6.1. What Is Known About the Depth
The idea that spacetime possesses a radial depth coordinate—a direction along which energy scale, clock rate, and curvature all vary monotonically—did not originate with the present work. It is implicit in the AdS/CFT correspondence, explicit in observational cosmology, and quantified by recent work on holographic complexity. The question is not whether such a coordinate exists, but what drives its evolution.
By “horizon” we mean a covariantly defined causal screen—an apparent horizon, holographic screen leaf, or analogous surface—that anchors a depth gradient for an interior observer. The term is used in this encompassing sense throughout this section.
In the AdS/CFT correspondence [
11], the radial coordinate
U of the anti-de Sitter bulk parameterizes holographic depth: energy scale in the boundary conformal field theory maps to radial position in the bulk geometry. The logarithmic depth variable
plays exactly the role of the logarithmic lapse
developed in
Section 5.1—a monotonic coordinate along which the effective energy scale, clock rate, and curvature all vary together. Maldacena thus establishes the structural fact that
a depth gradient exists, and that it is encoded by the lapse and its logarithmic derivatives.
But Maldacena’s throat is static. The geometry is a fixed background; the depth gradient does not evolve. In the real universe, bound structures sit at progressively greater depth relative to expanding voids, and the separation grows with cosmic epoch.
Wiltshire’s timescape cosmology [
14,
15] addresses precisely this dynamical aspect. Working entirely within classical GR, Wiltshire demonstrated that a two-scale partition of the universe into voids and walls, evolved via Buchert averaging [
13], produces a cumulative lapse difference sufficient to account for the apparent acceleration without a cosmological constant. That program has 18 years of observational development: fits to type Ia supernovae [
16], accommodation of BAO and CMB acoustic scales [
17], Sandage–Loeb drift predictions, and recent support from numerical-relativity void statistics [
18]. Timescape reproduces a wide range of observational datasets: the depth evolves, and the evolution mimics dark energy. Its observational program is quantitatively successful and extensive; what it lacks is not phenomenological precision but theoretical depth.
Specifically, timescape lacks a microphysical theory of mass—an explanation of why walls have positive intrinsic curvature while voids have negative curvature—and a derivation of the lapse difference from the Hamiltonian constraint rather than from a phenomenological partition. Implicitly, timescape treats virialized structures as static endpoints: once a wall equilibrates, its contribution to the lapse budget is fixed. But if matter is wound geometry, and the winding scale is conformally tied to the depth of the worldtube it inhabits, then equilibration of a structure does not mean the depth has finished evolving—it means the geometry has reached a local configuration from which further processing continues.
This is precisely what Susskind’s holographic complexity program [
24,
25] demonstrates in the black hole context. After a black hole thermalizes—after entanglement entropy has saturated and the system appears to have reached equilibrium—a geometric quantity continues to grow: the volume of the Einstein–Rosen bridge, identified with the computational complexity of the boundary quantum state. This growth is linear and persists for an exponentially long time. The geometry keeps taking more complex configurations, deepening relative to the exterior even though no classical observable registers a change at the horizon. The lesson extends beyond black holes: thermalization does not exhaust the geometry’s capacity to deepen. Local equilibria are way-stations, not endpoints; the total circuit depth of the system continues to increase as the internal state explores progressively more complex configurations of the same wound geometry.
None of these programs connects the depth to what matter is. If matter is wound spacetime (
Section 2 and
Section 3), and if the Compton–Schwarzschild reciprocity (
Section 5.2) conformally locks every inner confinement scale to an outer saturation scale at the Planck area, then neither horizon is truly static except in its own frame. From the interior of a gravitational well—riding the inward flow of the river model—the redshift of the cosmic horizon increases monotonically, the proper area of the causal boundary grows, and the luminosity distance to standard candles steepens beyond what constant-rate expansion would predict. The observable universe appears to accelerate. But this is the view from a frame whose clocks are progressively slowing and whose rulers are progressively contracting relative to the void. From the opposite vantage—an observer in the expanding void, holding the void scale fixed—those same galaxies are not flying apart. They are each shrinking into their own gravitational wells, compressing toward their respective depth axes, while the void between them passively dilates.
Neither description is preferred; both are readings of the same , differing only in which end of the depth gradient is called “at rest.” What we interpret as cosmic acceleration is the temporal drift of the logarithmic lapse, seen from the deep end. Maldacena’s static throat becomes dynamic once the interior observer’s measurement standards are recognized as participating in the depth they measure. Wiltshire’s differential clocks acquire a microphysical origin once the lapse difference is traced to conformal compression of wound geometry rather than to a phenomenological void/wall partition. Susskind’s monotonic complexity growth is structurally analogous to the macroscopic signature of irreversible virialization accumulating across cosmic time.
6.2. The Covariant Depth Functional
The contributions reviewed above all describe depth in foliation-dependent language: a radial coordinate, an expansion scalar, a lapse ratio. The question is whether there exists a covariant depth functional—defined directly from the 4-geometry, with no reference to a 3+1 slicing—that (i) is provably monotone under the Einstein equations plus standard energy conditions, and (ii) reduces to the lapse-drift variable when projected onto a cosmological foliation.
Such a functional exists. It is constructed from the holographic screen of a timelike observer, following Bousso and Engelhardt [
26,
27].
Let
be a future-directed timelike worldline representing the interior observer. At each proper time
, form the past light cone
. On that null hypersurface, consider compact spacelike 2-surfaces
(cuts of the cone), each with area
Define the
preferred leaf as the maximal-area cut of the past light cone:
The union
is the
past holographic screen of the observer
. This definition is purely 4-dimensional: it requires only causal structure and area. No foliation, no domain partition, no expansion scalar appears in the construction.
Proposition 1
(Bousso–Engelhardt area law).
Let be a past holographic screen foliated by leaves constructed as maximal-area cuts of . If the null curvature condition
holds (equivalent, via the Einstein equations, to the null energy condition on matter), and a genericity condition holds (the null congruence is not everywhere shear-free), then the leaf area is strictly increasing along the preferred flow on [26,27].
For past screens in expanding cosmologies, the preferred flow is future-directed: leaf area increases as the observer’s proper time advances. The preferred flow parameter is intrinsic to
, not an arbitrary 3+1 slicing. This is a theorem about general relativity, not a model-dependent claim. It holds for any spacetime satisfying the stated conditions, regardless of its matter content, topology, or symmetry. The proof relies on the same Raychaudhuri focusing that the present paper invokes in the 3+1 setting (
Section 6.4), but the screen construction packages the result covariantly: no foliation is needed, and the monotonicity is global.
Caveat. The Bousso–Engelhardt theorem is a result of classical general relativity. Semiclassical violations of the null energy condition (e.g. Hawking evaporation) can break area monotonicity; the present paper operates in the classical regime where such corrections are negligible at cosmological scales.
Since the leaf area is monotonically increasing, its logarithm defines a natural depth parameter. Define
where
is the Planck length. The choice of reference area
is a normalization convention: since only derivatives and ratios of
enter the observables (
Section 5.1), shifting
adds only a constant. The physically meaningful content is in the screen area
itself. By Proposition 1,
.
This is the covariant replacement for the foliation-dependent circuit depth that appears heuristically in the structure-formation literature. The IR end is now set by the screen area rather than the Hubble radius; the normalization is fixed at the Planck scale rather than read off a spatial slice.
The curvature 2-form
is the field strength (infinitesimal holonomy density) of the Lorentz connection. Null focusing is governed by the Ricci contraction
, obtained by contraction of the corresponding Riemann tensor. Via the Einstein equations,
Wound holonomy contributes to
and hence to null focusing, activating the area-law growth
. (No additional torsion degrees of freedom beyond the Levi-Civita connection are introduced; “winding” refers throughout to nontrivial holonomy of the standard metric-compatible connection within classical GR.)
The logical structure should be stated precisely. The Bousso–Engelhardt theorem guarantees screen-area monotonicity for
any spacetime satisfying the null energy condition; nontrivial Lorentz holonomy is not the mathematical cause of the theorem. What the holonomic framework provides is a concrete microphysical
mechanism for why focusing is present and persistent: matter is wound holonomy (
Section 2 and
Section 3), wound holonomy concentrates curvature, and concentrated curvature drives null focusing. The theorem provides the monotonicity; the holonomic framework provides the physical content that makes the monotonicity cosmologically active rather than kinematic.
6.3. Projection to the Cosmological Setting
The covariant functional is defined for a single observer worldline. To recover the cosmological lapse ratio , we evaluate it on two physically distinguished congruences: one threading void regions (), one threading wall regions (). We set in this subsection unless otherwise indicated.
The physically meaningful quantity is not the growth of either congruence’s screen area individually but the
difference between the two depth functionals:
The Planck-scale reference area in Equation (6.4) cancels in this difference; the relational depth depends only on the screen-area ratio. Any common rescaling of both areas—such as the passive growth of the Hubble radius in a uniformly expanding background—cancels identically and does not contribute to
. Depth separation arises only when the two congruences’ screen areas evolve at different rates, which happens whenever structure formation creates regions whose local geometry has deepened relative to the surrounding void.
In a spatially flat Friedmann–Lemaître–Robertson–Walker spacetime, the extremal-area cut of the past light cone coincides with the apparent horizon at areal radius
with screen area
.
1 The screen radius is the Hubble radius: the heuristic
of the foliation-dependent discussion is recovered as a special case of the covariant construction.
An observer comoving with virialized structure (wall congruence
) measures proper time
; the effective Hubble rate in that frame is
, yielding a screen radius
An observer in the expanding void (
) measures
, with screen radius
The lapse ratio is therefore a ratio of screen radii—equivalently, a ratio of square roots of screen areas:
The ordering
(hence
) follows from the deeper clock-rate environment of the wall congruence once virialization occurs. Equivalently,
, so the lapse ratio is an exponential of the relational depth (6.6). The lapse-drift variable used throughout this paper is the projection of the covariant depth functional onto two different observer congruences. The ADM lapse is a convenient 3+1 bookkeeping device; the underlying quantity is the screen-area ratio, which requires no foliation to define. The identification
is exact in FLRW and perturbatively stable under scalar inhomogeneities on the screen scale (see below); operationally, the Buchert averaging procedure (
Appendix A) extends the extraction to inhomogeneous spacetimes. Establishing the full nonlinear equivalence between the covariant screen construction and the Buchert coarse-graining remains open (
Section 7.3).
Since
with
, for any common matching parameter
labeling the comparison,
The lapse drift is controlled by the
relative logarithmic area-growth of the two screens, independent of any absolute “growth” attribution to either end. This is the covariant content behind the 3+1 identity that follows.
We choose a background time parameter
t adapted to the void congruence (so
in the simplest gauge), and define
. In the 3+1 description, the expansion scalar is
, and the differential law
is the rate at which the two congruences’ parametrizations of
diverge. Virialization locks the sign of
, so the lapse ratio
decreases monotonically once structure formation becomes significant; the screen-area theorem guarantees that the underlying depth variable
itself is monotone.
This differential law is precisely the kinematic identity on which Wiltshire’s timescape program is built [
14,
15]. The covariant depth functional provides the 4-dimensional object from which this 3+1 identity is projected; timescape operates at the projection level, while the present paper provides the object being projected.
The physically meaningful quantity is not the growth of either screen area individually but the ratio between the two congruences. In any expanding universe, both screen areas grow as the Hubble radius increases; that common growth cancels in the ratio and does not contribute to C. Depth separation arises when the two congruences’ screen areas evolve at different rates—which happens whenever structure formation creates regions whose local geometry has deepened relative to the surrounding void.
Before the first structures collapsed, both congruences shared the same screen area; . Each virialization event creates a new region of confined geometry—a wound structure whose Compton wavelength is shorter than any scale present in the homogeneous background—and shifts the wall congruence’s screen area relative to the void congruence’s. The cumulative effect is a monotonic decrease of as the population of virialized structures grows at every epoch. Local equilibration can stabilize certain 3+1 diagnostics (e.g. ) in a chosen slicing, but does not stabilize the screen-area ratio unless the causal surfaces defining for both congruences become stationary—a condition that is not met in an expanding universe with ongoing structure formation.
The identification was demonstrated above in exact FLRW. A realistic cosmology is not exactly homogeneous; the question is whether the identification survives under perturbations.
Write the scalar-perturbed FLRW metric in longitudinal gauge:
with
. The areal radius of a 2-sphere at coordinate radius
r is
. The apparent horizon—the maximal-area cut of the past light cone—satisfies
. Expanding to first order in the perturbation amplitudes gives
where
is evaluated on the scale of the screen leaf. The screen area is therefore
Evaluating on the two congruences and taking the ratio:
so that
On the scale of the maximal past-light-cone leaf—which by construction samples the Hubble-scale geometry, not local cluster-scale curvature—large-scale scalar perturbations satisfy
; observationally,
for the relevant modes. Because the screen leaf averages over Hubble-scale volumes, the perturbation amplitude entering Equation (6.17) is controlled by large-scale density contrast rather than nonlinear small-scale structure. The screen-area-ratio identification of
C is therefore perturbatively stable under realistic inhomogeneity, not a symmetry artifact of the exact FLRW limit.
6.4. The 3+1 Realization
The covariant result—monotonic depth growth projecting as lapse divergence—can be made quantitative in the standard ADM formalism; this subsection provides the calculation.
Given two congruences of comoving observers—one in voids (v), one in walls (w)—the relative lapse
evolves as (
Appendix A):
where
and
are the volume-averaged expansion scalars in void and wall domains respectively. Within a chosen foliation and Buchert-style domain partition, this kinematic identity relates the evolution of
to differential expansion.
The expansion scalar in each domain obeys the Raychaudhuri equation:
For voids, shear is negligible and density is low; the void expansion scalar approaches
. For walls, virialization halts expansion:
.
But the Hamiltonian constraint does not permit the expansion budget to simply vanish:
As
, the intrinsic curvature
(3)Rw must absorb what was formerly expansion budget. This is the critical distinction from a purely kinematic picture: walls are not passively non-expanding—they have actively converted expansion into spatial curvature. The Hamiltonian constraint ensures this conversion is exact.
A potential objection: the Hamiltonian constraint is an initial-value constraint, not an evolution equation. An individual virialized structure, once equilibrated, satisfies the constraint with and a stable (3)Rw; local 3+1 diagnostics stabilize in a chosen slicing. But the relational depth—the screen-area ratio —continues to evolve whenever (as emphasized in the relational depth definition (6.6), only the ratio is invariant). Equilibration stabilizes a structure’s local geometry without stabilizing its relational depth. At every epoch, new regions collapse and virialize, each converting its expansion budget into intrinsic curvature in a single, constraint-enforced event; the cumulative effect is a monotonic growth of the depth separation.
The differential expansion is
to leading order, once walls are effectively virialized and voids dominate the expansion. Since the cumulative drift is sourced by the growing population of virialized regions, the effective rate scales with the virialized volume fraction
up to an
efficiency factor. Substituting into the kinematic identity (6.18):
where
is the volume fraction of virialized structure and
absorbs the volume-weighted deviation of the actual
from
—a dimensionless factor expected to be
. The magnitude required by timescape phenomenology is of order
[
14]; the depth evolution equation naturally produces a cumulative drift of this order for realistic virialization histories, with the precise coefficient to be extracted from simulations (
Appendix A). The magnitude arises from the
persistence of confined structure over cosmic time, not from the depth of gravitational potentials at any single epoch.
Perturbative estimates of the Buchert backreaction scalar give
[
19], constraining
instantaneous expansion-scalar variance. This bound is not in tension with a cumulative lapse separation of order
. The distinction is between variance and drift:
measures the dispersion of expansion rates at a single epoch, while
measures the accumulated proper-time offset produced by a persistent sign-definite separation. The present argument operates in the regime where domains have gone fully nonlinear and the sign of
is locked by virialization.