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Pre-Big Bang (Gasperini-Veneziano) vs Simple Big Bounce

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Cyclic & Bouncing Cosmologies· within family

	Pre-Big Bang (Gasperini-Veneziano) Speculative	Simple Big Bounce Speculative
Proposed	1993	various
Key figures	Maurizio Gasperini, Gabriele Veneziano	Various authors
In one sentence	In string cosmology, a weakly-coupled contracting phase preceded the Big Bang, driven by the dilaton field.	The Simple Big Bounce treats our universe as the rebound of a previously contracting universe, with the Big Bang as the minimum-size moment of that transition.
Predictions	Specific tilted spectrum of gravitational waves (blue tilt) Before the Bang, a quiet low-energy state described by string theory, where all the forces were very weak (a weakly-coupled string vacuum) Universe is asymmetric across the Bang	The universe did not begin at a singularity but at a finite bounce point Some matter or radiation from the previous contracting phase may be detectable Specific signatures in the CMB depend on the bounce mechanism
Where it breaks	Pre-Bang gravity-wave spectrum not yet detected Mechanism of the bounce remains incompletely understood Less developed than LQC bounce	No direct observational evidence of a contracting pre-Big Bang phase Different bounce mechanisms make different predictions, making the framework difficult to test as a single theory Some bounce mechanisms require fine-tuning to avoid sending the universe into a singularity anyway
Key unresolved problem	The breakdown at the bounce: at the moment of greatest curvature the low-energy string equations stop working, and no one has an accepted replacement, so the switch from shrinking to expanding is not fully described.	The too-many-options problem: the big bounce idea is really a grab bag of different bounce mechanisms, each predicting something different, so it cannot be tested as one theory until you pick a specific version.
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Pre-Big Bang (Gasperini-Veneziano)

1993 · Speculative

Simple Big Bounce

various · Speculative

Proposed

1993

various

Key figures

Maurizio Gasperini, Gabriele Veneziano

Various authors

In one sentence

In string cosmology, a weakly-coupled contracting phase preceded the Big Bang, driven by the dilaton field.

The Simple Big Bounce treats our universe as the rebound of a previously contracting universe, with the Big Bang as the minimum-size moment of that transition.

Predictions

Specific tilted spectrum of gravitational waves (blue tilt)
Before the Bang, a quiet low-energy state described by string theory, where all the forces were very weak (a weakly-coupled string vacuum)
Universe is asymmetric across the Bang

The universe did not begin at a singularity but at a finite bounce point
Some matter or radiation from the previous contracting phase may be detectable
Specific signatures in the CMB depend on the bounce mechanism

Where it breaks

Pre-Bang gravity-wave spectrum not yet detected
Mechanism of the bounce remains incompletely understood
Less developed than LQC bounce

No direct observational evidence of a contracting pre-Big Bang phase
Different bounce mechanisms make different predictions, making the framework difficult to test as a single theory
Some bounce mechanisms require fine-tuning to avoid sending the universe into a singularity anyway

Key unresolved problem

The breakdown at the bounce: at the moment of greatest curvature the low-energy string equations stop working, and no one has an accepted replacement, so the switch from shrinking to expanding is not fully described.

The too-many-options problem: the big bounce idea is really a grab bag of different bounce mechanisms, each predicting something different, so it cannot be tested as one theory until you pick a specific version.

Reader vote

No votes yet