Compare · Before the Universe

Simple Big Bounce vs Ekpyrotic Cyclic Universe

Cyclic & Bouncing Cosmologies· within family

	Simple Big Bounce Speculative	Ekpyrotic Cyclic Universe Speculative
Proposed	various	2001
Key figures	Various authors	Paul Steinhardt, Neil Turok, Justin Khoury, Burt Ovrut
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.	Our universe was born from a collision between two parallel brane\|branes, and the collision repeats every trillion years.
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	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	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	Requires [[string theory]] and extra dimensions Brane-collision mechanism not fully derived Has its own initial-conditions problem
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 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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Simple Big Bounce

various · Speculative

Ekpyrotic Cyclic Universe

2001 · Speculative

Proposed

various

2001

Key figures

Various authors

Paul Steinhardt, Neil Turok, Justin Khoury, Burt Ovrut

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.

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

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

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

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

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

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 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