Our Universe appears one-directional. A hot Big Bang, expansion, the birth of stars and galaxies, the emergence of life — and ahead, if one believes the familiar picture, an ever emptier and colder space. But what if this story is not the only one? What if everything has already happened an infinite number of times and will happen again, down to every atom, every thought, every star?
It is precisely this possibility that physicists Sean Carroll, Nadia Diachenko and Saakshi Dulani consider in the preprint arXiv:2605.30405, published on 28 May 2026. They propose a quantum model of cosmology that is at once strictly periodic and solves one of the most unpleasant puzzles of modern physics — the problem of "Boltzmann brains."
The essence of the problem has long been known. If the space of possible states of the Universe is finite, and its evolution obeys quantum laws, then sooner or later the state will return almost exactly to what it began with. In such a picture, the Universe spends most of its time in thermal equilibrium — in a state of maximum entropy. Random downward fluctuations in entropy are inevitable, but small deviations occur far more often than large ones. And that means it is far easier to "assemble" a random brain with false memories than an entire low-entropy history like ours. An observer is with high probability precisely such a fleeting fluctuation-created being, and not the product of a long evolution from the Big Bang.
Carroll and his colleagues have found an elegant way out. If the energies of the possible states of the Universe are commensurable — that is, they differ from one another by integer multiples of one and the same base energy — evolution becomes not merely returning but strictly periodic. Everything repeats with one and the same period, like a perfectly tuned clock. The trajectory closes into a finite loop and traverses it again and again. This period may lie in a broad but entirely permissible range: from roughly 10 billion years to an monstrously large but still finite value at which "Boltzmann brains" do not yet manage to dominate over ordinary observers.
Moreover, if one starts from a state of minimum entropy, the system passes through a particularly deep and prolonged "excursion" of low entropy — far more pronounced than would follow from the ordinary statistics of random fluctuations. This excursion can be identified with our Big Bang. After it, the Universe evolves, reaches equilibrium, and then, still within the same cycle, contracts toward a Big Crunch, after which everything begins anew.
The authors define in detail the quantum analogue of Boltzmann entropy. The Hilbert space is partitioned into subspace macrostates, and the global entropy is computed as a weighted average of the logarithms of their dimensions. Minimizing this quantity at the initial moment guarantees that all branches after decoherence start from the lowest-entropy state. As a result, a typical observer appears precisely in the course of thermodynamically meaningful evolution after the low-entropy "bang," and not as a random fluctuation.
The model remains purely quantum. The fundamental object is the state of the Universe in a finite-dimensional space, which evolves according to the Schrödinger equation. Spacetime, matter and observers arise as effective structures. The energy spectrum in this case resembles the spectrum of a gigantic harmonic oscillator with a very special initial condition. The theory of everything itself, according to the authors, is "essentially a big harmonic oscillator."
In the section devoted to the spacetime interpretation, the researchers propose a possible picture. The low-entropy minimum corresponds to a "bounce" — the moment that joins the Big Crunch and the Big Bang. On either side of it run the ordinary cosmological phases of expansion and contraction, similar to our ΛCDM Universe. Most of the time the system spends in a high-entropy equilibrium phase, which can be represented as empty de Sitter space. The transition between these phases occurs not abruptly but through a gradual "excitation" of the vacuum, and all of this fits into a single periodic quantum rhythm.
The idea echoes thoughts expressed as far back as the late nineteenth century. Poincaré proved that systems with a bounded phase space sooner or later return almost to their initial state. Boltzmann proposed an eternal Universe with fluctuations. Eddington pointed to the fatal weakness of such a picture — the predominance of minimal deviations. The new work shows how quantum mechanics with a properly chosen spectrum can realize Boltzmann's dream without its main shortcoming.
If this picture is correct, then every moment of our life is not a unique event on the edge of infinity but an exact repetition of what has already happened countless times. And it will happen as many times again. The Universe turns out to be not random chaos but a flawlessly tuned mechanism in which low entropy, life and consciousness appear again and again as an integral part of a grand, eternal cycle. The scale of this process is hard to imagine: an infinite number of identical histories, nested within one and the same quantum rhythm. And yet mathematics indicates that this may be precisely how the world in which we exist is arranged.
