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

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Cyclic & Bouncing Cosmologies· within family

	Pre-Big Bang (Gasperini-Veneziano) Speculative	Loop Quantum Big Bounce Speculative
Proposed	1993	2001
Key figures	Maurizio Gasperini, Gabriele Veneziano	Abhay Ashtekar, Martin Bojowald, Parampreet Singh
In one sentence	In string cosmology, a weakly-coupled contracting phase preceded the Big Bang, driven by the dilaton field.	A previous contracting universe bounced, quantum geometry prevented the collapse from becoming a singularity.
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	Quantum bounce at Planck density Modified CMB power spectrum at large angular scales Pre-Bang contracting universe phase
Where it breaks	Pre-Bang gravity-wave spectrum not yet detected Mechanism of the bounce remains incompletely understood Less developed than LQC bounce	Effects only at Planck scale, not currently testable LQG itself is incomplete CMB corrections sub-dominant to cosmic variance
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 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.
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Pre-Big Bang (Gasperini-Veneziano)

1993 · Speculative

Loop Quantum Big Bounce

2001 · Speculative

Proposed

1993

2001

Key figures

Maurizio Gasperini, Gabriele Veneziano

Abhay Ashtekar, Martin Bojowald, Parampreet Singh

In one sentence

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

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

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

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

Where it breaks

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

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

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 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.

Reader vote

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100% · 1 vote