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Ekpyrotic Cyclic Universe vs Loop Quantum Big Bounce

Cyclic & Bouncing Cosmologies· within family

	Ekpyrotic Cyclic Universe Speculative	Loop Quantum Big Bounce Speculative
Proposed	2001	2001
Key figures	Paul Steinhardt, Neil Turok, Justin Khoury, Burt Ovrut	Abhay Ashtekar, Martin Bojowald, Parampreet Singh
In one sentence	Our universe was born from a collision between two parallel brane\|branes, and the collision repeats every trillion years.	A previous contracting universe bounced, quantum geometry prevented the collapse from becoming a singularity.
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	Quantum bounce at Planck density Modified CMB power spectrum at large angular scales Pre-Bang contracting universe phase
Where it breaks	Requires [[string theory]] and extra dimensions Brane-collision mechanism not fully derived Has its own initial-conditions problem	Effects only at Planck scale, not currently testable LQG itself is incomplete CMB corrections sub-dominant to cosmic variance
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 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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Ekpyrotic Cyclic Universe

2001 · Speculative

Loop Quantum Big Bounce

2001 · Speculative

Proposed

2001

Key figures

Paul Steinhardt, Neil Turok, Justin Khoury, Burt Ovrut

Abhay Ashtekar, Martin Bojowald, Parampreet Singh

In one sentence

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

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

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

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

Where it breaks

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

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

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 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

0% · 0 votes

100% · 1 vote