Introduction to 2D dilaton gravity - Cosmo-ufes€¦ · Introduction to 2D dilaton gravity...
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Introduction to 2D dilaton gravity
Dmitri Vassilevich
UFABC
Verão Quântico, Ubu, February 2019
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Contents
Why dilaton?General model and particular casesEven more general: Poisson sigma modelsAll classical solutionsAdS2/CFT1
Review: Grumiller, Kummer and D.V, Phys.Rept. (2002).
Support: CNPq, FAPESP
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Why dilaton?
The Einstein-Hilbert action in 2D∫d2x√−gR
has vanishing local variations and thus describes no dynamics.A remedy: include a scalar field X . For example:
S =
∫d2x√−g X (R − 2Λ).
This is the first 2D dilaton gravity - the Jackiw-Teitelboim (JT)model (1984). Classical solutions to this model are locally dS orAdS spacetimes.
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Generalizations
Let us add to the action a kinetic term for X and a potential
[Banks, ..., I. Shapiro, ..., around 1990-1992]:
L =
∫d2x√−g[1
2RX −12U(X )(∇X )2 + V (X )
]Important particular cases:Spherical reduction from D dimensions:
V = −(D − 2)(D − 3)XD−4D−2 U = − D − 3
(D − 2)X.
String gravity (CGHS):
V = −2X U = − 1X.
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First order formulation
The action above is equivalent to
L =
∫[X aDea + Xdω + εV(XaX
a,X )] ,
where ω is the spin-connection, ea is zweibein, a = ±,Dea = dea + aω ∧ ea, ε is the volume 2-form, X a are two auxiliaryfields that generate torsion constraints.
V = 12U(X )X aXa + V (X )
To prove the equivalence: solve the torsion constraints to express ωthrough e. The resulting action will depend on the metric ratherthan on the zweibein.
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Poisson sigma models
These are 2D sigma models with a target space being a Poissonmanifold with local coordinates X I (scalars from the 2D point ofview), gauge fields AI (one-forms from the 2D point of view) and aPoisson tensor P IJ(X ) satisfying the Jacobi identity
P IJ∂JPKL + cycl(IKL) = 0
The action [Schaller & Strobl, 1994]
L =
∫ [dX I ∧ AI + 1
2PIJAJ ∧ AI
]is invariant under the gauge transformations
δX I = P IJλJ
δAI = −dλI − (∂IPJK )λKAJ
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With the choice X I = (X ,X a), AI = (ω, ea) and
Pab = Vεab, PaX = X bε ab
one recovers 2D dilaton gravities in the 1st order formalism. Thegauge parameters λa correspond to diffeomorphisms, while λX
describes local Lorentz rotations.
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All classical solutions
[Bergamin, Grumiller, Kummer, DV, 2005]Consider a 1st order Euclidean dilaton gravity in complex variables∫ [
Y De + Y De + Xdω + εV(2Y Y ,X )]
whereY = (X 1 + iX 2)/
√2, e = (e1 + ie2)/
√2
Though the bar means a complex conjugation, it is convenient toview all fields as independent complex variables.Equations of motion:
dX − i Y e + iY e = 0, DY + ieV = 0
dω + ε ∂V∂X = 0, De + ε∂V∂Y
= 0
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By combining equations on the 1st line, we obtain
d(Y Y ) + VdX = 0
that is integrated to
dC = 0, C = w(X ) + eQ(X )Y Y
with
Q(X ) =
∫ X
U(z)dz , w(X ) =
∫ X
eQ(z)V (z)
This allows to express Y Y through X .The existence and the form of absolutely conserved quantity followsalso from the Poisson sigma model arguments.
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Similarly,d(e/Y ) = dX ∧ (e/Y )U(X ),
which yieldsd(e−Q/Y ) = 0
So thate−Q/Y = df
with some complex zero-form f . That is it,
e = YeQdf , e = idX
Y+ Y eQdf
ω = VeQdf − idY
Y
By imposing the reality condition one obtains a general classicalsolution depending of 3 arbitrary functions (gauge degrees offreedom) and one integration constant C (mass of the solution).
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AdS/CFT
According to the AdS/CFT conjecture gravity theories inasymptotically AdS spaces correspond to conformal theories at theasymptotic boundary. To implement this conjecture in 2D at theclassical level, one has to
define asymptotic conditions asymptotic symmetry algebrasdefine asymptotic degrees of freedomcompute the action for these degrees of freedom
Let us see, how the first step can be done [Grumiller, Salzer, DV,2015].
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Let us take the JT gravity, denote by ρ the radial coordinate, s.t.ρ→∞ corresponds to the asymptotic region of AdS2. Let ϕ be asecond coordinate. We require that at the asymptotics all fieldsbehave as listed below plus subleading terms
X 0 = XReρ − XLe
−ρ eϕ0 = 12eρ − 1
2Me−ρ
X 1 = X 1(ϕ) eρ1 = 1X = XRe
ρ + XLe−ρ ωϕ = −1
2eρ − 1
2Me−ρ
Other fields vanish. XR , XL and M are some functions of ϕ. Thiscomes from partially fixing the gauge, partially solving theequations of motion and by an educated guess. By equations ofmotion, XR,L and X 1 may be expressed through M.
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The asymptotic symmetry algebra is generated by bulk gaugetransformations that
Preserve the form of asymptotic conditionsChange M.
Parameters of these transformations depend on a single arbitraryfunction λ(ϕ)
λ0 = 12λe
ρ − 12(Mλ+ 2λ′′)e−ρ
λ1 = −λ′
λX = −12λe
ρ − 12(Mλ+ 2λ′′)e−ρ
M is transformed as
δM = −M ′λ− 2Mλ′ − 2λ′′′
This is the Virasoro algebra with a non-zero central charge.
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Q & A
Q: How can one make sure that M represents a true asymptoticdegree of freedom?A: It is enough to compute canonical boundary charges that haveto depend on asymptotic degrees of freedom and be finite.
Q: How can one obtain an action for asymptotic degrees offreedom?A: By substituting the solutions in JT action with a suitableboundary term. Depending on details, the resulting model may be ade Alfaro-Fubini-Furlan conformal quantum mechanics or aSchwarzian action [Maldacena et al, 2016].
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Further developments
By imposing looser asymptotic conditions in the JT model onecan get larger asymptotic symmetry algebras, as, e.g., the loopalgebra of sl(2), Virasoro plus an u(1)k , see [Grumiller et al,2017].There are higher spin and supersymmetric extensions.Extensions to generic dilaton gravities: work in progress.
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Dmitri Vassilevich 2D gravity