Multiverse models in canonical quantum cosmology and their testability
The idea that our universe is part of a multiverse that consists of a multitude of other universes, which might even possess different physical constants and laws, is intriguing, but will remain speculative as long as one cannot find a way that other parts of the multiverse can have an observable effect onto our universe.
In this talk, I will present multiverse models in the context of canonical quantum cosmology that are constructed within the framework of the so-called third quantization formalism, which results from a quantum-field-theoretical formulation of the Wheeler-DeWitt equation. These models will be analyzed with regard to their effects onto the cosmic microwave background (CMB) anisotropy spectrum.
In the first model based on eternal inflation, the third quantization formalism converts the eternally inflating universe into an ensemble of sub-universes that exhibit a distinctive pre-inflationary phase. Assuming that our observable universe is represented by such a sub-universe, we calculate the effect of the pre-inflationary phase onto the primordial scalar power spectrum and find that there is a suppression of power on the largest scales followed by a bump leading to an enhancement. In order to get a sizable effect for the suppression to explain the observed quadrupole anomaly in the CMB, the bump is enhanced too much to be compatible with the CMB data.
The second model involves an explicit quantum interaction between the sub-universes. Here, we obtain a different pre-inflationary phase that also leads to a suppression on large scales, but does not exhibit a prominent enhanced bump. Thus it fits the CMB data better and might even lead us towards an explanation of the CMB quadrupole discrepancy.