Science › Creation theories
Cyclic universes (ekpyrotic and Penrose's CCC)
2001· Theory
SpeculativeA serious idea with no experimental proof yet.
Models in which our Big Bang is one of many: the universe passes through repeated cycles or 'aeons', with no first beginning.
See it on the Science map →How it works
Ekpyrotic theory (Justin Khoury, Burt Ovrut, Paul Steinhardt and Neil Turok, 2001; Greek ekpyrosis means conflagration) puts our universe on a membrane, or 'brane', that collides with a parallel one; the collision is a hot big bang. Steinhardt and Turok made it cyclic in 2002, with a collision roughly every trillion years. Roger Penrose's Conformal Cyclic Cosmology (about 2005) instead joins aeons end to end. Hindu and Stoic thought imagined cosmic cycles long before.
Why it matters
They offer an alternative to inflation's story and, in some versions, a past with no first moment. Their predictions differ from inflation's, so coming observations can check them.
How we know
Untested. Ekpyrotic models predict essentially no primordial gravitational waves, while inflation predicts some, so detecting them would rule ekpyrosis out. Claimed traces of Penrose's earlier aeons are disputed.
Still a mystery
Can a universe pass through a crunch smoothly? The ekpyrotic bounce still lacks a complete theory, and no cyclic model has yet explained why our Big Bang was so orderly.
Connections (4)
▸Cyclic universes (ekpyrotic and Penrose's CCC) answer "an earlier cycle" to What came before the Big Bang?Speculative
The relationship
Each version says our Big Bang had a predecessor. In the ekpyrotic cycle, a previous phase of slow contraction smoothed and flattened space, doing inflation's job, before the branes collided. In Penrose's scheme, the previous aeon expanded for an immense time until only radiation remained, then became our Big Bang. Either way, 'before' was another cosmic era.
How the link was found
Khoury, Ovrut, Steinhardt and Turok (2001); Steinhardt and Turok (2002); Penrose from about 2005, set out in his 2010 book 'Cycles of Time'.
Is it proven?
Not established. Both fit current data only with adjustments, and neither has passed a test that inflation fails. Coming searches for primordial gravitational waves in the microwave background could rule out the ekpyrotic version.
Open questions and mysteries
Did the cycles themselves have a beginning, as the Borde–Guth–Vilenkin theorem suggests for expanding phases? How does structure pass from one cycle to the next?
▸Roger Penrose proposed one version of Cyclic universes (ekpyrotic and Penrose's CCC)Speculative
The relationship
Penrose noticed that in the far future, after black holes evaporate, only massless particles like light remain. They experience no time or distance, so the universe loses any sense of scale. The same holds in the earliest universe. Rescaled, an infinite, empty late universe looks just like a hot, tiny early one, so one can become the next.
How the link was found
Penrose developed Conformal Cyclic Cosmology from about 2005, motivated by his 1979 argument that the Big Bang was extraordinarily orderly: by his estimate, one chance in 10 raised to the power 10¹²³.
Is it proven?
Unconfirmed. Penrose and colleagues reported concentric circles (2010) and 'Hawking points' (2018), spots supposedly left by an earlier aeon's evaporated black holes. Several independent teams found such patterns consistent with chance.
Open questions and mysteries
It requires electrons and every other massive particle eventually to lose their mass, which no known physics provides. How information passes between aeons is also unclear.
▸String theory inspired the colliding-branes version of Cyclic universes (ekpyrotic and Penrose's CCC)Speculative
The relationship
In one form of string theory, M-theory, our three-dimensional space can be a membrane, or brane, floating in a higher-dimensional space beside a parallel brane. A spring-like force pulls the two together. When they collide, the energy of their motion turns into hot matter and radiation: a big bang. They then separate, and the cycle repeats.
How the link was found
Petr Hořava and Edward Witten described the two-brane setup in 1996. Khoury, Ovrut, Steinhardt and Turok built the ekpyrotic universe on it in 2001.
Is it proven?
Unproven, as is string theory itself. Renata Kallosh, Lev Kofman and Andrei Linde argued in 2001 that the model needs as much fine-tuning as it removes. Later cyclic models (Ijjas and Steinhardt, 2019) dropped the branes.
Open questions and mysteries
What happens at the moment of collision, when the extra dimension briefly shrinks to zero, is not well understood.
▸Cyclic universes (ekpyrotic and Penrose's CCC) must explain the reset of Entropy: disorder always growsOpen question
The relationship
Entropy only grows, so each cycle should start more disordered than the last, and cycles would grow longer and larger. Run backward, that implies a first cycle, defeating the point. The ekpyrotic answer: each cycle's expansion spreads entropy so thin that any observable region starts fresh. Penrose argues entropy is effectively reset as black holes evaporate and scale disappears.
How the link was found
Richard Tolman identified the problem for oscillating universes in 1931–34. Steinhardt and Turok (2002) and Penrose (from 2005) proposed their resets in response.
Is it proven?
Neither reset is accepted. The ekpyrotic dilution is calculated within its model; Penrose's relies on information being destroyed in black holes, which most physicists now doubt.
Open questions and mysteries
Why was our Big Bang so low in entropy? Every cyclic model must answer that, and none has convinced most cosmologists.