VOS Model: Interface Dynamics

• The VOS model is a unified framework that describes macroscopic interface dynamics by tracking a characteristic length and representative velocity with energy loss mechanisms.
• It bridges microscopic field theoretic and kinetic equations with statistical evolution, enabling analysis of both relativistic (cosmological) and nonrelativistic (material science) regimes.
• The model provides actionable insights through scaling laws and diagnostic tools, helping distinguish between frictionless cosmic expansion and friction-dominated systems affecting dark energy scenarios.

A velocity-dependent one-scale (VOS) model is a coarse-grained dynamical framework for describing the macroscopic evolution of complex networks of interfaces, such as domain walls, cosmic strings, or cell boundaries, by tracking both a single characteristic length (or correlation length) and a representative velocity, supplemented by velocity-dependent energy loss or dissipation mechanisms. The VOS formalism connects microscopic field-theoretic (or kinetic) equations to macroscopic statistical evolution, enabling a unified description of interface dynamics in both relativistic and nonrelativistic regimes, as well as in material systems and cosmology.

1. Fundamental Framework and Equations

The velocity-dependent one-scale model emerges from a multicomponent field theory with discrete symmetry. For relativistic interfaces, the starting action is:

$$S = \int d^4x\, \sqrt{-g} \left( X - V(\phi_e) \right) , \quad X = -\frac{1}{2} \partial_\mu \phi_e \partial^\mu \phi_e$$

where $V(\phi_e)$ has (at least) two degenerate minima so domain walls can form. Varying this action in an expanding, homogeneous, and isotropic universe with metric $ds^2 = -dt^2 + R^2(t)\,d\vec{x}\cdot d\vec{x}$ and taking the thin-wall limit leads to a microscopic interface velocity equation:

$$\dot{v} + (1 - v^2) \left( \frac{v}{\ell_d} - \kappa \right) = 0$$

where:

• $v$ is the local microscopic wall/interface velocity,
• $\ell_d$ is the damping length ($\ell_d^{-1} = 3H$ plus frictional damping if present),
• $\kappa$ is the local curvature.

Taking appropriate energy-weighted averages over the network, the macroscopic evolution equations become:

$$\frac{d\bar{v}}{dt} + (1 - \bar{v}^2)\left( \frac{\bar{v}}{\ell_d} - \bar{\kappa} \right) = 0$$

$$\dot{L} = H L + \frac{L}{\ell_d}\bar{v}^2 + \tilde{c}\,\bar{v}$$

where $L$ is the characteristic (macroscopic) length, $\bar{v}$ is the root-mean-square (RMS) velocity, and $\tilde{c}$ is a phenomenological parameter representing energy loss from interactions and collapse, to be calibrated numerically.

In the frictionless limit ($\ell_f \to \infty$) with power-law expansion $R\propto t^{\alpha}$, a scaling (attractor) solution is:

$$L / t = \sqrt{ \frac{k(k + \tilde{c})}{3\alpha(1-\alpha)} }$$

$$\bar{v} = \sqrt{ \frac{1-\alpha}{3\alpha}\cdot\frac{k}{k + \tilde{c}} }$$

where $k = \bar{\kappa} L$ is a dimensionless curvature parameter.

2. Connection with Phase Field and Nonrelativistic Models

Phase field models (PFMs) are used to describe diffuse interface dynamics in material science (e.g., grain growth, soap froth). When order parameter is non-conserved, PFMs predict $L \sim t^{1/2}$ in friction-dominated regimes. In this limit, the microscopic velocity–curvature relation simplifies:

$v \simeq \kappa \ell_f$

where $\ell_f$ is the friction scale. The corresponding attractor becomes:

$$L = \sqrt{2 \tilde{c} k \ell_f t} , \quad \bar{v} \ll 1$$

This recovers the $t^{1/2}$ scaling observed in nonrelativistic systems, showing natural incorporation of PFMs into the VOS formalism.

3. Contrasting Relativistic and Nonrelativistic Dynamics

The VOS model highlights distinct features in interface network scaling depending on the physical regime:

• Relativistic (Cosmological):
  • Expansion: $a(t) \propto t^\alpha$, $\alpha<1$
  • Friction is negligible ($\ell_f \to \infty$).
  • Scaling: $L \propto t$, with persistent, subluminal velocities.
  • Local velocity is no longer simply tied to curvature; the system preserves memory of field-theoretic origins.
• Nonrelativistic (Friction-dominated):
  • No expansion or $H = 0$.
  • Local dynamics: $v \propto \kappa \ell_f$ (direct velocity–curvature coupling).
  • Scaling: $L \propto t^{1/2}$.
  • The “von Neumann law” applies: in 2D, cell area evolves as $da_n/dt \propto (n-6)$.

Despite these dynamic differences, a single simulation snapshot at fixed $L$ does not suffice to discriminate between these regimes due to statistical similarities in network morphology.

4. Topological Diagnostics: Probability Distribution of Domain Edges

A key statistical diagnostic is the PDF $f_n$ for the number of edges $n$ bounding domains in the network:

• In nonrelativistic systems, $f_n$ is peaked at $n=5$ (as in real soap froths); the average is $n=6$ for Y-type junctions.
• For both relativistic and nonrelativistic interface networks, $f_n$ for $n>6$ domains match closely, but for $n\leq 6$, small-domain elimination is faster when $v=\kappa \ell_f$ due to velocity–curvature coupling.
• The PDF $f_n$ thus carries crucial information about the dynamics and constraints of the network, and is central for distinguishing statistical properties across regimes.

A statistical analog of von Neumann's law is verified for relativistic dynamics:

$$\frac{d(a_n)}{dt} = \frac{\pi}{3} \ell_d R^2 (n - 6)$$

which connects growth/shrinkage rate to the number of edges.

5. Implications for Cosmological Domain Walls and Dark Energy

To make cosmic domain walls a viable dark energy source, one would require a frustrated (quasi-static) network characterized by:

• extremely low velocities ($\bar{v} \sim 10^{-5}$)
• highly regular (hexagonal, $n=6$) domain distributions with narrow $f_n$.

However, both relativistic and nonrelativistic simulations demonstrate broad $f_n$—with many unstable small-$n$ domains forcibly eliminated—precluding network frustration. Thus, the statistical network properties directly rule out cosmological domain walls as a significant dark energy candidate. Furthermore, laboratory studies of nonrelativistic interfaces, showing the same lack of frustration, can be used as proxies to eliminate domain wall dark energy scenarios.

6. Unified Picture and Generalization

The velocity-dependent one-scale paradigm thus offers a common theoretical scaffold that:

• incorporates both sharp (relativistic) and diffuse (nonrelativistic, phase field) interface treatments,
• accommodates both frictionless cosmic evolution and friction-dominated material science scenarios,
• predicts and unifies the dynamic scaling of characteristic length and velocities ($L \propto t$ vs $L \propto t^{1/2}$),
• relates local curvature, damping, and network statistics to global macroscopic laws,
• provides a bridge between microscopic (field-theoretic or curvature-based) dynamics and observable, statistical network properties.

This unified approach enables the use of experimental results from material systems to draw robust cosmological conclusions, highlights the essential difference in network freezing/frustration needed for dark energy, and underlines the statistical tools required for distinguishing dynamical regimes.

7. Summary Table: Core Features of VOS Model in Interface Dynamics

Regime	Scaling Law for $L$	Velocity–Curvature Coupling	Key Diagnostic
Relativistic (Cosmo)	$L \propto t$	Nonlocal/microscopic	$f_n$ matches nonrelativistic for $n>6$; von Neumann law holds statistically
Nonrelativistic (Fric)	$L \propto t^{1/2}$	$v \propto \kappa \ell_f$	$f_n$ peaked at $n=5$; rapid elimination for $n \leq 6$; direct von Neumann law

The VOS model’s strength lies in its synthesis of field-theoretic microdynamics, statistical geometry, and macroscopic evolution equations, providing a powerful, predictive, and testable framework for a diverse array of interface-dominated physical systems (1006.3564).