Pre-Big Bang (Gasperini-Veneziano)
String theory predicts a dilaton-driven contracting phase before the Bang.
Placeholder for a 3D visualisation of Cyclic & Bouncing Cosmologies. The interactive scene will land in Phase 3. There is no absolute beginning. Our universe is the latest phase in an eternal cycle. Variants disagree on the mechanism: a conformal transition (Penrose), a quantum bounce (LQC), a brane collision (Ekpyrotic), or a dilaton-driven bounce (Pre-Big Bang).
In one sentence
In string cosmology, a weakly-coupled contracting phase preceded the Big Bang, driven by the dilaton field.
The claim
The Pre-Big Bang scenario emerges from low-energy string theory. The string action contains a scalar field called the dilaton, which controls the strength of all forces. In the Pre-Big Bang model, the universe began in an empty, cold, weakly-coupled state with the dilaton at very negative values. The dilaton-driven dynamics led to accelerating expansion ("dilaton-driven inflation") that ended at a high-curvature bounce.
After the bounce, the universe entered the standard hot Big Bang phase. The asymmetry between the pre-Bang and post-Bang phases is a defining feature: cold and contracting before, hot and expanding after.
The family stance
A previous cycle, aeon, contracting phase, or alternate-brane state existed before our universe. The "before" is a physically connected predecessor, not nothing or another arena.
Predictions
- Specific tilted spectrum of gravitational waves (blue tilt)
- Pre-Bang phase has weakly-coupled string vacuum
- Universe is asymmetric across the Bang
Evidence
- Naturally derived from string theory low-energy action
- Avoids singularity via high-curvature regime
Counterpoints
- Pre-Bang gravity-wave spectrum not yet detected
- Mechanism of the bounce remains incompletely understood
- Less developed than LQC bounce
Variants in this family
▸Go deeperTechnical detail with proper terminology
The Gasperini-Veneziano scenario uses the duality symmetries of string theory. The pre-Bang and post-Bang phases are related by scale-factor duality: a(t) ↔ 1/a(-t). The dilaton φ runs from -∞ to a finite value across the bounce.
The high-curvature bounce regime is where string-scale physics dominates and the low-energy effective action breaks down. Various proposed completions (loop corrections, string-theoretic completions) attempt to describe this transition, but none is universally accepted.
References
- EstablishedGasperini & Veneziano (1993). Astropart. Phys. 1, 317
- EstablishedGasperini & Veneziano (2003). Phys. Rep. 373, 1 (review)
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