on the phase structure of quantum gravity€¦ · a.c. longhitano, phys.rev.d22:1166,1980. the...
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The gravitational Higgs phenomenon GraviGUT The main question NCG
On the phase structure of quantum gravity
Roberto Percacci1
1SISSA, Trieste, Italy
Quantum gravity in ParisMarch 23, 2017
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The gravitational Higgs phenomenon GraviGUT The main question NCG
Outline
1 The gravitational Higgs phenomenon
2 GraviGUT
3 The main question
4 NCG
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The gravitational Higgs phenomenon GraviGUT The main question NCG
Basic question
Why is the gravitational connection not dynamical?
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The gravitational Higgs phenomenon GraviGUT The main question NCG
Higgs mechanism I
Certain gauge fields are experimentally seen to be massive.How to reconcile a mass with gauge invariance?Higgs field φ with values in some vectorspace VW a G-invariant potential with minima in G/H ⊂ VAdapted coordinates in V : φ = (ρ, σ)Dynamics gives 〈φ2〉 = ρ2
0Can choose “unitary gauge” φ = (ρ, σ0)
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The gravitational Higgs phenomenon GraviGUT The main question NCG
Higgs mechanism II
DecomposeA = A|L(H) + A|P
where L(G) = L(H)⊕ P
Dφ = ∂φ+ Aφ = (Dρ,Dσ) where
Dρ = ∂ρ and Dσ = ∂σ + AiKi(σ)
in particular Dσ0 = Ai |PKi(σ0)
in unitary gauge
(Dφ)2 7→ (∂ρ)2 + ρ20(A|P)2
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The gravitational Higgs phenomenon GraviGUT The main question NCG
Higgs mechanism III
only Goldstone bosons are necessary for HiggsmechanismHiggs particle “only” necessary for perturbativerenormalizability
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The gravitational Higgs phenomenon GraviGUT The main question NCG
Higgsless Higgs mechanism
Higgs field h = ρ− ρ0 also has mass ≈ λρ0W = λ
4 (φ2 − ρ20)2
limλ→∞W with ρ0=const
ρ decouples in the limit, leaves gauged nonlinear sigma model
T. Appelquist, C.W. Bernard, Phys.Rev.D22:200,1980.A.C. Longhitano, Phys.Rev.D22:1166,1980.
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The gravitational Higgs phenomenon GraviGUT The main question NCG
Low Energy EFT
For p2 � m2h,
ρ = ρ0
For p2 � m2A,
A|P = 0
orDσ = 0
Example: Meissner effect in superconductivity
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The gravitational Higgs phenomenon GraviGUT The main question NCG
Gravity with more Variables
Spacetime manifold M, dimM = 4
frame field (a.k.a. soldering form) θaµ, detθ 6= 0
pseudo-fiber metric, γab signature +,+,+,−linear connection, Aµa
b (structure group GL(4))
First two carry nonlinear realizations of GL(4)
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The gravitational Higgs phenomenon GraviGUT The main question NCG
Induced structures in TM
If we think of θ : TM → EE real vectorbundle with fiber dimension 4local bases {∂µ} in TM and {ea} in E
thengµν = θa
µ θbν γab
Γλµν = θ−1
aµAλa
bθbν + θ−1
aµ∂λθ
aν
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The gravitational Higgs phenomenon GraviGUT The main question NCG
Torsion and Nonmetricity
Θµaν = ∂µθ
aν − ∂νθa
µ + Aµab θ
bν − Aνa
b θbµ
∆λab = −∂λγab + Aλca γcb + Aλc
b γac
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The gravitational Higgs phenomenon GraviGUT The main question NCG
Gauge invariance
G = AutGL(4)E
θaµ(x) 7→ θ′
aµ(x ′) = Λ−1a
b(x) θbν(x)
∂xν
∂x ′µ
γab(x) 7→ γ′ab(x ′) = Λca(x) Λd
b(x) γcd (x)
Aµab(x) 7→ A′µ
ab(x ′) =
∂xν
∂x ′µ(Λ−1a
c(x)Aνcd (x)Λd
b(x)
+Λ−1ac(x)∂νΛc
b(x))
0→ AutGL(4)M E → AutGL(4)E → DiffM → 0
is split: θ∗ : DiffM → AutGL(4)Eθ∗(f ) = θ ◦ Tf ◦ θ−1
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The gravitational Higgs phenomenon GraviGUT The main question NCG
Goldstone Bosons
AutGL(4)E acts transitively on metric and soldering form
γ(x) ∈ GL(4)/SO(3,1)
γ ∈ {fiber metrics} ≈ AutGL(4)E/AutSO(3,1)E
θ ∈ {isomorphisms TM → E} ≈ AutGL(4)E/DiffM
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The gravitational Higgs phenomenon GraviGUT The main question NCG
Metric gauge
θaµ = δa
µ
unbroken group DiffMgµν = γµν , Γλ
µν = Aλµν
Θµaν = Γµ
aν − Γν
aµ
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The gravitational Higgs phenomenon GraviGUT The main question NCG
Vierbein gauge
γab = ηab
unbroken group AutSO(3,1)Mgµν = θa
µ θbν ηab
∆λab = Aλab + Aλba
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The gravitational Higgs phenomenon GraviGUT The main question NCG
Metric and vierbein gauge
Not enough freedom to fix both simultaneously
In general gauge: connection and two “Goldstone bosons”In either of the two “unitary” gauges: connection and one“Goldstone boson”
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The gravitational Higgs phenomenon GraviGUT The main question NCG
Low energy action
S(A, γ, θ) = SG(A, γ, θ) + Sm(A, γ, θ)
where
SG =
∫d4x
√|g|[M2
P θaµθb
νFµνab + . . .]
Sm =
∫d4x
√|g|[M2(Θ ·Θ + ∆ ·∆ + Θ ·∆) + . . .
]
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The gravitational Higgs phenomenon GraviGUT The main question NCG
The Higgs Mechanism v.I
flat background: A = 0, θ = 1, γ = η
Θµaν = Aµa
ν − Aνaµ
∆µab = Aµab + Aµba
Fµνab = ∂µAνa
b − ∂νAµab + Aµa
c Aνcb − Aνa
c Aµcb
S contains
12
∫d4x
√|det g|M2 Q(A,A)
generically Q is non-degenerate.There is a more general point of view.
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The gravitational Higgs phenomenon GraviGUT The main question NCG
Levi–Civita Connection
given θ, γ, there is a unique A s.t. Θ = 0, ∆ = 0
Aabc =12(Eabc + Ecab − Ebac
)+
12(Cabc + Cbac − Ccab
)where
Eabc = θ−1aλ ∂λκbc
Cabc = γad θdλ
(θ−1
bµ ∂µθ
−1cλ − θ−1
cµ ∂µθ
−1bλ)
Any connection A can be split uniquely in A = A + Φ
then S(A, γ, θ) = S(A(θ, γ) + Φ, θ, γ) = S′(Φ, θ, γ)
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The gravitational Higgs phenomenon GraviGUT The main question NCG
The Higgs Mechanism v.II
Θµaν = Φµ
aν − Φν
aµ
∆µab = Φµab + Φµba
Fµνab = Fµνa
b + ∇µφνab − ∇νφµa
b + φµa
c φνc
b − φνac φµ
cb
therefore
SP(A, γ, θ) = SP(A + Φ, γ, θ) = SH(γ, θ) + SQ(Φ, γ, θ)
where
SQ(φ, γ, θ) =12
∫d4x
√|det g| M2
P QP(Φ,Φ)
andSm(φ, γ, θ) =
12
∫d4x
√|det g| M2 Qm(Φ,Φ)
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The gravitational Higgs phenomenon GraviGUT The main question NCG
Gravitational Higgs Phenomenon
Generically all components of φ are massive.(Not true for the Palatini action, since QP has nontrivial kernel)At energy scales p2 � M2
P
φ = 0⇐⇒ {Θ = 0 and ∆ = 0}
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The gravitational Higgs phenomenon GraviGUT The main question NCG
Gravitational Higgs Phenomenon
gravity is a gauge theory of GL(4) with two Goldstonebosonsthere are two unitary gaugesHiggsless Higgs phenomenon occurs at Planck scale,giving mass to Φ (equivalently A)at low energy A = A(θ, γ)
Θ = 0 and ∆ = 0 means that the theory is in a “Higgs”phase
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The gravitational Higgs phenomenon GraviGUT The main question NCG
Questions
why is the metric nondegenerate?what is the dynamical origin of the Planck scale?does the connection propagate at ultra-Planckian scales?
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The gravitational Higgs phenomenon GraviGUT The main question NCG
Grand Unification
use Higgs phenomenon with G1 ×G2 ⊂ Gto do list:
identify GUT group Gfit particles in irreps of Gwrite G-invariant actionexplain symmetry breaking (select order parameter, orbit,potential)check that new particles not seen at low energy have highmass
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The gravitational Higgs phenomenon GraviGUT The main question NCG
Grand unification: SO(10)
(eL , νL ,eR , νR ,ur ,g,bL ,d r ,g,b
L ,ur ,g,bR ,d r ,g,b
R )
16 complex 2 component Weyl spinors of Lorentz4 doublets and 8 singlets of SU(2)LRepeat three times. (nν = 2.984± 0.008 measured at LEP)
Fit exactly in the 16 of SO(10)!explains hypercharge assignments
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The gravitational Higgs phenomenon GraviGUT The main question NCG
A symmetry breaking chain
SO(10)↓
SO(4)× SO(6) ≈ SU(2)R × SU(2)L × SU(4)↓
SU(2)R×SU(2)L×SU(3)C×U(1)B−L↓
U(1)EM × SU(2)L × SU(3)C↓
U(1)EM × SU(3)C
requires at least 45, 16 and 10
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The gravitational Higgs phenomenon GraviGUT The main question NCG
GraviGUT I
use gravitational Higgs phenomenon to constructunified theory of gravity and all other interactions.to do list:
identify GraviGUT group Gfit particles in irreps of Gwrite G-invariant actionexplain symmetry breaking (select order parameter, orbit,potential)check that new particles not seen at low energy have highmass
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The gravitational Higgs phenomenon GraviGUT The main question NCG
GraviGUT II
G1 = SO(1,3), G2 = SO(10),
=⇒ G = SO(1,13) or G = SO(3,11)
keep dimM=4,enlarge fibers of E to have dimension N > 4order parameter is soldering form
γ =
[η 00 1N−4
], θ is 4× N matrix, e.g. 〈θ〉 =
[140
]
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The gravitational Higgs phenomenon GraviGUT The main question NCG
Fermions I
F. Nesti, R.P., Phys. Rev. D 81, 025010 (2010) arXiv:0909.4537 [hep-th]
Assume γab = ηab, G = SO(3,11)
In SO(10) GUT one family isη ∈ 2C × 16C of SO(3,1)× SO(10)
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The gravitational Higgs phenomenon GraviGUT The main question NCG
Fermions II
Let BΣ∗ij = ΣijB and ψc = Bψ∗. Define ψ± by (ψ±)c = ±ψ±(Majorana spinors)Define ψL/R by γψL/R = ∓ψL/R (Weyl spinors).In signature (3,11) [γ,B] = 0 so we can define Majorana-Weylspinors ψL/R±. These have 64 real components.
Decomposing ψL+ under SO(3,1)× SO(10) ⊂ SO(3,11) wefind it is equivalent to η.:
64R = 2C × 16C
Remark: SO(1,13) has Weyl 64C
64C = 2C × 16C + 2C × 16C
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The gravitational Higgs phenomenon GraviGUT The main question NCG
Fermions III
DµψL+ =
(∂µ +
12
AijµΣ
(3,11)L ij
)ψL+
let Σ†ijA = −A Σij
then ψ†L+(Aγ i)LDψL+ is one-form in 14 of SO(3,11)
S =
∫ψ†L+(Aγ i)LDψL+ ∧ θj ∧ θk ∧ θ` φijk` .
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The gravitational Higgs phenomenon GraviGUT The main question NCG
Fermions IV
Assuming the following VEVs:{φmnrs = εmnrsφijk` = 0 otherwise
{θmµ = Mem
µ
θaµ = 0 otherwise
one gets
S =
∫d4x√
g η†σµ∇µη ,
where now
∇µ = D(4+10)µ = ∂µ +
12
Γabµ Σ
(3,1)ab +
12
Aabµ (10)Σ
(10)ab
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The gravitational Higgs phenomenon GraviGUT The main question NCG
GraviGUT III
Gravitational Higgs phenomenon:
A =
[A(4) HHT A(10)
]kinetic term of θ gives mass to A(4), H,SO(10) remains unbroken
R.P. Phys. Lett. B 144, 37 (1984), Nucl. Phys. B 353, 271, (1991).
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The gravitational Higgs phenomenon GraviGUT The main question NCG
Status of GraviGUT
kinematics well understoodfermionic content and dynamics okbosonic action for broken phase can be writtenhard to write action that works in both phases
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The gravitational Higgs phenomenon GraviGUT The main question NCG
The main question for a QT of spacetime
What makes the VEV of the soldering form/metric nonzero?
Related questions:- How does an extended spacetime arise?- What is the origin of the Planck scale?
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The gravitational Higgs phenomenon GraviGUT The main question NCG
This conference
Causal dynamical triangulations (Jurkiewicz)String theory (West)LQG (Geiller)NCG (Steinacker, Barrett)Asymptotic safety (Wetterich)
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The gravitational Higgs phenomenon GraviGUT The main question NCG
What you see is what you get
Theory is subject to constraints also at high energy.A.H. Chamseddine, V.Mukhanov, “On Unification of Gravity andGauge Interactions” JHEP 1603 (2016) 020, arXiv:1602.02295
Recently extended to GUTsG.K.Karananas, M. Shaposhnikov, “Gauge coupling unificationwithout leptoquarks” arXiv:1703.02964 [hep-ph]
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The gravitational Higgs phenomenon GraviGUT The main question NCG
Pick the right solution
Dynamics admits solutions with θ = 0 and also with θ 6= 0.Just pick the one having θ of maximal rank.
G. Lisi, L. Smolin, S. Speziale, J.Phys. A43 (2010) 445401arXiv:1004.4866 [gr-qc]
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The gravitational Higgs phenomenon GraviGUT The main question NCG
Conformal models
S =
∫d4x√
g[
12
(∇φ)2 − V (φ) + ξφ2R]
→ VEV of φ is the Planck mass
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The gravitational Higgs phenomenon GraviGUT The main question NCG
Ad-hoc models
Quantum graphityT. Konopka, F. Markopoulou, S. SeveriniPhys.Rev. D77 (2008) 104029 arXiv:0801.0861 [hep-th]
Self-organizing networksC. Trugenberger- Phys.Rev. D92 (2015) 084014 arXiv:1501.01408 [hep-th] |- Phys.Rev. E92 (2015) no.6, 062818 arXiv:1507.01820 [hep-th]- arXiv:1610.05934 [hep-th]
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The gravitational Higgs phenomenon GraviGUT The main question NCG
Self-consistent mean-field theory
Assuming 〈θ〉 = θ 6= 0, use θ in action to construct V (θ; θ).Check self-consistency a posteriori
R. Floreanini, R.P., E. Spallucci, Class. and Quantum Grav. 8, L193,(1991).R. Floreanini, R.P. Phys. Rev. D 46, 1566 (1992).
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The gravitational Higgs phenomenon GraviGUT The main question NCG
Bi-metric approach/asymptotic safety
Bi-metric actions appear in the functional RG approach toquantum gravity.Recent work by B. Knorr shows that at least in some cases
〈gµν − gµν〉 = 0
Otherwise, this approach is well-suited to discuss the Higgsphase.Unlikely to have access to the “unbroken” phase.
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The gravitational Higgs phenomenon GraviGUT The main question NCG
Lattice approaches
Lattice simulations of gravity see rich phase structure
Regge calculusEuclidean dynamical triangulationsCausal dynamical triangulations
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The gravitational Higgs phenomenon GraviGUT The main question NCG
New entry
A new class of multi-matrix models motivated by NCG
J. Barrett„ J.Math.Phys. 56 (2015) no.8, 082301 arXiv:1502.05383[math-ph]J. Barrett, L. Glaser, J.Phys. A49 (2016) 245001 arXiv:1510.01377[gr-qc]L. Glaser, arXiv:1612.00713 [gr-qc]
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The gravitational Higgs phenomenon GraviGUT The main question NCG
Fuzzy geometries
Subclass of matrix geometriesSpectral triple (A,H,D) with H = S ⊗M(n,C),(S spinor space of signature (p,q))with inner product(ψ1 ⊗m1, ψ2 ⊗m2) = (ψ1, ψ2) Trm†i m2andA = M(n,R),M(n,C),M(n/2,H)acts on H by ρ(a)(ψ ⊗m) = ψ ⊗ (am)Dirac operator
D(ψ ⊗m) =∑
i
ΓiLψ ⊗ [Li ,m] +
∑i
ΓiHψ ⊗ {Hi ,m}
where Γi ∈ C(p,q), ΓiL, Li antihermitian, Γi
H , Hi hermitian.
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The gravitational Higgs phenomenon GraviGUT The main question NCG
Path integral
Z =
∫(dD)e−S(D)
S(D) = g2 trD2 + g4 trD4
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The gravitational Higgs phenomenon GraviGUT The main question NCG
Observables
Measure the expectation value of the eigenvalues of D
〈λi〉 =
∫(dD)λi e−S(D)
the distribution of eigenvalues and the “order parameter”
F =
∑i(trHi)
2
n tr∑
i H2i
calculated by Monte Carlo methodP.Labus and R.P., to appear
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The gravitational Higgs phenomenon GraviGUT The main question NCG
TYPE (1,0)
D = {H, ·}
S = g4
(2NTrH4 + 8TrHTrH3 + 6(TrH)2
)+g2
(2NTrH2 + 2(TrH)2
)single-trace part of the action solvableE. Brezin, C. Itzykson, G. Parisi, J. B. Zuber, Commun. Math. Phys.59 (1978) 35.G. M. Cicuta, L. Molinari, E. Montaldi, Mod. Phys. Lett. A 1 (1986)125.
-3 -2 -1 1 2 3
0.1
0.2
0.3
0.4
0.5
0.6
-3 -2 -1 1 2 3
0.1
0.2
0.3
0.4
0.5
0.6
-3 -2 -1 1 2 3
0.2
0.4
0.6
0.8
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The gravitational Higgs phenomenon GraviGUT The main question NCG
TYPE (1,1)
Phase transition at g2 ≈ −2.4
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The gravitational Higgs phenomenon GraviGUT The main question NCG
TYPE (1,1)
-4 -2 2 4
0.1
0.2
0.3
0.4
0.5
g2 = −2.2,−2.4,−2.6
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The gravitational Higgs phenomenon GraviGUT The main question NCG
TYPE (1,1)
0 50 100 150 200
2
4
6
8
10
g2 = −2.2,−2.4,−2.6
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The gravitational Higgs phenomenon GraviGUT The main question NCG
TYPE (3,0)
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The gravitational Higgs phenomenon GraviGUT The main question NCG
TYPE (3,0)
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The gravitational Higgs phenomenon GraviGUT The main question NCG
Conclusions
A quantum theory of spacetime has to generate anextended geometry dynamically.Why is there a non-degenerate metric?Desirable to control phases of theory be a tunable“potential”. Models of fuzzy geometry seem to giveprecisely such a tool.There are hints from Monte Carlo simulations that suchmodels have a phase transition.Simplest models seem to give low-dimensional geometries.Hopefully some such model can produce d = 4Search just begun!