A Changing Doorway Across the Universe

The fabric of the cosmos is stretching, and inside that expansion, the rules for building a doorway across the universe may be changing. For decades, the theoretical "wormhole" has been the white whale of general relativity—a shortcut through space-time that usually requires unstable "exotic matter."

But what if the very expansion of our universe provides the scaffolding to keep these tunnels open?

A New Theoretical Framework

A new framework, utilizing a minimal four-scalar field configuration, suggests phantom fluids or "ghost" particles are not needed for stability. Instead, the background expansion of a Friedmann-Lemaître-Robertson-Walker (FLRW) universe can naturally "dilute" the energy violations usually required to keep a wormhole throat from snapping shut.

This matters because it moves wormholes from the realm of "mathematical impossibilities" into "cosmological probabilities."

The Stability Condition

The study finds that in a non-phantom universe where the Hubble parameter is decreasing, two essential energy conditions can be satisfied:

The Null Energy Condition (NEC)
The Weak Energy Condition (WEC)

The key stability criterion is given by: $-dH/dt > e^{-2N}/r_0^2$ .
If a wormhole scales with the universe under this condition, it could theoretically grow large enough to be traversable.

Implications for Cosmic Fate and Dark Matter

The implications extend beyond travel, influencing our understanding of the universe's end and its invisible structure.

Near the End of Time

The team modeled wormhole behavior near "Big Rip" or "Sudden" singularities.
Their math shows a wormhole could act as a cosmic life raft, connecting a crumbling singular universe to a non-singular one.
This allows for timelike trajectories that pass through the throat before the singularity hits, emerging safely on the other side.

Re-evaluating Dark Matter

By applying the model to Navarro-Frenk-White (NFW) profiles, they confirmed that supporting such wormhole geometries requires non-vanishing pressure.
This challenges the common "cold" dark matter assumption, suggesting the "invisible" glue of our galaxy might be more complex than a simple, pressureless gas.

Current Limitations & Open Questions

The team remains cautious, noting significant theoretical gaps that remain.

Thermodynamics: A consistent thermodynamical description is still missing. Wormholes lack an event horizon, making it impossible to calculate phenomena like Hawking radiation.
Energy Conditions: While the math holds for expanding space, the conditions required to satisfy the Dominant Energy Condition (DEC) are described as "somewhat complicated" and haven't been fully mapped.

For now, the stars remain distant, but the math suggests the universe itself might be helping us build the bridges to reach them.

Reference: Summary based on: "Wormhole spacetimes in an expanding universe: Energy conditions and future singularities" by Taishi Katsuragawa, Shin’ichi Nojiri, and Sergei D. Odintsov (KEK-TH-2774, KEK-Cosmo-0396). Available at [arXiv:2511.03275v2].