How unitary cosmology generalizes thermodynamics and solves the inflationary entropy problem
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Tegmark-2012-How unitary cosmology generalizes thermodynamics.pdf
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Author(s)
Tegmark, Max Erik
Date Issued
June 2012
Journal
Physical Review D
Publisher
American Physical Society
Citation
Tegmark, Max. “How Unitary Cosmology Generalizes Thermodynamics and Solves the Inflationary Entropy Problem.” Physical Review D 85.12 (2012): 123517. © 2012 American Physical Society.
Version
Final published version
Abstract
We analyze cosmology assuming unitary quantum mechanics, using a tripartite partition into system,
observer, and environment degrees of freedom. This generalizes the second law of thermodynamics to
‘‘The system’s entropy cannot decrease unless it interacts with the observer, and it cannot increase unless
it interacts with the environment.’’ The former follows from the quantum Bayes theorem we derive. We
show that because of the long-range entanglement created by cosmological inflation, the cosmic entropy
decreases exponentially rather than linearly with the number of bits of information observed, so that a
given observer can reduce entropy by much more than the amount of information her brain can store.
Indeed, we argue that as long as inflation has occurred in a non-negligible fraction of the volume, almost
all sentient observers will find themselves in a post-inflationary low-entropy Hubble volume, and we
humans have no reason to be surprised that we do so as well, which solves the so-called inflationary
entropy problem. An arguably worse problem for unitary cosmology involves gamma-ray-burst constraints
on the ‘‘big snap,’’ a fourth cosmic doomsday scenario alongside the ‘‘big crunch,’’ ‘‘big chill,’’
and ‘‘big rip,’’ where an increasingly granular nature of expanding space modifies our life-supporting laws
of physics. Our tripartite framework also clarifies when the popular quantum gravity approximation
G
8 GhT
i is valid, and how problems with recent attempts to explain dark energy as gravitational
backreaction from superhorizon scale fluctuations can be understood as a failure of this approximation.
MIT Department
Massachusetts Institute of Technology. Department of Physics
MIT Kavli Institute for Astrophysics and Space Research
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DOI of Published Version
https://doi.org/10.1103/PhysRevD.85.123517
