Before the Big Bang: Wormholes at the Dawn of the Universe ✦
Euclidean wormholes are proposed as a contribution to the initial quantum state of the Universe. Compared to the Hartle-Hawking no-boundary approach, they show conceptual kinship but make a genuine breakthrough: while preserving the importance of Euclidean saddles for encoding the properties of cosmological wave functions, wormholes expand the class of regular saddles relevant for inflationary universes and solve problems that limit the no-boundary model. The main result is an enriched semiclassical landscape of initial conditions, physically rich and consistent with holographic principles. This opens the way to constructing early Universe models in UV-complete theories of quantum gravity.
Context
The problem of initial conditions for the Big Bang remains one of the key puzzles in quantum field theory and gravity. While inflation successfully explains the observed large-scale structure, it doesn't answer what triggered the expansion. The no-boundary proposal by Hartle and Hawking was an elegant step forward, but it encountered phenomenological difficulties. New studies suggest that wormholes — topological tunnels in spacetime — may naturally define the initial state.
Methods
To describe the quantum birth of the Universe, the formalism of the Euclidean path integral was used, where the dominant contribution comes from saddle-point configurations — classical solutions of Einstein's equations in imaginary time. The model included the Standard Model of particle physics, with the Higgs field acting as the inflaton, along with additional sectors: radiation and axion fields. Analytical and numerical solutions with a throat were constructed — the so-called 'wine-glass' wormholes (wormholes with a characteristic scale-factor profile). After the analytic continuation of time τ → it, such geometries transition into an expanding Lorentzian Universe.
Results
It turned out that wormholes are stable and dominant saddle-point contributions for realistic parameter values. The nucleation probability of the Universe in a particular state is determined by the ratio of transition amplitudes, and the contribution of wine-glass wormholes naturally provides the initial conditions for inflation: zero velocity of the scalar field and accelerated expansion with a minimum scale factor. This resolves the fine-tuning problem characteristic of the no-boundary model. The spacetime curvature near the throat is regulated by the balance between gravitational attraction and the repulsive effect of axion fields, keeping the geometry regular and singularity-free.
Implications
This approach links fundamental quantum cosmology with phenomenology: changing the weights of initial states could manifest in the statistics of the cosmic microwave background and the distribution of large-scale structure. This opens a path to testing the hypothesis of multiple topological channels for the Universe's birth and to discriminating between scenarios using observational data.
Future development
The next step is to develop a unified program that incorporates all regular saddle-point contributions — both no-boundary and wormhole types — within a single formalism. Plans include refining the integration contours in the complex plane, analyzing the stability of all solution classes, and developing pipelines to compare predictions with cosmological observations.
Impact
The results impact quantum cosmology, inflation theory, and particle physics, connecting low-energy models to the ultraviolet completion of quantum gravity.
Next steps
Immediate next steps involve computing fluctuations around the saddle points for all topological classes and deriving concrete predictions for the spectrum of the cosmic microwave background.
Key open problems
The wormhole scenario is directly connected to the unification of quantum theory and gravity, as well as to explaining the very mechanism of inflation without resorting to ad hoc initial conditions.
🎯 Euclidean wormholes are named after the shape of a 'wine glass': their geometry in imaginary time has a narrow throat connecting two asymptotically anti-de Sitter regions. Under analytic continuation, the throat becomes the moment of birth of a hot Universe with accelerated expansion.
🎬 The image of a tunnel connecting 'nothing' to our Universe echoes the sci-fi idea of a wormhole as a portal (as in 'Interstellar'), but here it finds a rigorous mathematical embodiment in quantum cosmology.
Key numbers
- Exponent n for radiation: 4
- Exponent n for axion field: 6
- Required number of e-folds of inflation: >60
- Number of new topological classes of saddle points: at least 3