Quantum Quantum Gravity Gravity
and emergent and emergent metric metric
Why do we need Why do we need quantum gravity for quantum gravity for
cosmology ?cosmology ? gravitational equations provide gravitational equations provide
fundamental framework for cosmologyfundamental framework for cosmology gravity is coupled to quantum matter gravity is coupled to quantum matter
and radiationand radiation energy momentum tensor is a quantum energy momentum tensor is a quantum
objectobject can one have an equation with classical can one have an equation with classical
metric field on one side and a quantum metric field on one side and a quantum object on the other side ?object on the other side ?
Can one have an equation with Can one have an equation with classical metric field on one side classical metric field on one side
and a quantum object on the other and a quantum object on the other side ?side ?
yes : equation for expectation values !
metric is expectation metric is expectation value of quantum fieldvalue of quantum field
One only needs to assume that One only needs to assume that somesome quantum quantum theory exists for which an observable with theory exists for which an observable with properties of metric exists and has a nonzero properties of metric exists and has a nonzero expectation valueexpectation value
formalism : quantum effective action – exact field formalism : quantum effective action – exact field equations follow from variation of action equations follow from variation of action functionalfunctional
IfIf effective action takes form of Einstein-Hilbert effective action takes form of Einstein-Hilbert action ( with cosmological constant ) the Einstein action ( with cosmological constant ) the Einstein field equations followfield equations follow
This would be sufficient for cosmology !This would be sufficient for cosmology !
Einstein gravityEinstein gravity
Is Einstein Hilbert action sufficient ?Is Einstein Hilbert action sufficient ? It cannot be the exact effective action It cannot be the exact effective action
for a theory of quantum gravity !for a theory of quantum gravity ! Can it be a sufficiently accurate Can it be a sufficiently accurate
approximation for the quantum approximation for the quantum effective action ?effective action ?
Answer to this question needs Answer to this question needs consistent theory of consistent theory of quantum quantum gravity gravity !!
Einstein gravity as effective Einstein gravity as effective theory for large distance theory for large distance scales or small momentascales or small momenta
diffeomorphism symmetrydiffeomorphism symmetry derivative expansionderivative expansion
zero derivatives : cosmological constantzero derivatives : cosmological constant
two derivatives : curvature scalar Rtwo derivatives : curvature scalar R
four derivatives : Rfour derivatives : R22, two more tensor , two more tensor structuresstructures
higher derivatives are expected to be higher derivatives are expected to be induced by quantum fluctuations induced by quantum fluctuations
short distance short distance modificationsmodifications
coefficient Rcoefficient R22 order one order one
( typical quantum contribution 1/16( typical quantum contribution 1/16ππ22 ) : ) :
higher order derivative terms play a role only higher order derivative terms play a role only onceonce
curvature scalar is of the order of squared curvature scalar is of the order of squared PlanckPlanck
massmass
singularity of black holes , inflationary singularity of black holes , inflationary cosmologycosmology
no analytic behavior expected : Rno analytic behavior expected : R22 ln(R) etc. ln(R) etc.
long distance long distance modifications ??modifications ??
non- local termsnon- local terms f( R ) with huge coefficients of Taylor f( R ) with huge coefficients of Taylor
expansionexpansion
this could modify late time behavior of this could modify late time behavior of cosmology and be related to dark energycosmology and be related to dark energy
possible explanation why cosmological possible explanation why cosmological constant is zero or small ?constant is zero or small ?
need for quantum gravityneed for quantum gravity
before judgment one needs at least before judgment one needs at least one consistent model of quantum one consistent model of quantum gravitygravity
will it be unique ? probably not !will it be unique ? probably not !
Quantum gravityQuantum gravity
Quantum field theoryQuantum field theory Functional integral formulationFunctional integral formulation
Symmetries are crucialSymmetries are crucial Diffeomorphism symmetryDiffeomorphism symmetry
( invariance under general coordinate ( invariance under general coordinate transformations )transformations )
Gravity with fermions : local Lorentz Gravity with fermions : local Lorentz symmetrysymmetry
Degrees of freedom less important :Degrees of freedom less important :
metric, vierbein , spinors , random triangles ,metric, vierbein , spinors , random triangles ,
conformal fields…conformal fields…
Graviton , metric : collective degrees of freedom Graviton , metric : collective degrees of freedom
in theory with diffeomorphism symmetryin theory with diffeomorphism symmetry
Regularized quantum Regularized quantum gravitygravity
①① For finite number of lattice points : For finite number of lattice points : functional integral should be well definedfunctional integral should be well defined
②② Lattice action invariant under local Lattice action invariant under local Lorentz-transformationsLorentz-transformations
③③ Continuum limit exists where Continuum limit exists where gravitational interactions remain presentgravitational interactions remain present
④④ Diffeomorphism invariance of continuum Diffeomorphism invariance of continuum limit , and geometrical lattice origin for limit , and geometrical lattice origin for thisthis
scalar gravityscalar gravity
quantum field theory for scalarsquantum field theory for scalars d=2 , two complex fields i=1,2d=2 , two complex fields i=1,2 non-linear sigma-modelnon-linear sigma-model
diffeomorphism symmetry of actiondiffeomorphism symmetry of action
with D.Sexty
lattice regularizationlattice regularization
collective metric collective metric observableobservable
metric correlation metric correlation functionfunction
response of metric to response of metric to sourcesource
Spinor gravitySpinor gravity
is formulated in terms of is formulated in terms of fermionsfermions
Unified TheoryUnified Theoryof fermions and bosonsof fermions and bosons
Fermions fundamentalFermions fundamental Bosons collective degrees of freedomBosons collective degrees of freedom
Alternative to supersymmetryAlternative to supersymmetry Graviton, photon, gluons, W-,Z-bosons , Higgs Graviton, photon, gluons, W-,Z-bosons , Higgs
scalar : all are collective degrees of freedom scalar : all are collective degrees of freedom ( composite )( composite )
Composite bosons look fundamental at large Composite bosons look fundamental at large distances, distances,
e.g. hydrogen atom, helium nucleus, pionse.g. hydrogen atom, helium nucleus, pions Characteristic scale for compositeness : Planck Characteristic scale for compositeness : Planck
massmass
Massless collective fields Massless collective fields or bound states –or bound states –
familiar if dictated by familiar if dictated by symmetriessymmetries
for chiral QCD :for chiral QCD :
Pions are massless bound Pions are massless bound states of states of
massless quarks !massless quarks ! for strongly interacting electrons :for strongly interacting electrons :
antiferromagnetic spin wavesantiferromagnetic spin waves
Geometrical degrees of Geometrical degrees of freedomfreedom
ΨΨ(x) : spinor field ( Grassmann (x) : spinor field ( Grassmann variable)variable)
vielbein : fermion bilinearvielbein : fermion bilinear
Emergence of geometryEmergence of geometry
vierbeinmetric
/ Δ
Possible ActionPossible Action
contains 2d powers of spinors d derivatives contracted with ε - tensor
SymmetriesSymmetries
General coordinate transformations General coordinate transformations (diffeomorphisms)(diffeomorphisms)
Spinor Spinor ψψ(x) : transforms (x) : transforms as scalaras scalar
Vielbein : transforms Vielbein : transforms as vectoras vector
Action S : invariantAction S : invariantK.Akama, Y.Chikashige, T.Matsuki, H.Terazawa (1978)K.Akama (1978)D.Amati, G.Veneziano (1981)G.Denardo, E.Spallucci (1987)A.Hebecker, C.Wetterich
Lorentz- transformationsLorentz- transformations
Global Lorentz transformations: Global Lorentz transformations: spinor spinor ψψ vielbein transforms as vector vielbein transforms as vector action invariantaction invariant
Local Lorentz transformations:Local Lorentz transformations: vielbein does vielbein does notnot transform as vector transform as vector inhomogeneous piece, missing covariant inhomogeneous piece, missing covariant
derivativederivative
1) Gravity with 1) Gravity with globalglobal and not local Lorentz and not local Lorentz
symmetry ?symmetry ?Compatible with Compatible with
observation !observation ! 2)2) Action with Action with locallocal Lorentz Lorentz symmetry ? symmetry ? Can be Can be constructed !constructed !
Two alternatives :
Spinor gravity with Spinor gravity with local Lorentz symmetrylocal Lorentz symmetry
Spinor degrees of Spinor degrees of freedomfreedom
Grassmann variablesGrassmann variables Spinor indexSpinor index Two flavorsTwo flavors Variables at every space-time pointVariables at every space-time point
Complex Grassmann variablesComplex Grassmann variables
Action with local Lorentz Action with local Lorentz symmetrysymmetry
A : product of all eight spinors , maximal number , totally antisymmetric
D : antisymmetric product of four derivatives ,L is totally symmetricLorentz invariant tensor
Double index
Symmetric four-index Symmetric four-index invariantinvariant
Symmetric invariant bilinears
Lorentz invariant tensors
Symmetric four-index invariant
Two flavors needed in four dimensions for this construction
Weyl spinorsWeyl spinors
= diag ( 1 , 1 , -1 , -1 )
Action in terms of Weyl - Action in terms of Weyl - spinorsspinors
Relation to previous formulation
SO(4,C) - symmetrySO(4,C) - symmetry
Action invariant for arbitrary complex transformation parameters ε !
Real ε : SO (4) - transformations
Signature of timeSignature of time
Difference in signature between space and time :
only from spontaneous symmetry breaking , e.g. byexpectation value of vierbein – bilinear !
Minkowski - actionMinkowski - action
Action describes simultaneously euclidean and Minkowski theory !
SO (1,3) transformations :
Emergence of geometryEmergence of geometry
Euclidean vierbein bilinearMinkowski -vierbein bilinear
GlobalLorentz - transformation
vierbeinmetric
/ Δ
Can action can be Can action can be reformulated in terms of reformulated in terms of
vierbein bilinear ?vierbein bilinear ?
No suitable W exists
How to get gravitational How to get gravitational field equations ?field equations ?
How to determine How to determine geometry of space-time, geometry of space-time,
vierbein and metric ?vierbein and metric ?
Functional integral Functional integral formulation formulation
of gravityof gravity CalculabilityCalculability
( at least in principle)( at least in principle) Quantum gravityQuantum gravity Non-perturbative formulationNon-perturbative formulation
Vierbein and metricVierbein and metric
Generating functional
IfIf regularized functional regularized functional measuremeasure
can be definedcan be defined(consistent with (consistent with
diffeomorphisms)diffeomorphisms)
Non- perturbative Non- perturbative definition of definition of quantum quantum
gravitygravity
Effective actionEffective action
W=ln Z
Gravitational field equation for vierbein
similar for metric
Gravitational field equationGravitational field equationand energy momentum and energy momentum
tensortensor
Special case : effective action depends only on metric
Symmetries dictate general Symmetries dictate general form of effective action and form of effective action and gravitational field equationgravitational field equation
diffeomorphisms !diffeomorphisms !
Effective action for metric : curvature scalar R + additional terms
Lattice spinor gravityLattice spinor gravity
Lattice regularizationLattice regularization
Hypercubic latticeHypercubic lattice Even sublattice Even sublattice Odd sublatticeOdd sublattice
Spinor degrees of freedom on points Spinor degrees of freedom on points of odd sublatticeof odd sublattice
Lattice actionLattice action
Associate cell to each point y of even Associate cell to each point y of even sublattice sublattice
Action: sum over cellsAction: sum over cells
For each cell : twelve spinors For each cell : twelve spinors located at nearest neighbors of y located at nearest neighbors of y ( on odd sublattice )( on odd sublattice )
cellscells
Local SO (4,C ) symmetryLocal SO (4,C ) symmetry
Basic SO(4,C) invariant building blocksBasic SO(4,C) invariant building blocks
Lattice actionLattice action
Lattice symmetriesLattice symmetries
Rotations by Rotations by ππ/2 in all lattice planes /2 in all lattice planes ( invariant )( invariant )
Reflections of all lattice coordinates Reflections of all lattice coordinates ( odd )( odd )
Diagonal reflections e.g zDiagonal reflections e.g z11↔z↔z2 2 ( odd )( odd )
Lattice derivativesLattice derivatives
and cell averagesand cell averages
express spinors in terms of derivatives and express spinors in terms of derivatives and averagesaverages
Bilinears and lattice Bilinears and lattice derivativesderivatives
Action in terms ofAction in terms of lattice derivatives lattice derivatives
Continuum limitContinuum limit
Lattice distance Δ drops out in continuum limit !
Regularized quantum Regularized quantum gravitygravity
For finite number of lattice points : For finite number of lattice points : functional integral should be well definedfunctional integral should be well defined
Lattice action invariant under local Lattice action invariant under local Lorentz-transformationsLorentz-transformations
Continuum limit exists where Continuum limit exists where gravitational interactions remain presentgravitational interactions remain present
Diffeomorphism invariance of continuum Diffeomorphism invariance of continuum limit , and geometrical lattice origin for limit , and geometrical lattice origin for thisthis
Lattice diffeomorphism Lattice diffeomorphism invarianceinvariance
Lattice equivalent of diffeomorphism symmetry Lattice equivalent of diffeomorphism symmetry in continuumin continuum
Action does not depend on positioning of Action does not depend on positioning of lattice points in manifold , once formulated in lattice points in manifold , once formulated in terms of lattice derivatives and average fields terms of lattice derivatives and average fields in cellsin cells
Arbitrary instead of regular latticesArbitrary instead of regular lattices Continuum limit of lattice diffeomorphism Continuum limit of lattice diffeomorphism
invariant action is invariant under general invariant action is invariant under general coordinate transformationscoordinate transformations
Positioning of lattice Positioning of lattice pointspoints
Lattice action and Lattice action and functional measure functional measure of spinor gravity are of spinor gravity are
lattice diffeomorphism lattice diffeomorphism invariant !invariant !
Next tasksNext tasks
Compute effective action for Compute effective action for composite metriccomposite metric
Verify presence of Einstein-Hilbert Verify presence of Einstein-Hilbert term term
( curvature scalar )( curvature scalar )
ConclusionsConclusions
Unified theory based only on fermions Unified theory based only on fermions seems possibleseems possible
Quantum gravity – Quantum gravity –
functional measure can be regulatedfunctional measure can be regulated Does realistic higher dimensional Does realistic higher dimensional
unifiedunified
model exist ?model exist ?
end
Lattice derivativesLattice derivatives
Cell average :
Lattice diffeomorphism Lattice diffeomorphism invarianceinvariance
ContinuumLimit :
Lattice diffeomorphism Lattice diffeomorphism transformationtransformation
Unified theory in higher Unified theory in higher dimensionsdimensions
and energy momentum and energy momentum tensortensor
Only spinors , no additional fields – no genuine Only spinors , no additional fields – no genuine sourcesource
JJμμm m : expectation values different from vielbein : expectation values different from vielbein
and and incoherentincoherent fluctuations fluctuations
Can account for matter or radiation in Can account for matter or radiation in effective four dimensional theory ( including effective four dimensional theory ( including gauge fields as higher dimensional vielbein-gauge fields as higher dimensional vielbein-components)components)
Gauge symmetriesGauge symmetries
Proposed action for lattice spinor gravity Proposed action for lattice spinor gravity has also has also
chiral SU(2) x SU(2) local gauge symmetry chiral SU(2) x SU(2) local gauge symmetry
in continuum limit , in continuum limit ,
acting on flavor indices.acting on flavor indices.
Lattice action : Lattice action :
only global gauge symmetry realizedonly global gauge symmetry realized
Gauge bosons, scalars …
from vielbein components in higher dimensions(Kaluza, Klein)
concentrate first on gravity