Sean Carroll proposes exactly periodic universe — SkimNews
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- Sean Carroll, with collaborators Nadiia Diachenko and Saakshi Dulani at Johns Hopkins University, published a paper proposing a cyclic cosmology where each cycle plays out identically — galaxies reform, lives unfold again and conversations repeat word for word.
- The model starts from quantum mechanics rather than general relativity, assuming a finite Hilbert space and applying Poincaré's recurrence theorem to derive an exactly periodic universe.
- Each cycle spans roughly a googolplex years — incomprehensibly longer than the universe's current ~10^10-year age — with the full history of the cosmos repeating an infinite number of times.
- The model sidesteps the Boltzmann brain problem because the universe's quantum evolution can be tuned along a path that never visits states containing spontaneously formed self-aware brains with false memories.
- In the model, the universe expands, empties into empty space, then plays the big bang in reverse — particles compress and blueshift, white holes spew matter outward, and a bounce triggers the next identical cycle.
- Carroll says whether the picture feels liberating or disquieting is a matter of taste; he frames it either as endlessly repeating cycles or as a single universe that consumes its own tail.
Why it matters: Carroll's approach flips cosmology's standard method — starting from quantum mechanics rather than general relativity — and offers a specific mathematical exit from the Boltzmann brain problem, which threatens the evidentiary foundations of any eternal-universe model by implying most observers are random fluctuations with unreliable memories. If the theory holds, the roughly 10^10-year-old universe we inhabit is one of infinitely many identical iterations.
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