Compare · Before the Universe

Loop Quantum Big Bounce vs Pre-Big Bang (Gasperini-Veneziano)

Cyclic & Bouncing Cosmologies· within family

	Loop Quantum Big Bounce Speculative	Pre-Big Bang (Gasperini-Veneziano) Speculative
Proposed	2001	1993
Key figures	Abhay Ashtekar, Martin Bojowald, Parampreet Singh	Maurizio Gasperini, Gabriele Veneziano
In one sentence	A previous contracting universe bounced, quantum geometry prevented the collapse from becoming a singularity.	In string cosmology, a weakly-coupled contracting phase preceded the Big Bang, driven by the dilaton field.
Predictions	Quantum bounce at Planck density Modified CMB power spectrum at large angular scales Pre-Bang contracting universe phase	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	Effects only at Planck scale, not currently testable LQG itself is incomplete CMB corrections sub-dominant to cosmic variance	Pre-Bang gravity-wave spectrum not yet detected Mechanism of the bounce remains incompletely understood Less developed than LQC bounce
Key unresolved problem	The out-of-reach test: the bounce in loop quantum cosmology only shows up at Planck-scale densities, energies far beyond any experiment we can imagine building, so the prediction stays untestable for now.	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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Loop Quantum Big Bounce

2001 · Speculative

Pre-Big Bang (Gasperini-Veneziano)

1993 · Speculative

Proposed

2001

1993

Key figures

Abhay Ashtekar, Martin Bojowald, Parampreet Singh

Maurizio Gasperini, Gabriele Veneziano

In one sentence

A previous contracting universe bounced, quantum geometry prevented the collapse from becoming a singularity.

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

Predictions

Quantum bounce at Planck density
Modified CMB power spectrum at large angular scales
Pre-Bang contracting universe phase

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

Effects only at Planck scale, not currently testable
LQG itself is incomplete
CMB corrections sub-dominant to cosmic variance

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

Key unresolved problem

The out-of-reach test: the bounce in loop quantum cosmology only shows up at Planck-scale densities, energies far beyond any experiment we can imagine building, so the prediction stays untestable for now.

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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