Minds don’t just flare up Frédéric Lecloux/Agence VU/Camerapress

TRUST your senses. Any theory that lets bizarre brains randomly pop into existence can’t be a valid description of the universe.

That might seem obvious, but such conscious observers, called Boltzmann brains, are inevitable in certain versions of cosmology. New work that claims to banish such theories not only suggests your brain isn’t such an oddity, but tells us which frameworks for the cosmos are the most sound.

The notion of a Boltzmann brain is built on 19th-century physicist Ludwig Boltzmann’s idea that the entropy of a closed system – a measure of its disorder – always increases. There are far more ways to be disorderly than orderly, so it’s much more likely that the system will move towards disorder. But there is always an infinitesimal probability that a system will suddenly fluctuate from disorder to order.


What’s more, we know that the expansion of our universe is accelerating, and the standard view is that mysterious dark energy is responsible. If dark energy remains constant throughout time, the universe will expand forever.

“If you have literally forever to wait, you’ll get essentially every single possible thing fluctuating into existence,” says Sean Carroll at the California Institute of Technology in Pasadena. That includes Boltzmann brains.

The idea is that given infinite time, more brains will fluctuate into existence than evolve, so most conscious observers would be the result of fluctuations. In such an old universe, then, the odds are that we are such brains, too.

Carroll isn’t a fan of Boltzmann brains, and now he thinks he can show they are a bridge too far.

If our brains spontaneously fluctuated into existence, he reasons, then we must be living in the very far future, since the universe needs a near-infinite time for such fluctuations to become a reality. But our measurements suggest that the universe began a mere 14 billion years ago.

That discrepancy means that if we truly are Boltzmann brains in an old universe, then our perceptions are befuddled, too. “We’d have no reason to believe that our memories of the past are accurate,” says Carroll.

He calls this paradox “cognitive instability”: the inability to trust your own processes of reasoning and memory. That should be enough to rule out such universes – and the cosmological models that produce them, he says (arxiv.org/abs/1702.00850).

This has implications for theories of dark energy. For example, if dark energy weakens over time, then the universe could contract and end up in a “big crunch” – and it would never get old enough for Boltzmann brains to form. If dark energy is instead constant over time, then Boltzmann brains could crop up eventually. But whether or not the universe will fluctuate in the necessary way depends on the particulars of a theory of quantum gravity, which have yet to be worked out.

Tossing out those theories that lead to Boltzmann brains may help us decide between competing ideas, Carroll says. On that basis, it’s reasonable that the universe might be headed for a big crunch, for example.

Raphael Bousso at the University of California at Berkeley has wracked his brains over this problem, and is torn about Carroll’s ideas.

“If a theory predicts that the overwhelming majority of observers are Boltzmann brains, then that theory is ruled out,” he says. But he thinks Carroll’s argument introduces an unnecessary mystique. “There is no need for fancy notions like ‘cognitive instability’.”

This article appeared in print under the headline “Reject universes that lead to cosmic brains”