AdS₃ Freelance Holography

• AdS₃ Freelance Holography is a framework that extends conventional AdS₃/CFT₂ duality by allowing the boundary to be defined on any timelike hypersurface with flexible conditions.
• It utilizes the covariant phase space formalism to incorporate ambiguities (W, Y, and Z freedoms) that translate into multitrace deformations, novel RG flows, and modified symmetry algebras.
• The approach yields new insights into induced gravity, soft hair, and bulk solution spaces, offering fresh perspectives on black hole physics and quantum gravity.

AdS$_3$ Freelance Holography is a modern framework that generalizes the gauge/gravity correspondence—most notably, the AdS$_3$/CFT$_2$ duality—by relaxing foundational constraints on both the location of the “boundary” and the nature of boundary conditions in the bulk. Instead of confining the boundary theory to the conformal boundary of anti–de Sitter space with Dirichlet (fixed) boundary values, freelance holography allows one to define the boundary theory on any timelike codimension-one hypersurface in AdS$_3$, with completely general (including non-Dirichlet) boundary conditions for the bulk fields. This construction systematically relates changes in the bulk variational principle (via the addition of boundary and corner terms in the action) to deformations of the dual field theory, thereby substantially enlarging the landscape of admissible bulk/boundary pairs. The resulting holographic dictionary encodes new classes of RG flows, emergent induced gravity, and expanded surface charge algebras, and has significant implications for black hole physics, symmetry algebras, and multitrace deformations.

1. Generalization of Gauge/Gravity Duality

Traditional AdS$_3$/CFT$_2$ holography fixes the CFT on the conformal boundary (as $r\to\infty$ in Fefferman–Graham coordinates), imposing Dirichlet boundary conditions for bulk fields. Freelance holography sets both the location of the holographic dual field theory and its couplings “free”:

• The boundary can be any timelike codimension-one surface $\Sigma_r$ at finite $r$.
• The boundary conditions can be arbitrary—modern implementations utilize a parametrized set of renormalized boundary conditions that extend beyond Dirichlet, including Neumann, conformal, and more exotic, possibly non-covariant, variants (Parvizi et al., 12 Mar 2025, Taghiloo, 24 Sep 2025, Sheikh-Jabbari et al., 12 Oct 2025).

These generalizations are realized by modifying the bulk action through the addition of total-derivative (boundary) terms:

$$S^{(W)}_{\text{bulk}}[J; \mathcal{M}] = S^{(D)}_{\text{bulk}}[J; \mathcal{M}] + \int_{\partial\mathcal{M}} \partial_\mu W^\mu$$

and induce, in the dual theory, deformations of the action via

$$S_{\text{bdry}}^{(W)}[\mathcal{J};\Sigma] = S_{\text{bdry}}^{(D)}[\mathcal{J};\Sigma] + W[\Sigma].$$

Arbitrariness in $\Sigma$ and $W$ corresponds to arbitrary slicing and boundary conditions, respectively.

2. Covariant Phase Space Formalism and Boundary Condition Freedoms

The formal apparatus underpinning freelance holography is the Covariant Phase Space Formalism (CPSF). This formalism systematically identifies ambiguities—termed $W$, $Y$, and $Z$ freedoms—that reflect the choice of boundary and corner terms in the bulk Lagrangian (Parvizi et al., 12 Mar 2025):

• $W$-freedom: Governs the addition of boundary terms, translating, in holography, to multi-trace deformations or source redefinitions in the dual boundary theory.
• $Y$-freedom: Ambiguities in the symplectic potential; although not affecting bulk symplectic form, these terms become surface contributions on the boundary, encoding information about “edge modes” or intrinsic boundary dynamics.
• $Z$-freedom: Determines the “corner” (codimension-2) Lagrangian, critically influencing the choice of slicing and phase space variables for the dual theory.

The on-shell variation of the bulk action,

$$\delta S_{\text{bulk}}\approx\int_\Sigma \Theta_{\text{bulk}}, \qquad \Theta_{\text{bulk}} = \Theta_D + \delta W + dY,$$

shows how these freedoms specify the symplectic structure and the variational principle, and hence, the dual field theory’s deformation structure and dynamical content (Parvizi et al., 12 Mar 2025, Taghiloo, 24 Sep 2025).

3. Explicit Classification and Solution of Bulk Geometries

AdS$_3$ Einstein gravity is integrable, enabling an explicit parametric solution for the bulk metric even with arbitrary (including non-covariant) boundary conditions (Sheikh-Jabbari et al., 12 Oct 2025). In adapted Fefferman–Graham gauge,

$$h_{ab}(x, r) = \left( \frac{r}{\ell} \right)^2 Q_{ac} q^{cd} Q_{db}, \qquad Q_{ab} = q_{ab} - \frac{6\pi \ell^4}{c r^2} \widetilde{T}_{ab},$$

where $q_{ab}$ is the (arbitrary) boundary metric and $\widetilde{T}_{ab}$ is the (modified) boundary energy–momentum tensor. The solution depends on two arbitrary functions of one variable after constraint equations—arising from the Hamiltonian and momentum constraints—are imposed:

$$\delta \int_\Sigma \sqrt{-q} \left(T + \frac{c}{24\pi} R \right) = 0.$$

This freedom results directly from the arbitrary choice of boundary condition and location, with Dirichlet conditions reproducing the Bañados geometries, and Neumann or general choices yielding solutions with enhanced “soft-hair” content and distinct phase space structures (Sheikh-Jabbari et al., 12 Oct 2025).

4. Holographic RG Flow and Multitrace Deformations

Moving the boundary hypersurface $\Sigma_{r_c}$ inwards (i.e., decreasing $r_c$) induces an RG flow in the dual field theory. The flow of the boundary effective action $S_\text{bdry}[\Sigma_{r}]$ satisfies a differential equation linked to the chosen boundary term $W$ (Parvizi et al., 12 Mar 2025, Taghiloo, 24 Sep 2025):

$$r \frac{d}{dr} S^{(D)}_{\text{bdry}}[\Sigma_r] = -\ell \int_{\Sigma_r} \sqrt{-h}\, \mathcal{O}^2,$$

where $\mathcal{O}^2$ is tied to the Brown–York tensor, often taking the explicit form

$$\mathcal{O}^2 \equiv T^{ab} T_{ab} - \frac{1}{d-1} T^2.$$

In gravity, the Hamiltonian constraint relates this flow to induced gravity dynamics on the boundary:

$$r \frac{d}{dr} S_{\text{bdry}}^{(D)}[\Sigma(r)] = \ell \int_{\Sigma(r)} \sqrt{-h}(R + \Lambda).$$

A key result is that every choice of $W$ and boundary slicing corresponds to a different RG trajectory, interpreted as a deformation (a multitrace operator, $T\bar{T}$-type, or more general) of the dual field theory (Taghiloo, 24 Sep 2025, Sheikh-Jabbari et al., 12 Oct 2025).

5. Surface Charges, Symmetry Algebras, and Soft Hair

Relaxed boundary conditions alter the phase space and allowed asymptotic symmetries. The surface charges, computed via the covariant phase space (Lee–Wald) prescription,

$$\delta Q_{\xi} = \int d\phi ( T\,\delta L + Y\,\delta J ),$$

depend sensitively on the chosen boundary condition and slicing. For Dirichlet, the familiar result is two Virasoro algebras with Brown–Henneaux central charge $c = (12\pi\ell)/\kappa$. For Neumann, conformal, or non-covariant choices, the algebra can be a semi-direct sum including an abelian $u(1)$ current, or even more intricate mixtures. The general symmetry bracket takes the form

$$\{ Q_\xi, Q_\zeta \} = Q_{[\xi, \zeta]_\text{adj}} + K(\xi, \zeta),$$

with the central extension $K$ determined by the corner ambiguities and the corresponding boundary condition. The existence of extra “soft” degrees of freedom—modeled as extra free functions in the space of solutions—manifests as additional symmetries (“soft hair”) (Sheikh-Jabbari et al., 12 Oct 2025).

6. Induced Gravity and Boundary Condition Flow

Allowing arbitrary boundary conditions not only maps to multitrace deformations but also generates induced gravity dynamically on the cutoff boundary. The radial flow equations, after applying the Hamiltonian constraint, become

$$r \frac{d}{dr} S_{\text{bdry}}^{(D)}[\Sigma(r)] = \ell \int_{\Sigma(r)} \sqrt{-h}(R + \Lambda),$$

with $R$ the Ricci scalar on $\Sigma(r)$ and $\Lambda$ an effective cosmological constant (Taghiloo, 24 Sep 2025). This result means that starting from a non-gravitational (field) theory, holographic renormalization dynamically “induces” gravity as part of the RG evolution. This mechanism is critical for understanding the emergence of boundary gravity, dynamical black hole horizons, and entropy-related phenomena in cutoff holography.

7. Impact and Applications

AdS$_3$ Freelance Holography enables a unified, flexible implementation of gauge/gravity duality:

• Facilitates the paper of dual field theories not only at conformal infinity but also at fixed radial cutoffs, relevant for effective field theories with UV modifications or near-horizon physics.
• Connects the choice and evolution of boundary conditions to multitrace and other RG deformation flows in the dual QFT, thus broadening the reach of the AdS/CFT paradigm (Taghiloo, 24 Sep 2025, Sheikh-Jabbari et al., 12 Oct 2025).
• Promotes a more complete understanding of the phase space of gravity in AdS$_3$, the role of soft symmetry extensions, and the microstate counting necessary for quantum black hole entropy.
• Suggests that the “dictionary” between bulk and boundary is not unique, but can be “dialed” via changes in the variational principle (boundary terms), with significant consequences for bulk solution spaces and dual field theories.
• Provides a route for interpolating between conventional AdS holography and flat/BMS holography, and offers impetus for future investigations into non-AdS holographic correspondences.

This comprehensive viewpoint is exemplified by the explicit reconstruction of bulk solutions in terms of arbitrary boundary data, first-order holographic flow equations for cutoff duals, and the analysis of central extensions in varied surface charge algebras. Freelance holography therefore sets the stage for a systematic exploration of non-standard boundary conditions, induced actions, and RG flows in AdS$_3$ gravity and its quantum extensions.