Compare · Before the Universe

Loop Quantum Big Bounce vs Ekpyrotic Cyclic Universe

Cyclic & Bouncing Cosmologies· within family

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

2001 · Speculative

Ekpyrotic Cyclic Universe

2001 · Speculative

Proposed

2001

Key figures

Abhay Ashtekar, Martin Bojowald, Parampreet Singh

Paul Steinhardt, Neil Turok, Justin Khoury, Burt Ovrut

In one sentence

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

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

Predictions

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

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

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

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

Key unresolved problem

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.

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

100% · 1 vote

0% · 0 votes