Cyclic‑universe chatter resurfaces

Cosmologists have argued for decades over whether the universe began once or keeps renewing itself, and that debate flared again in social posts on April 22 and April 23. (x.com) The standard picture says the universe was once hot and dense, then expanded and cooled. NASA says the cosmic microwave background — the leftover light from that early era — was released about 380,000 years after the Big Bang and still fills the sky today. (science.nasa.gov) Particle physicists treat that hot early phase as strongly supported by two observations: the cosmic microwave background and the measured abundances of light elements. The Particle Data Group says those observations established the hot Big Bang model as the leading description of the universe’s past. (pdg.lbl.gov) The online argument centered on entropy, a measure of how spread out energy and disorder become over time. In ordinary thermodynamics, entropy rises, so any model with endless cycles has to explain why each new cycle does not simply inherit more disorder than the last. (link.springer.com) One response comes from cyclic or “bounce” models, which replace a single beginning with repeated phases of contraction and expansion. Paul Steinhardt’s research page describes the Big Bang in those models as a “big bounce” connecting an earlier contracting phase to the present expanding one. (paulsteinhardt.org) A modern cyclic version dates to a 2001 paper by Paul Steinhardt and Neil Turok. Their model proposed endless epochs, each ending in a crunch and restarting in a bang, with slow contraction and accelerated expansion doing work that inflation handles in the standard picture. (arxiv.org) Another version is Roger Penrose’s conformal cyclic cosmology, which does not rely on a simple crunch-and-rebound story. A March 31, 2025 paper by Penrose and Krzysztof Meissner describes one cosmic “aeon” flowing into the next through a conformal rescaling of the universe’s very late, dilute future. (arxiv.org) That Penrose paper also argues for possible traces in the cosmic microwave background, including proposed “Hawking spots.” Those claims remain disputed, and the standard inflationary Big Bang framework still dominates textbooks, data analysis, and mission planning. (arxiv.org) (science.nasa.gov) The timing of the new chatter followed a broader burst of attention around cyclic cosmology this month. New Scientist reported on April 17 that the idea had regained notice amid fresh discussion of dark energy and the long-term fate of cosmic expansion. (newscientist.com) What resurfaced online was not a new measurement but an old fault line in cosmology: whether the universe needs one exceptionally low-entropy beginning, or a mechanism that lets beginnings happen again and again. (pdg.lbl.gov) (paulsteinhardt.org)