Dark Start: How Vacuum Decay Gave Birth to the Big Bang ✦
Context
The origin of the hot Big Bang remains a mystery. The standard scenario assumes that after inflation, the inflaton field transfers energy to Standard Model particles through 'reheating'. However, the nature of dark matter and dark energy points to hidden sectors that interact with ours only gravitationally. A new hypothesis, developed in this work, ties these ideas together: perhaps ordinary matter is born from the decay of a false vacuum, triggered by the evolution of the dark sector. The concept of the Big Bang, proposed by Georges Lemaître and confirmed by observations of Edwin Hubble, gains a new dimension.
Methods
The researchers modeled the phase transition dynamics within field theory. As soon as the rate of expansion of the Universe drops to a critical value, quantum tunneling triggers bubble nucleation. The bubble walls are accelerated by the energy difference between the vacua, reaching Lorentz factors γ ~ 10¹⁵ — nearly the speed of light. To calculate the gravitational waves, a bulk flow model was used, where the walls are treated as thin shells that freely pass through each other after collisions. It was taken into account that the bubbles are of the size of the Hubble horizon, and their evolution is embedded in the expanding spacetime curvature.
Results
The main result is the shape of the gravitational wave spectrum. Its peak amplitude reaches ΩGW ~ 3×10⁻⁸ for phase transition energy scales from TeV to 10¹¹ GeV. The infrared slope of the spectrum depends on the equation of state of the dark sector: if it behaves like kinetic energy, then ΩGW ∝ k²; for a radiation-like case, it's ∝ k³ log² k. The peak frequency lies in the range of future detectors — from ground-based (Einstein Telescope, Cosmic Explorer) to space-based (LISA, DECIGO). The simulations also showed that the process does not lead to overproduction of primordial black holes (according to the collapse criterion by Stephen Hawking), provided the latent energy fraction α < 0.05.
Implications
The proposed scenario turns gravitational waves into a direct probe of the pre-thermal history of the Universe. By measuring the spectrum, one can recover the equation of state of the dark sector and the scale of the phase transition. Moreover, collisions of the walls can produce heavy particles, such as right-handed neutrinos, whose decay can explain the baryon asymmetry (leptogenesis). This links the origin of the Big Bang to the origin of matter.
Future development
Future observatories, such as next-generation LIGO, Einstein Telescope, and the space interferometer LISA, will be able to test the predictions. Advancing models of the dark sector (e.g., with non-standard symmetries) will refine the phase transition parameters and predict additional signals — from the cosmic microwave background to direct searches for dark matter particles.
Impact
The scenario impacts cosmology, particle physics, and gravitational-wave astronomy. It offers a new solution to the initial conditions problem for the Big Bang and provides motivation to search for the stochastic background in unexplored frequency ranges.
Next steps
Detailed numerical simulations with specific field potentials and thermalization taken into account are needed. It is also important to study how the scenario fits into specific models of inflation and dark sector dynamics.
Key open problems
The work directly addresses the cosmological constant problem (vacuum landscape mechanism), the nature of dark matter, and baryon asymmetry. The key question — why is the entropy of the observable Universe so large — gets a natural explanation through the thermalization of bubble walls.
🎯 The Lorentz factor of the walls in this model reaches 10¹⁵ — meaning that from a stationary observer's perspective, the wall thickness is compressed by a thousand billion times, becoming nearly Planckian (~10⁻³⁵ m). These are perhaps the most extreme macroscopic objects in the history of the Universe.
🎬 The idea of vacuum decay as a cosmic catastrophe is explored in Greg Egan's novel 'Schild's Ladder', where a false vacuum decay threatens to consume the Universe. However, in this scenario, the decay doesn't destroy but rather gives birth to our world — a sort of creative apocalypse.
Key numbers
- peak GW energy density: ~3×10⁻⁸
- Lorentz factor of walls: ~10¹⁵
- phase transition energy range: from 1 TeV to 10¹¹ GeV
- latent energy fraction (α): ≤0.05
- number of e-folds until thermalization: ~5–7