Compare · Before the Universe

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

Cyclic & Bouncing Cosmologies· within family

	Simple Big Bounce Speculative	Pre-Big Bang (Gasperini-Veneziano) Speculative
Proposed	various	1993
Key figures	Various authors	Maurizio Gasperini, Gabriele Veneziano
In one sentence	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.	In string cosmology, a weakly-coupled contracting phase preceded the Big Bang, driven by the dilaton field.
Predictions	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	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
Where it breaks	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	Pre-Bang gravity-wave spectrum not yet detected Mechanism of the bounce remains incompletely understood Less developed than LQC bounce
Key unresolved problem	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.	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.
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Simple Big Bounce

various · Speculative

Pre-Big Bang (Gasperini-Veneziano)

1993 · Speculative

Proposed

various

1993

Key figures

Various authors

Maurizio Gasperini, Gabriele Veneziano

In one sentence

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.

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

Predictions

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

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

Where it breaks

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

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

Key unresolved problem

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.

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.

Reader vote

No votes yet