Anthropic Reasoning in Quantum Cosmology
The temperature of the CMB varies with time from the big bang. A prediction of the probability for the outcome of an observation of the CMB temperature must therefore be conditioned on the time we make the observation. More generally any prediction made by theory for our observations of the universe must be conditioned on a description of the observational situation. This includes a description of us as physical observing systems within the universe. We therefore have zero probability to observe what is inconsistent with our existence --- for example, a CMB temperature above the surface temperature of the sun. That is a very simple example of an anthropic constraint on observed quantities. The papers in this section are concerned with such anthropic reasoning and how it fits into quantum cosmology.
The Return of the Observer (coming)
Anthropic Reasoning in Quantum Cosmology [129]
Describes prediction in general in quantum cosmology and anthropic prediction in particular. There are no predictions of any kind including anthropic ones that do not depend on the quantum state of the universe at some level.
Are We Typical ? [137]
(with Mark Srednicki) Anthropic prediction generally requires a typicality assumption ---- that what we observe is typical of what other observers in some class observe. However, We have no observational evidence that as human observers we are typical of any class of objects in the universe, and there is no reason to believe that the laws of physics have to be such as to make our observations typical of others that might be made in the universe. An assumption that we are atypical is a testable hypothesis.
Science in a Very Large Universe, The Xerographic Distribution [144a,144b]
(with Mark Srednicki) Inflation can make the universe large enough that there is significant probability that we are replicated as physical systems at other locations in spacetime. Predictions of our future observations then require an assumed probability distribution for our location among the possible ones (the xerographic distribution) in addition to the probabilities arising from the quantum state. It is the combination of fundamental theory plus the xerographic distribution that can be predictive and testable by further observations.
Replication Regulates Volume Weighting in Quantum Cosmology [143]
(with Thomas Hertog) This paper shows that the quantum state of the universe defines a measure for the prediction of the probabilities for observation that is well behaved for spatially large of infinite universes when the probabilities that we are replicated elsewhere in the large universes are taken into account. There is a greater probability to observe the properties of large universes than small ones because in a large universe there are more replications of us than in a small one.
Anthropic Bounds on Lambda from the No-Boundary Quantum State [d]
(with Thomas Hertog) Anthropic selection emerges naturally and inevitably in quantum cosmology when probabilities for our observations of the universe are calculated. The results depend on the prior implied by the universe's quantum state. We illustrate this by computing the probabilities specified by the no-boundary wave function for observations of the values of Lambda and Q in an inflationary landscape model in which both quantities vary. Within the anthropic range of values the no-boundary state yields an approximately flat distribution on Lambda and strongly favors small values of Q. This restores Weinberg's successful prediction of Lambda.