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Pre-Big Bang (Gasperini-Veneziano) vs Ekpyrotic Cyclic Universe

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Cyclic & Bouncing Cosmologies· within family

	Pre-Big Bang (Gasperini-Veneziano) Speculative	Ekpyrotic Cyclic Universe Speculative
Proposed	1993	2001
Key figures	Maurizio Gasperini, Gabriele Veneziano	Paul Steinhardt, Neil Turok, Justin Khoury, Burt Ovrut
In one sentence	In string cosmology, a weakly-coupled contracting phase preceded the Big Bang, driven by the dilaton field.	Our universe was born from a collision between two parallel brane\|branes, and the collision repeats every trillion years.
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	Almost no primordial gravitational waves: the tensor-to-scalar ratio r, the strength of gravitational-wave ripples relative to density ripples, sits near zero (r ≈ 0) The spectrum of primordial density ripples is slightly stronger at large scales than small, a so-called red tilt Each cycle ~10¹² years or longer
Where it breaks	Pre-Bang gravity-wave spectrum not yet detected Mechanism of the bounce remains incompletely understood Less developed than LQC bounce	Requires [[string theory]] and extra dimensions Brane-collision mechanism not fully derived Has its own initial-conditions problem
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 missing derivation: the brane collision driving the model, two higher-dimensional sheets striking each other, has never been worked out from string theory's basic rules, so the bounce itself stays physically unspecified.
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Pre-Big Bang (Gasperini-Veneziano)

1993 · Speculative

Ekpyrotic Cyclic Universe

2001 · Speculative

Proposed

1993

2001

Key figures

Maurizio Gasperini, Gabriele Veneziano

Paul Steinhardt, Neil Turok, Justin Khoury, Burt Ovrut

In one sentence

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

Our universe was born from a collision between two parallel brane|branes, and the collision repeats every trillion years.

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

Almost no primordial gravitational waves: the tensor-to-scalar ratio r, the strength of gravitational-wave ripples relative to density ripples, sits near zero (r ≈ 0)
The spectrum of primordial density ripples is slightly stronger at large scales than small, a so-called red tilt
Each cycle ~10¹² years or longer

Where it breaks

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

Requires [[string theory]] and extra dimensions
Brane-collision mechanism not fully derived
Has its own initial-conditions problem

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 missing derivation: the brane collision driving the model, two higher-dimensional sheets striking each other, has never been worked out from string theory's basic rules, so the bounce itself stays physically unspecified.

Reader vote

No votes yet