Could Thin-Shell Wormholes Hide Within the Universe’s Emptiest Cosmic Voids?

Wormholes Carved from Cosmic Voids Could Stay Open, New Study Finds

Traversable wormholes—shortcuts threading space-time like a tunnel through a mountain—have haunted theoretical physics for nearly a century, and every serious attempt to build one has collided with the same wall: the throat appears to require exotic matter no laboratory has ever produced. Now a team of theorists has proposed an unexpected place to look for such objects and an unexpected recipe for keeping them open. In a study published in The European Physical Journal C, Jonathan Rebouças, Edson Otoniel and Francisco S. N. Lobo construct wormholes whose throats are carved from black holes embedded inside cosmic voids—the vast underdense basins that dominate the architecture of the cosmic web—and then pose the question that separates wormhole physics from wormhole fantasy: is the object stable? Their answer is nuanced but striking. When the exotic matter smeared over the throat is modeled as a modified cosmic Chaplygin gas, the configuration can hold itself together, provided the gas carries a sufficiently large linear pressure term.

The conceptual foundations were laid in 1935, when Albert Einstein and Nathan Rosen found that the Schwarzschild solution, sliced in a particular way, contains a bridge joining distant regions of space-time—a bridge that pinches shut too quickly for anything to cross. The modern era opened in 1988, when Michael Morris and Kip Thorne specified what a wormhole a traveler could actually survive would require. Their analysis produced the field’s defining embarrassment: keeping a throat open violates the classical energy conditions, the inequalities that normally forbid, among other things, a locally negative energy density. The culprit is geometric. The flare-out condition—the requirement that the funnel open back up rather than close—forces matter at the throat into behavior no known substance exhibits. In the decades since, theorists have tried to shrink, localize or disguise this exotic ingredient, embedding wormholes in phantom energy, Casimir vacuum, dark matter halos, loop-quantum-gravity corrections and the effective geometries of modified gravity.

Thin-shell wormholes are the most economical realization of that program. Instead of spreading strange matter through the bulk of space, the cut-and-paste construction takes two identical copies of a well-behaved seed geometry and glues them along a spherical surface—the shell—which becomes the wormhole’s throat. All the exotic material is then concentrated on that two-dimensional interface, like surface tension on a soap bubble. The price of gluing is computed with the Darmois–Israel junction conditions, the relativistic bookkeeping that converts the jump in extrinsic curvature across the shell into a surface stress tensor. From those conditions the authors read off the two numbers characterizing the throat’s supporting fluid: a surface energy density and a tangential pressure. Whether the object survives its own perturbations is decided by the linearized stability analysis introduced for Schwarzschild shells by Eric Poisson and Matt Visser and refined since for charged, cosmological, rotating, higher-dimensional and quantum-corrected backgrounds. Squeeze the throat slightly, and the question becomes brutally simple: does it spring back, or does it run away?

The novelty of the new work is the environment. Cosmic voids are the emptiest regions of the Universe: expanses where the matter density plunges far below the cosmic average, wrapped in walls and filaments of galaxies and together occupying a substantial fraction of the volume of space. Because they are weak-field, weakly screened environments, voids amplify subtle gravitational signatures that are difficult to isolate in dense clusters, which has made them a favorite hunting ground for dark energy and modified-gravity effects. The team anchors its geometry in the universal density profile of Hamaus, Sutter and Wandelt, a phenomenological formula that captures both the underdense core of a typical void and the compensating overdense wall around it. The profile is set by a mean background density, a negative density contrast, a scale radius, a void radius and two shape parameters controlling the inner and outer slopes. Crucially, the void’s contribution imprints a de Sitter-like character on the gravitational field, so the environment behaves on large scales like the exponentially expanding space associated with a positive cosmological constant.

Drop a black hole into that profile and something qualitatively new appears. The lapse function—the quantity that governs how clocks and radial distances are warped—acquires two roots instead of one. The inner root is an ordinary black-hole horizon; the outer root is a cosmological-like horizon generated not by a true cosmological constant but by the de Sitter-like character of the void itself, in close analogy with the Schwarzschild–de Sitter solution. Between the two horizons lies a finite region where the lapse function is positive, and it is precisely there that the authors perform their surgery. Following the cut-and-paste recipe, they take two copies of this black-hole-in-void spacetime, excise everything beyond a chosen radius and sew the remaining pieces throat to throat. The result carries no exotic matter in the bulk at all: whatever strange substance holds it open lives entirely on the shell, and the shell is forbidden from approaching either horizon. The throat must sit strictly inside the window between the black-hole horizon and the void’s cosmological-like boundary.

Because the seed geometry is not isolated—its mass function carries the void’s density profile inside it—every quantity on the shell inherits that cosmic fingerprint. The surface energy density and tangential pressure, and through them the null, weak, dominant and strong energy-condition combinations, can be written explicitly in terms of the void mass function and density profile. The degree of exoticity demanded at the throat is therefore not a free parameter; it is dictated by how empty, how large and how steeply walled the surrounding void happens to be. That ties together three ingredients usually studied in isolation: the statistical structure of the cosmic web, the horizon structure of compact objects and the classical stability theory of wormholes. It also sharpens a conceptual distinction. Unlike a continuous Morris–Thorne wormhole supported by a fluid filling space, this object’s exotic matter is confined to a junction surface whose admissible radius is boxed in on both sides by horizons born of the environment.

The authors also take the thermodynamics of the shell seriously. A static shell hovering at a fixed radius possesses an associated temperature—an Unruh-type temperature felt by observers stationed at the throat—and the paper derives a first law for the configuration. The striking part is what the first law connects. The shell’s entropy is tied directly to the entropies of the two horizons that bracket it: the black-hole horizon on the inside and the cosmological-like horizon on the outside. The throat’s thermodynamic ledger is therefore not self-contained; it knows about the large-scale void through the outer horizon. This dovetails with a recently developed unified thermodynamic framework for thin-shell wormholes, in which a generalized first and second law relate the shell’s temperature to Hawking-like particle creation. In the void setting, the framework gains an environmental dial: alter the void’s density contrast or size, and the thermodynamic budget of the throat shifts with it.

Stability is where the study earns its keep. The radial motion of the throat is recast as a particle rolling in a one-dimensional effective potential; a static shell is an equilibrium point of that potential, and the sign of its second derivative there decides everything. A positive sign means a small squeeze or stretch is resisted; a negative sign means the perturbation runs away toward collapse or explosive expansion. To close the dynamical system, the fluid on the shell needs an equation of state, and the authors test two cousins of the Chaplygin gas, a fluid long used by cosmologists as a tractable stand-in for exotic behavior. In both the generalized cosmic Chaplygin gas and the modified cosmic Chaplygin gas, the integration constant B is not free; it is fixed by the static junction condition itself. The stability verdict is therefore handed to the void geometry and to the remaining equation-of-state parameters—the exponents γ and ω and, in the modified model, the linear coefficient A multiplying the surface energy density.

The numerical verdicts split cleanly. Scanning configurations with a void density contrast of −0.95, black-hole masses from 1 to 10 in geometrized units, γ values from 0.1 to 0.999 and ω values from −0.1 down to −1.5, the authors find that throats supported by the generalized cosmic Chaplygin gas are unstable across the entire sampled parameter range: the effective potential always curves the wrong way. The modified version tells a different story. Because it carries an explicit linear term A in its pressure, the fluid can stiffen in exactly the way the throat needs; for sufficiently large A, the second derivative of the effective potential turns positive and the static configuration becomes a genuine local minimum. Stability, in other words, is not a marginal accident here. It emerges from a competition between the void’s de Sitter-like environment, which fixes the available window of throat radii between the two horizons, and the equation of state of the exotic surface fluid, which decides whether that window contains a valley or a hilltop.

None of this means astronomers should begin scanning voids for tunnels. The construction is exact but mathematical, a solution of Einstein’s equations in a phenomenological void background, and the exotic matter on the shell remains hypothetical. What the paper delivers is a controlled arena for a question that is maturing quickly in gravitational physics: how does the large-scale environment rewrite the behavior of compact objects? Compact bodies are habitually modeled as isolated, yet the Universe is structured on scales far larger than galaxies, and this analysis shows that a void’s underdensity does not merely decorate the metric. It creates an extra horizon, constrains where a throat may live, fixes the surface stresses and co-signs the stability verdict. The framework offers a starting point for cataloguing stable and unstable wormhole configurations between the black-hole and cosmological-like horizons of void spacetimes, and the authors point toward extensions involving rotation, higher-curvature gravity and observational signatures such as gravitational lensing. If wormholes exist, they may prefer the emptiest neighborhoods of the cosmos—and there is now a formalism for saying which ones would stay open.