Just curious as to what you think about this recent paper from Max Tegmark:(adsbygoogle = window.adsbygoogle || []).push({});

How unitary cosmology generalizes thermodynamics and solves the inflationary entropy problem

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 can't decrease unless it interacts with the observer, and it can't increase unless it interacts with the environment." 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-neglible 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 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 it is valid to make the popular quantum gravity approximation that the Einstein tensor equals the quantum expectation value of the stress-energy tensor, and how problems with recent attempts to explain dark energy as gravitational backreaction from super-horizon scale fluctuations can be understood as a failure of this approximation. "

Additional quotes:

"The main goal of this paper is to investigate the entropy problem in unitary quantum mechanics more thoroughly. We will see that this fundamentally transforms the problem, strengthening the case for inflation."

...

"This shows that with unitary quantum mechanics, the conventional phrasing of the entropy problem is too simplistic, since a single pre-inflationary quantum state evolves into a superposition of many different semiclassical post-inflationary states. The careful and detailed analysis of the entropy problem in [36] is mainly performed within the context of classical physics, and quantum mechanics is only briefly mentioned, when correctly stating that Liouville's theorem holds quantum mechanically too as long as the evolution is unitarity. However, the evolution that is unitary is that of the total quantum state of the entire universe."

Any thoughts?

**Physics Forums | Science Articles, Homework Help, Discussion**

Join Physics Forums Today!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# How unitary cosmology solves the inflationary entropy problem

Can you offer guidance or do you also need help?

Draft saved
Draft deleted

**Physics Forums | Science Articles, Homework Help, Discussion**