Could the universe be a cycle?

Why cosmologists consider cycles

The standard Big Bang model describes the universe expanding from an extremely hot, dense state roughly 13.8 billion years ago, but it does not explain what, if anything, came before that state or why it had the particular properties it did. Cyclic models were developed partly to avoid positing an absolute beginning and partly to explain features of the universe, like its large-scale smoothness, using repeating dynamics instead of a one-time inflationary event.

Ekpyrotic and cyclic models

The ekpyrotic scenario, developed by physicists including Paul Steinhardt and Neil Turok in the early 2000s, proposes that the Big Bang was not an absolute beginning but the result of a collision between two higher dimensional membranes, or branes, in a larger space. In the cyclic extension of this idea, such collisions recur over enormous timescales, with each collision triggering a new phase of expansion that looks, from inside, like a new Big Bang.

These models were constructed partly as alternatives to standard cosmic inflation, aiming to reproduce the same observational successes, such as the near uniform temperature of the cosmic microwave background, through a different mechanism involving slow contraction rather than rapid inflationary expansion.

Penrose's conformal cyclic cosmology

Roger Penrose has proposed a distinct cyclic model called conformal cyclic cosmology, in which the extremely late, cold, and dilute future of one aeon becomes mathematically equivalent to the hot, dense Big Bang of the next aeon through a technique called conformal rescaling, which treats the overall scale of the universe as unphysical. Penrose and collaborators have pointed to claimed circular patterns in the cosmic microwave background as possible signatures of events from a previous aeon, though these claims are contested and not generally accepted by the wider cosmology community.

Status of the evidence

The standard inflationary Big Bang model, combined with the cosmological constant to account for accelerating expansion, remains the consensus framework because it fits an enormous range of observations, including the cosmic microwave background, the abundance of light elements, and the large-scale distribution of galaxies. Cyclic alternatives have not matched this record and currently rely more on theoretical motivation than on observational confirmation.