Sean Carroll is a Research Professor of theoretical physics at the California Institute of Technology. He received his Ph.D. in 1993 from Harvard University. His research focuses on fundamental physics and cosmology, especially issues of dark matter, dark energy, spacetime symmetries, and the origin of the universe. Recently, Carroll has worked on the foundations of quantum mechanics, the arrow of time, and the emergence of complexity. He is the author of The Particle at the End of the Universe, From Eternity to Here: The Quest for the Ultimate Theory of Time, and the textbook Spacetime and Geometry: An Introduction to General Relativity, as well as several lecture courses for The Great Courses.
The quantum state of a universe dominated by vacuum energy has a nonzero temperature, much like that of a black hole. Under the right conditions, thermal fluctuations can lead to the appearance of freak, random observers, known as "Boltzmann Brains." The total number of observers in such a universe can easily be dominated by such fluctuations, rather than "ordinary observers" living in the warm aftermath of a Big Bang. I will discuss why such a situation should be considered as a severe problem for cosmological models, including standard LCDM, and also how it can be avoided, depending on details of the full quantum theory describing cosmological evolution.