Compare · Before the Universe

Ekpyrotic Cyclic Universe vs String-Gas Cosmology

Cyclic & Bouncing Cosmologies· within family

	Ekpyrotic Cyclic Universe Speculative	String-Gas Cosmology Speculative
Proposed	2001	1989
Key figures	Paul Steinhardt, Neil Turok, Justin Khoury, Burt Ovrut	Robert Brandenberger, Cumrun Vafa
In one sentence	Our universe was born from a collision between two parallel brane\|branes, and the collision repeats every trillion years.	String-Gas Cosmology proposes that the Big Bang was the dynamics of a gas of fundamental strings, with cosmic structure emerging from string thermodynamics.
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	The number of large spatial dimensions is constrained by string dynamics (predicts three) Specific spectrum of cosmological perturbations from string thermodynamics Distinct signatures in CMB power spectrum that differ from inflation Relationship between primordial gravitational waves and density perturbations
Where it breaks	Requires [[string theory]] and extra dimensions Brane-collision mechanism not fully derived Has its own initial-conditions problem	String theory itself has not been experimentally verified Specific predictions are model-dependent and not yet decisive The mechanism for emerging from the high-temperature string gas phase is not fully worked out
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 missing handoff: no one has fully worked out how the hot early string-gas phase, a dense tangle of vibrating strings, gives way to the ordinary expanding universe we see, so the link to observed physics is incomplete.
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Ekpyrotic Cyclic Universe

2001 · Speculative

String-Gas Cosmology

1989 · Speculative

Proposed

2001

1989

Key figures

Paul Steinhardt, Neil Turok, Justin Khoury, Burt Ovrut

Robert Brandenberger, Cumrun Vafa

In one sentence

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

String-Gas Cosmology proposes that the Big Bang was the dynamics of a gas of fundamental strings, with cosmic structure emerging from string thermodynamics.

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

The number of large spatial dimensions is constrained by string dynamics (predicts three)
Specific spectrum of cosmological perturbations from string thermodynamics
Distinct signatures in CMB power spectrum that differ from inflation
Relationship between primordial gravitational waves and density perturbations

Where it breaks

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

String theory itself has not been experimentally verified
Specific predictions are model-dependent and not yet decisive
The mechanism for emerging from the high-temperature string gas phase is not fully worked out

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 missing handoff: no one has fully worked out how the hot early string-gas phase, a dense tangle of vibrating strings, gives way to the ordinary expanding universe we see, so the link to observed physics is incomplete.

Reader vote

No votes yet