Compare · Before the Universe

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

Cyclic & Bouncing Cosmologies· within family

	Ekpyrotic Cyclic Universe Speculative	Pre-Big Bang (Gasperini-Veneziano) Speculative
Proposed	2001	1993
Key figures	Paul Steinhardt, Neil Turok, Justin Khoury, Burt Ovrut	Maurizio Gasperini, Gabriele Veneziano
In one sentence	Our universe was born from a collision between two parallel brane\|branes, and the collision repeats every trillion years.	In string cosmology, a weakly-coupled contracting phase preceded the Big Bang, driven by the dilaton field.
Predictions	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	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	Requires [[string theory]] and extra dimensions Brane-collision mechanism not fully derived Has its own initial-conditions problem	Pre-Bang gravity-wave spectrum not yet detected Mechanism of the bounce remains incompletely understood Less developed than LQC bounce
Key unresolved problem	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.	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	No votes yet

Ekpyrotic Cyclic Universe

2001 · Speculative

Pre-Big Bang (Gasperini-Veneziano)

1993 · Speculative

Proposed

2001

1993

Key figures

Paul Steinhardt, Neil Turok, Justin Khoury, Burt Ovrut

Maurizio Gasperini, Gabriele Veneziano

In one sentence

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

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

Predictions

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

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

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

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

Key unresolved problem

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.

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