A New Quantum Model Suggests the Universe Could Repeat Forever
What if the universe doesn't simply expand toward an endless
future, but instead follows a cosmic cycle that repeats again and again?
A new theoretical paper by physicists Sean M. Carroll,
Nadiia Diachenko, and Saakshi Dulani proposes a quantum cosmological model in
which the universe could be “exactly periodic”—potentially avoiding one of the
strangest problems associated with an eternal universe: the Boltzmann Brain
paradox. The paper, Toward a Phenomenologically Acceptable Quantum Cyclic
Universe, was posted to arXiv in May 2026.
The idea is highly speculative. It does not establish that
our universe actually repeats forever. Instead, the researchers show that a
particular quantum model can produce this behavior under specific assumptions.
The strange problem of the Boltzmann Brain
To understand why the researchers developed the model, it helps to start with a bizarre thought experiment in statistical physics.
Imagine a universe that lasts forever and eventually reaches
an extremely high-entropy equilibrium state. If the universe has enough time,
extraordinarily unlikely fluctuations can occur.
In principle, one such fluctuation could randomly assemble
the particles necessary to create a functioning brain—with apparent memories,
thoughts and perceptions.
This hypothetical observer is called a Boltzmann Brain.
The problem is statistical. If a universe lasts eternally,
tiny fluctuations could potentially occur vastly more often than a fluctuation
large enough to produce an entire low-entropy universe filled with galaxies,
stars, planets and evolved observers.
That creates an uncomfortable question:
- If the universe were eternal and dominated by random fluctuations, why should we expect to be observers living in a coherent universe rather than isolated Boltzmann Brains?
- Carroll and his collaborators investigate whether quantum dynamics could provide a different way out of this problem.
From recurrence to an exact cosmic cycle
The starting point of their model is quantum mechanics.
The researchers consider a universe described by a quantum
state evolving **unitarily** in a finite-dimensional Hilbert space. Under those
assumptions, quantum evolution is recurrent: given enough time, the state can
return arbitrarily close to an earlier state.
But the researchers focus on something stronger than
ordinary recurrence.
They show that if the differences between the relevant
energy eigenvalues are commensurable, the quantum evolution can become exactly periodic rather than merely returning approximately to previous
configurations.
In simple terms, the universe could theoretically follow a
repeating quantum trajectory:
Beginning → evolution → high entropy → contraction → low
entropy → beginning again
And then the entire sequence repeats.
Not approximately.
Exactly.
At least, that's what the mathematical model permits.
Where does the Big Bang fit?
This is where the proposal becomes particularly interesting.
The researchers consider a quantum state that begins at minimum
thermodynamic entropy. Instead of immediately behaving like a typical random
fluctuation, the system can undergo a distinctive, large excursion away from
equilibrium.
That excursion could potentially correspond to something
resembling our Big Bang.
The universe would then evolve away from its low-entropy
beginning, producing the familiar thermodynamic arrow of time and allowing
complex structures to develop.
Eventually, according to the speculative picture, the
universe could reach a future high-entropy state before undergoing a Big Crunch.
The cycle would then begin again.
Big Bang → cosmic evolution → Big Crunch → Big Bang → cosmic
evolution → Big Crunch...
Forever.
Why this helps with the Boltzmann Brain problem
The key feature isn't simply that the universe repeats.
It's that the proposed quantum dynamics can make the large,
structured entropy excursion associated with a Big Bang much more prominent
than ordinary random fluctuations would suggest.
The paper argues that this can result in relatively few
Boltzmann fluctuations during the high-entropy portion of the cycle before the
universe undergoes another large entropy excursion.
That changes the usual statistical picture.
Instead of an eternal universe spending essentially all of
its existence in equilibrium while occasionally producing random observers, the
model creates a **distinguished recurring cosmic history**.
Our universe-like state would therefore not need to be a
freak statistical accident.
Does this mean everything has happened before?
This is where the popular interpretation can go beyond what
the paper actually establishes.
If the model describes a genuinely exactly periodic quantum
universe, then the complete quantum state would repeat after some enormous
period.
That could lead to a startling philosophical consequence: if
the physical state of the universe—including the physical configurations
responsible for our memories and experiences—were reproduced exactly, then
events within that cycle would recur.
In that hypothetical scenario, a conversation happening
today could occur again in a later cycle.
And again.
And again.
But the paper does not demonstrate that your individual
experiences have already happened an infinite number of times. It proposes a
mathematical framework in which an exactly periodic quantum cosmology is
possible under particular assumptions.
There is still a huge gap between the mathematics and reality
Perhaps the most important point is that this is a theoretical
model, not an observation that the universe actually undergoes these cycles.
The researchers explicitly describe their work as a quantum
model and say they speculate on its spacetime interpretation.
For the scenario to describe our actual universe, many
difficult questions would have to be answered.
Would the required quantum system correspond to realistic
cosmological physics? Can a physically realistic universe have the required
finite-dimensional Hilbert space? Can the proposed quantum dynamics be
connected consistently to spacetime and gravity? And, most importantly, could
the model make observational predictions that distinguish it from other
cosmological scenarios?
Those questions remain open.
A universe without an ultimate beginning?
The most fascinating implication is therefore not that
scientists have discovered we are trapped in an eternal loop.
They haven't.
Instead, the new work demonstrates a possible route toward a
universe in which the Big Bang isn't necessarily a unique beginning.
It could be one point on an enormous quantum cycle.
If such a universe were physically realized, cosmic history
would have no final chapter. There would be no first cycle and no last
cycle—only an endlessly repeating quantum history.
Our universe would not be a one-time event.
It would be one repetition in an eternal pattern.
And if the cycle were truly exact, the most unsettling
question might not be "What happened before the Big Bang?"
It might be:
"How many times has this exact universe already existed?"
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