Despite the gloomy prognosis given above, there are some specific cases where the existence of a multi-domain scenario can be disproved. These are when we live in a 'small universe', where we have already seen all the way around it [45,46]. In this case, the universe closes up on itself in a single FLRW-like domain, so there can be no further domains that are causally connected to us.
This proposal is observationally testable; indeed it has been suggested that the power spectrum of the microwave background fluctuations (in particular, the lack of power on the largest angular scales) might already support it . The proposal can be further tested by looking for identical circles in the microwave sky or for an alignment of the quadrupole and octopole axes. (A preliminary identification of the first effect has been made by Roukema and colleagues ), and evidence for the second effect is also good according to Katz and Weeks ). Confirmation of the small universe hypothesis would disprove the usual multi-domain chaotic inflation scenario but not a true multiverse proposal, since that cannot be observationally falsified or verified.
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