radial geodesics of expp from y1 to y2. As both points are in the convex domain, there must be ans ∈R such thate0+e(α2) = s(e0+e(α1)). Due to the construction ofe, e(αi) and e0 are orthogonal and we conclude s = 1. Therefore e(α1) = e(α2) and since e is a diffeomorphism we get α1 = α2. Finally y1 and y2 coincide and we have y1 = Φ(t,y1). By Lemma1.1.19maximal integral curves of gradσ originating in or heading for a vertex are complete and non-self-intersecting such that we get the desired result, namelyt=0.
FinallyS, f and Φcomply with the requirements of Lemma5.1.19such that ϕ−1 is a
diffeo-morphism and henceϕis a chart as claimed.
Lemma5.1.31. Consider the chartϕ:D ⊂Σ→Rn−1defined above. Let X∈CpM, s∈ Rsuch that X and sX are in the domain ofexpp andexpp(X), expp(sX)∈ D. Denote ϕ = (ϕ0, . . . ,ϕn−2), then we find the following properties
∀i6=0 : ϕi
expp(X)= ϕi
expp(sX) (5.13) and withu˜∈U˜ as defined above (in particularexpp(u˜) =x=ϕ−1(0)) we get
rank d
expp
˜
u:Tu˜ CpM
→Tϕ−1(0)Σc
=n−1. (5.14) Proof: The second statement follows from the definition of the coordinates. The point ˜u is in the neighbourhood of 0∈ TpM where the exponential map is a diffeomorphism such that the restriction to a submanifold is a diffeomorphism too.
For the first statement considerXandsas required. Let
expp(X) =ϕ−1(κ0,α) =Φ(κ0, expp(e0+e(α))) (5.15) be the unique coordinate representation of expp(X)and define the null geodesicη(t) =expp(tX) for t ∈ [1,s]. Then due to the assumptions we have η(t) ∈ Σc for all t ∈ [1,s]. Therefore
104 Chapter 5: almost einstein structures with vanishing almost scalar curvature
˙
η(t) ∝ gradση(t) 6= 0 such that there is an integral curve of gradσ connecting expp(X) and expp(sX). Hence
expp(sX) = Φ(κ1, expp(X))
(5.15)
= Φ
κ1,Φ(κ0, expp(e0+e(α)))
= Φκ0+κ1, expp(e0+e(α))
= ϕ−1(κ0+κ1,α).
In general the exponential map is not a diffeomorphism on its domain and it is convenient to consider only the subset of the tangent space where it is a diffeomorphism. Assume p to be a vertex of Σ, then we get the following lemma on the intersection of the latter domain and the tangent null coneCpM. domain.
Lemma 5.1.32. Let (M,g,σ) be an almost Einstein structure and Σ its singularity set. Consider p ∈ Σd and let γ : (0, 1] → Σc be the null geodesic defined byγ(t) = expp(t u)with u ∈ CpM and such that there is no further vertex inγ( (0, 1] ). Then there is a setΩ ⊂ CpM∪ {0}such that
(i) Ωis star shaped andΩ\ {0}is open inCpM (ii) ∀t ∈ [0, 1] : γ(t)∈ expp(Ω)and
(iii) expp :Ω\ {0} → Σcis a diffeomorphism onto its image.
Proof: Basically we will use the same construction methods as above. The first step will be to construct the neighbourhood Ω of the the preimage of the geodesic. Then we will show expp to be a diffeomorphism on it.
First we point out that the null geodesic γcan be extended to valuest ∈ (0−e, 1+e). The extension to valuest < 0 is possible, since exppis a local diffeomorphism in p. The extension to values t > 1 is possible, since for γ(1) ∈ Σc the gradient gradσγ(1) does not vanish.
Therefore the integral curve inγ(1)can locally be extended in both directions. Since it is a null pregeodesic tangent toγ, it can be reparametrised to a null geodesic tangent toγ. By an affine transformation of the parameter this gives the desired extension ofγ beyond 1.
We will now use the notation introduced in the preliminaries of Lemma5.1.30. So let Ube a convex neighbourhood of the origin inTpM, such that p is the only critical point of gradσ in exp(U). Then gradσexp
p(X) = 0 for X ∈ U if and only if X = 0. The end point of γ will again be denotedq = expp(u)and is an element of Σc. We define
Ω1:= (CpM∩U)∪ {0}
to be the tangent null cone including its vertex. If we restrict the exponential map toΩ1without the vertex, i.e.
expp : Ω1\ {0} → Σc,
this is a restriction to an(n−1)-dimensional submanifold ofTpMand hence a diffeomorphism onto its image. By convexity ofUwe get thatΩ1is star shaped with respect to the origin.
We recall the preceding definition, i.e. there is a ˜u ∈ U with x = expp(u˜) ∈ Σc such that u = su. The gradient grad˜ σ does not vanish alongγand is tangent to it. By Lemma5.1.6(iiv) γ can be reparametrised to an integral curve of gradσ and hence there is a t0 ∈ R such that γ(1) = expp(u) = Φ(t0,x). We consider the case where p is a repeller and thereforet0 > 0.
For the other case we have negativet0and the roles of xand qhave to be swapped. By Lemma 5.1.30there is a coordinate map
ϕ :We →(−e,t0+e)× V ⊂ Rn−1
5.1 topology of the conformal boundary Σ 105
with We := ϕ−1( (0−e,t0+e)× V) such that the segment of γ containing x and q = γ(1) is contained inW. By construction the preimagee ϕ−1(t,α)is in ˜U for t ∈ (−e, 0] such that in particular ϕ−1(t,α) ∈ expp U˜
⊂ expp(Ω1) for all t ≤ 0. Since ϕ(·,α) =Φ(·,x) are null pregeodesics tangent to radial null geodesics starting at p, the exponential map is defined onto points inW. That is whye
Ω2 := exp−p1 We
is a well defined quantity. Again we remark that by construction of ϕ−1 the vector field gradσ does not vanish alongWe. We define
Ω := Ω1∪Ω2.
The exponential map is defined for all X ∈ Ω as seen above and we will show that Ω has the desired properties.
We find
gradσexp
p(X)= 0 ⇔ X= 0, (5.16)
since this holds for X ∈ Ω1 and X ∈ Ω2. Also Ω1\ {0}and Ω2 are by construction open in CpMand so is their union.
Now we showΩ to be star shaped with respect to 0. The claim is clear for X ∈ Ω1. Now considerX ∈ Ω2. Then expp(X) = ϕ−1(t,α) and hence
expp(X) =Φ(t,f(α) ) = Φ(t, expp(Y) )
for someY ∈ Ω1, since im(f) ⊂ Ω1. This implies that there is a null pregeodesic in We from expp(Y)to expp(X). HenceX = µYfor someµ> 1 and therefore[1,µ]Y ⊂ Ω2. Combining it with the last observation, this leads to[0,µ]Y = [0, 1]X ⊂ Ω.
Second we show expp : Ω → Σ to be bijective onto its image. Assume expp(X1) = expp(X2)forXi ∈ Ωand define the geodesicsγi(t) := expp(t Xi). SinceΩ is star shaped we havet Xi ∈ Ω for all 0 ≤ t ≤ 1. By construction gradσ is non-vanishing along the geodesics, except in p = γi(0). Therefore there are non-vanishing functions fi : (0, 1] → R± along the geodesics such that fi(t)γ˙i(t) = gradσγi(t). By Lemma 5.1.8 p is an attractor or an repeller of gradσ. Since gradσ is tangent to null geodesic radiating from p this means that f1and f2 must have the same sign such that in particularµ := ff1(1)
2(1) is strictly positive. Without loss of generality assumeµ ∈ (0, 1] and interchange the roles of X1and X2else. At γ1(1) = γ2(1) we therefore have
˙
γ2(1) = 1
f2(1)gradσγ1(1) = µγ˙1(1).
Now consider the geodesicη(t) := µ1(µ(t−1) +1). Then it holds η(1) = γ1(1) = γ2(1) and ˙η(1) = µγ˙1(1) = γ˙2(1). By uniqueness of geodesics we then have η(t) = γ2(t)for all t ∈ [0, 1]. In particular p = µ2(0) = η(0) = µ1(1−µ)with 1−µ < 1. Since we assumed X1 ∈ Ω, by condition (5.16) this is equivalent to µ = 1 and henceµ1(t) = µ2(t)for all t. We concludeX1= X2.
The third part of the proof will be to show thatdh exppi
u is an isomorphism for an arbitrary u ∈ Ω \ {0}. It suffices to show ker
dh exppi
u
= {0}. Since Ω\ {0} is open in CpM, there is an e > 0 such that γ : t 7 → expp(t u) is well defined for all t ∈ (−e, 1+e). Then according to Lemma5.1.30there is an open setUq ⊂ Σcand coordinates ϕ : Uq →Rn−1 such that the properties of Lemma5.1.31are fulfilled. Following the preceding construction there is a orthonormal frame{ei}of TpMand ˜u ∈ CpMsuch that
u = su˜ expp(u˜) = ϕ(0)
˜
u = e0+e1 expp(u) = ϕ(t0, 0)
106 Chapter 5: almost einstein structures with vanishing almost scalar curvature andαn ∈ Rboth sufficiently small. Then by construction ofe(see Equation (5.12)) the curves
ηn(t) := (1+αnt) (e0+e1) In the following we will identify the tangent spacesTu CpM
and Tu˜ CpM
with each other.
For null tangent vectors ˙ηn we find dϕexp
p(u)is an isomorphism, the last line does not vanish. Furthermore we have dϕexp
for some smooth map g, since the coordinates along the geodesic γ have only a non-vanishing component at the first position. From the previous calculation we conclude that ˙f(0) 6= 0.
Moreover, we point out that this calculation is in principle valid for arbitrary base points as dh
exppi
u(u) is tangent to the geodesic specified by expp(t u).
For spacelike tangent vectors ˙ηs we then find for the i-th component (i 6= 0) of its image under this composition could be added to give a Zero vector, which would contradict dϕexp
p(u)˜ ◦dh
5.1 topology of the conformal boundary Σ 107
The last lemmata can be summarised to classify the maximal domain where the restricted exponential map is a diffeomorphism.
Corollary5.1.33. Let p ∈ Σd and U˜ open inCpM such thatU˜ ∪ {0} is star shaped with respect to the origin and expp
˜
U : ˜U → Σcis a diffeomorphism. Let DΦ ⊂ R× M be the maximal domain of the flowΦofgradσ. ThenU˜m a x := exp−1p Φ R×U˜ ∩ DΦ is a well defined extension ofU˜ and
expp
U˜ : ˜Um a x →Σcis a diffeomorphism.
The maximal subset cannot be bigger since every radial null geodesic that leaves a connected component ofΣc must leave it at a vertex by Lemma 5.1.7. More precisely as long as the null geodesic has values inΣcit can be reparametrised to an integral curve of gradσ. Since there is a point where the null geodesic is not inΣc, the reparametrised integral curve cannot be extended beyond that point and therefore must be maximal. Now Lemma5.1.7can be applied.
Remark. We assert that any subset of Σthat is a spacelike (n−2)-dimensional submanifold of Mmust not contain any of the vertices inΣd. This obviously has the same reason that preventΣ from being a submanifold of M at those points. Hence a spacelike submanifold of Σimplicitly is a submanifold ofΣc.
Proposition5.1.34. Let N be an(n−2)-dimensional spacelike submanifold ofΣcandγ :[0,t0] →Σ a null geodesic withγ(0) := q ∈ N,γ(t0) =: p ∈ Σd andγ( [0,t0) ) ⊂ Σc. In particular there is no other vertex betweenγ(0)and γ(t0). Thenγ(t0)is a focal point of N with respect toγ.
Proof: First for e > 0 being sufficiently small, γ can be extended to values t ∈ (−e,t0+e). Moreover, since γ(t0) = p, there is a u ∈ CpM with γ(t) = exp( (t−t0)u). We are now in the setting of Lemma5.1.32 and there is an Ω ⊂ CpM containing γ( [0,t0] ) such that the exponential map is a diffeomorphism onΩ\ {0}.
We will now construct a geodesic variation δ γ normal to N with vanishing Jacobi field at γ(t0) = p. Considerδ >0 sufficiently small and a curveα : (−δ,δ) → N withα(0) = γ(0), α(s) ∈ Ω for alls ∈ (−δ,δ) andα0(0) 6= 0. We define
X : (−δ,δ) → CpM
s 7 → exp−p1(α(s) ).
Due to Lemma5.1.32 this map is well defined and depends smoothly on the parameter s. Its covariant derivative with respect toαis
X0(0) = ∇
d sX(0) = dh
exp−1p i
α(0) α0(0)
and it is non-vanishing as the exponential map is a diffeomorphism and α0(0) 6= 0. Up to rescaling ofδand e, the geodesic variation of γ
δ γ: (−e,t0+e)×(−δ,δ) → Σ (t,s) 7 → exppt
0−t
t0 X(s).
is well defined. By definition we haveδ γ(0,s) = α(s)and δ γ(t0,·) ≡ p. In additionδ γ(·,s) are null geodesics inΣfor fixedsand therefore normal to N. We now define its variation vector field
J : [0,t0] → T M t 7 → ∂sδ γ(t, 0). More explicitly this reads
J(t) = ∂s|s=0expp
t0−t t0 X(s)
=dh exppi
t0−t t0 X(0)
t0−t t0 X0(0)
.
108 Chapter 5: almost einstein structures with vanishing almost scalar curvature
We remark that X0(0) 6= 0 is tangent to CpM and t0t−t
0 X(0) ∈ Ω. Therefore dexpp is an isomorphism at least on the tangent space of the null cone and we get J(t) = 0 if and only if t= t0. Summarising the facts we conclude that Jis aN-Jacobi field onγandγ(t0) = pa focal
point as claimed.
Now summarising Propositions5.1.29and5.1.34gives the following theorem.
Theorem 5.1.35. Let (M,g,σ) be an almost Einstein structure with S[g,σ] = 0. Let N be an (n−2)-dimensional spacelike submanifold of Σc and γ : [0,t0] → Σ a null geodesic with γ(0) := q∈ N and γ( [0,t0) )⊂ Σc. In particular there is no vertex betweenγ(0)andγ(t0). Thenγ(t0)is a focal point of N with respect toγif and only if it is a vertex ofΣ. In other words
γ(t0)is a focal point of N ⇐⇒ γ(t0)∈Σd.