singularity theorems in general relativity
TRANSCRIPT
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Singularity Theoremsin General RelativitySingularity Theoremsin General Relativity
José M M Senovilla
8th June 2005
José M M Senovilla
8th June 2005
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Be careful!Be careful!
Gato ≠ Liebre !!
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What do ST say?What do ST say?
They predict, under certain circumstances, the
existence of singularitiessingularitiesin classical spacetimes.But, what is this thing they predict? Singularities?
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SingularitySingularityThe intuitive ideas (such as “infinite concentration” of
matter/energy) are not what the ST predict!
The main idea: they imply the suddenappearance, or disappearance, of physical
particles.
In mathematical terms, they imply the existence ofincomplete endless causal geodesics
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SingularitySingularityOops!
This particle (a rocket, orlight,…) has reached the“edge” of the spacetime;In finite time and distance!
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Singularity SingularityOops!
This particle (a rocket, orlight,…) comes from the“edge” of the spacetime;In finite time and distance!
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Para ver esta película, debedisponer de QuickTime™ y de
un descompresor Sorenson Video 3.
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Para ver esta película, debedisponer de QuickTime™ y de
un descompresor Sorenson Video 3.
Para ver esta película, debedisponer de QuickTime™ y de
un descompresor Sorenson Video 3.
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SingularitiesSingularities
This means that one can reach in finite time, something which is at finite distance
and where the spacetime
ENDSBEGINS
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PeoplePeople
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Pattern singularity theoremPattern singularity theorem• If the spacetime is smooth and
satisfies:
• A condition on the curvature• A causality condition• And a Boundary/Initial condition
• THEN, it has incomplete causalgeodesics (i.e., Singularities).
• If the spacetime is smooth and satisfies:
• A condition on the curvature• A causality condition• And a Boundary/Initial condition
• THEN, it has incomplete causalgeodesics (i.e., Singularities).
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How do these conditionswork?
How do these conditionswork?
• What are the effects andconsequences of each of the abovehypotheses?
• What are they needed for?
• And what do they imply?
• What are the effects andconsequences of each of the abovehypotheses?
• What are they needed for?
• And what do they imply?
Understandable with elementary geometry !
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What is the shortest curve?What is the shortest curve?
!
!
It depends on the “length element”!!
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What is the shortest curve?What is the shortest curve?
!
!
If the length is “euclidean”, dl2=dx2 +dy2 .
y
xx
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What is the shortest curve?What is the shortest curve?
!
!
If the length is “riemannian/gaussian”:e.g. dl2=F2(x,y) dx2 + G2(x,y) dy2 .
y
x
This could well be the shortest!
?
x
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How can one control this?
How can we properly define “shortest”?
How can one control this?
How can we properly define “shortest”?
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LENGTHLENGTH
∫→ =q
p
dlqpLγ
γ
)(
!
!
p
q
γ
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Minimum length?Minimum length?
!
!
γ1
γ2
γ3
p
q
0)( =→qpL γ
δγδ Geodesics
Geodesics are equivalently characterizedby a vanishing proper curvature vector (or”acceleration” for physicists). Observe thatthis is an extrinsic property!
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The geodesic condition is LOCAL
The geodesic condition is LOCAL
For instance, in a spherethe geodesics are segmentsof a great ”circle” (ortodrome).
This means that the greatcircles minimize, locally, thedistance between any two of their points. But, what happensat large?
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CORNERS, CONJUGATE/FOCAL POINTS
CORNERS, CONJUGATE/FOCAL POINTS
N
S
So, geodesics arenot unique for givenpoints.
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CORNERS, CONJUGATE/FOCAL POINTS
CORNERS, CONJUGATE/FOCAL POINTS
N
S
As a matter of fact,ALL geodesics startingfrom S will meet at N!!
N is a focal point for the geodesic familyemanating from S
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CORNERS, CONJUGATE/FOCAL POINTS
CORNERS, CONJUGATE/FOCAL POINTS
S
pq
The curve p " S " qis geodesic… but with a corner !!
Corners makedistance larger
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CORNERS, CONJUGATE/FOCAL POINTS
CORNERS, CONJUGATE/FOCAL POINTS
N
S
There are twogeodesics withoutcorners from S to p.
p
The shortest hasnot crossed a focalpoint. The other HAS.
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Minimum LengthMinimum Length
A curve from p to q has the minimum length between p and q if and only if it is a geodesic, with no corners, and no focal/conjugate
points from p.
A curve from p to q has the minimum length between p and q if and only if it is a geodesic, with no corners, and no focal/conjugate
points from p.
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Shortest?Shortest?
!
!
If the length is “minkowskian”, dl2=dx2 - dt2 .
t
x
!dl2<0!
OKx
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LENGTH/TIMELENGTH/TIME
L( pγ
→q) = dl2
γp
q
∫
If dl2 > 0 all along γ, L gives distanceIf dl2 < 0 all along γ, L gives time
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Shortest?Shortest?
!
!
If the length is “minkowskian”, dl2=dx2 - dt2 .
t
x
!
!
OK
L=0 !x
dl2<0, butL(=Time)>0
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Shortest?Shortest?There cannot be a minimum L when L is time!!!
!q
!p
This curve, for instance, has manycorners but total L = 0 !
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What are the meaning of geodesics in these
SPACE-TIMES?
What are the meaning of geodesicsgeodesics in these
SPACE-TIMES?
0)( =→qpL γ
δγδ
Of course, this geodesic condition implies that they are extremal curves with respect to L. But, what about the
second variation?
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Shortest/Longest !Shortest/Longest !
!
!
If the length is “minkowskian”, dl2=dx2 - dt2 .
t
x
!
!
Shortest for dl2|γ>0
L=0 !x
Longest fordl2|γ<0
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Maximum Time !Maximum Time !
A curve from p to q has the maximum time between p and q if
and only if it is a (causal) geodesic, with no corners, and no
focal/conjugate points from p.
A curve from p to q has the maximum time between p and q if
and only if it is a (causal) geodesic, with no corners, and no
focal/conjugate points from p.
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Twin ”paradox”Twin ”paradox”
!q
!p
Twin 3
Twin 1
Twin 2The oldest twin is always the one in freefall (= geodesic)
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Pattern singularity theoremPattern singularity theorem• If the spacetime is smooth and
satisfies:
• A condition on the curvature• A causality condition• And a Boundary/Initial condition
• THEN, it has incomplete causalgeodesics (i.e., Singularities).
• If the spacetime is smooth and satisfies:
• A condition on the curvature• A causality condition• And a Boundary/Initial condition
• THEN, it has incomplete causalgeodesics (i.e., Singularities).
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The condition on thecurvature
The condition on thecurvature
It is needed to ensure the focalization of geodesics.
N
SN is a focal point for the geodesic familyemanating from S
Recall :
We can see, intuitively, that the presence of focal points arises due to the curved form of the sphere, i.e., to the
presence of CURVATURE.
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The condition on thecurvature
The condition on thecurvature
is many times disguised as an “energy condition”
= +
Riemann Weyl Ricci-Curbastro
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The condition on thecurvature
The condition on thecurvature
is many times disguised as an “energy condition”
= +
Energy/mass distribution
Einstein’s equations
“genericity”
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Pattern singularity theoremPattern singularity theorem• If the spacetime is smooth and
satisfies:
• A condition on the curvature
•A causality condition• And a Boundary/Initial condition
• THEN, it has incomplete causalgeodesics (i.e., Singularities).
• If the spacetime is smooth and satisfies:
• A condition on the curvature
•A causality condition• And a Boundary/Initial condition
• THEN, it has incomplete causalgeodesics (i.e., Singularities).
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The causality conditionThe causality condition
It is necessary in two respects:
(i) To prohibit avoiding the future, the passing of time;
(ii) To ensure the existence of maximal geodesics
The strongest (and more effective for the ST) causality condition is called
Global Hyperbolicity
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Therefore, there always existsat least one curve with the
maximum L (=time).
The causality conditionThe causality condition
A spacetime is said to be globally hyperbolic if the set ofall causal paths between any two points is compact
!
!
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The causality + curvatureconditions
The causality + curvatureconditions
Observe:
(i) Curvature implies the existence of focal points for allendless geodesics
(ii) Global hyperbolicity ensures the existence of maximalcurves between any two (causally related) points.
YET, these two conditions by themselves do NOT provide a contradiction
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Pattern singularity theoremPattern singularity theorem• If the spacetime is smooth and
satisfies:
• A condition on the curvature• A causality condition
•A Boundary/Initial condition
• THEN, it has incomplete causalgeodesics (i.e., Singularities).
• If the spacetime is smooth and satisfies:
• A condition on the curvature• A causality condition
•A Boundary/Initial condition
• THEN, it has incomplete causalgeodesics (i.e., Singularities).
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The boundary/initial conditionThe boundary/initial condition
• It may adopt several forms:(i) The existence of a Trapped Surface(ii) An instant of time where ALL the
universe is strictly expanding(iii)Enough curvature (or matter) such
that the light cones refocusecompletely
• It may adopt several forms:(i) The existence of a Trapped Surface(ii) An instant of time where ALL the
universe is strictly expanding(iii)Enough curvature (or matter) such
that the light cones refocusecompletely
It is absolutely essential
(ii) and (iii) are easily understandable, but what about (i) ?
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The boundary/initial conditionThe boundary/initial condition
The concept of closed trapped surface is a breakthrough in science,
a beautiful and powerfulidea, due to Penrose, who had
an extraordinary insight and changedrelativistic science for ever more.
What is less known, and hardly mentioned, is that theconcept of trapped surface is purely geometricpurely geometric
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PASEN…, Y VEAN!PASEN…, Y VEAN!
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Evolving surfacesEvolving surfaces
S
Sin
Sout
Of course,Sin < Sout
But, Sin < S ?Or S < Sout ?
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Trapped surfaces? Trapped surfaces?
S(to)
Sin(t)
Sout(t)
t > to
Sin(t) < S(to)?S(to)< Sout(t)?
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Expanding world!Expanding world!
S(to)
S(t)
Sin(t)
Sout(t)
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Contracting world!Contracting world!
S(t)
Sin(t)
S(to)Sout(t)
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Trapped surfaces!Trapped surfaces!
“Futuretrappedsurface”
Sout(t) < S(to)is possibleContracting:
“Past trappedsurface”
Sin(t) > S(to)is feasibleExpanding:
(t>to)
Of course, this (expansion/contraction) may change with time t!
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Trapped surfaces (mathematically revisited)
Trapped surfaces (mathematically revisited)
Less known is the fact that the concept of trapped surface is purely geometric, and closelyrelated to that of minimal surfaces(and geodesics).
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What surface has the minimum area?
What surface has the minimum area?
Q
P
In analogy with the shortest pathdiscussed before, one can ask, for given lines such as P and Q on the left,which surface with boundary on P and Qhas the minimum area?
Of course, again the answer dependson the “length element”!!
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Minimum area?Minimum area?
If the line element is “euclidean”, dl2=dx2 +dy2 +dz2.
z
yx
PP
CATENOID (discovered by Meusnier in 1776)
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AREAAREAQ
P
S ∫→ =Q
PS
S
dlQPA )(
(Observe that this is a double integral)
If the length is riemannian, e.g.:dl2=F2(x,y,z) dx2 + G2(x,y,z) dy2 + H2(x,y,z) dz2
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Minimum area?Minimum area?Q
P
S 0)( =→QPSL S
δδ
Minimal surfacesMinimal surfaces are equivalently characterized by the vanishing of the meancurvature (vector). Observe that again, this is an extrinsic property!
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Examples of minimal surfaces in flat space
Examples of minimal surfaces in flat space
Helicoid
Catalan
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Costa
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Schwarz
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What about higher dimensions?What about higher dimensions?
The mean curvature looks like a scalar (the mean of theprincipal curvatures) due to the existence of only ONEextra dimension. If the co-dimension is > 1, then it is verysimple to see that the mean curvature is a (normal)vector: it may have components along all directionsorthogonal to the surface.
This is exactly as with geodesics, which in 3-dimensionalspace, as is well known, have a curvature vector (theacceleration) which is orthogonal to the curve.
The number of independent components of these vectorsis in general the co-dimension.
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Four (or higher) dimensions!
Four (or higher) dimensions!
Imagine the space has dimension > 3. If the length ispositive definite, e.g.
dl2 = dx2 + dy2 + dz2 + … + dw2
(so, no time!).
Then, the mean curvature vector H, as any other vector, hasa non-negative norm, and the norm vanishes if and only if the mean curvature vector vanishes:
H ! H ≥ 0 ; furthermore, H ! H = 0 H = 0
The only distinguished case is that of minimal surfaces!!
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Four (or higher) dimensions!
Four (or higher) dimensions!However, if there is time
dl2 = - dt2 + dx2 + dy2 + dz2 + … + dw2
Then, vectors may have negative and vanishing norm!
# H ! H < 0 ; These are TRAPPED SURFACES!!
# H ! H = 0, BUT H ≠0 ; These are “marginallytrapped”: they form the
HORIZONS !
There appear TWO new possibilities:
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Do ST apply to actual, physical, situations?
Do ST apply to actual, physical, situations?
Who cares?
Yes, mainly to two real physical situations:
(i) Gravitational collapse (and formation of “black holes”.)(ii) Cosmology (and the classical origin or end of everything.)
Why? How?
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Gravitational collapseGravitational collapse
The gravitational
field affects light!
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Bending of light by a largemass
Bending of light by a largemass
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Gravitational lenses!Gravitational lenses!
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The Einstein CrossThe Einstein Cross
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Abell 2218: A GalaxyCluster Lens
Abell 2218: A GalaxyCluster Lens
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Gravitational collapseGravitational collapse
The gravitational
field affects light!
Trapped surface?
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The formation of trappedsurfaces in realistic
processes of gravitationalcollapse is one the mainareas of research andinterest in numerical
relativity.
The formation of trappedsurfaces in realistic
processes of gravitationalcollapse is one the mainareas of research andinterest in numerical
relativity.
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CosmologyCosmology
The ST may apply to the Universe as a whole, The ST may apply to the Universe as a whole, and imply tand imply the (classical) singular origin he (classical) singular origin ------ and and possibly endpossibly end------ oof the entire matter and energy f the entire matter and energy
content of the Universe due to THREE content of the Universe due to THREE observed factsobserved facts
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1) Expansion1) Expansion
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2) Isotropy of MWBR2) Isotropy of MWBR
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3) Homogeneity + origin of light elements
3) Homogeneity + origin of light elements
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The Friedman-LRW geometries
The Friedman-LRW geometries
dl2 = -dt2 + a2(t) [dr2+S2(r,k)dΩ2]
Scale factor
Curvatureindex
k = 0, ±1
Take any 2-sphere defined byt=T , r=R
with T and R constants.
A trivial calculation provides:
S = sinh r, r, sin rfor k= -1 , 0, 1
r H = 1
a2(T)−Ý a (T)a(T) ∂
∂t+
′ S (R)S(R)
∂∂r
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The Friedman-LRW geometries
The Friedman-LRW geometries
dl2 = -dt2 + a2(t) [dr2+S2(r,k)dΩ2]
⇒
r H •
r H = 1
a2
′ S 2
S2 − Ý a 2
=
1a2
1S2 − k − Ý a 2
Using Einstein’s equations
⇒
r H •
r H = 1
a 2
′ S 2
S 2 − Ý a 2
=
1a 2S 2 −
ρ3
Matterdensity
r H = 1
a2(T)−Ý a (T)a(T) ∂
∂t+
′ S (R)S(R)
∂∂r
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The Friedman-LRW geometries
The Friedman-LRW geometries
r H •
r H ≤ 0 ⇔ a(T)S(R) ≥
3ρ(T)
This is, of course, always possible for big enough R(provided the matter density is positive!!)
ALL FRWL models have trapped spheres !
The Universe?
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CONCLUSIONSCONCLUSIONS
??
There is a LOT of nice, intriguing, beautiful, appealing, interesting,
(differential) geometry behindGRAVITATION.
And many unexplored zones!!