Guth, Kaiser and Nomura: inflation after Planck 2013

Primary source: Alan H. Guth, David I. Kaiser and Yasunori Nomura, “Inflationary paradigm after Planck 2013”, Physics Letters B 733 (2014), pp. 112–119. An explicit reply to Ijjas, Steinhardt and Loeb’s argument.

Claim: They define the paradigm minimally as an early period of accelerated expansion that explains observable features. They identify approximate spatial flatness, near scale-invariant scalar tilt, predominately Gaussian fluctuations unless inflaton potential or initial conditions are tuned, and coherent acoustic modes among the characteristic predictions of simple inflation. A chosen potential constrains quantitative observables; the observations can exclude chosen potentials without excluding accelerated expansion itself.

Where the sides really disagree: Their response says the detailed dynamics that initiate inflation need not be the same as the final observable stage, and volume weighting is not an unquestioned probability rule. They explicitly acknowledge that the probability measure for eternal inflation is not known: their position is that the difficulty computing a unique multiverse-wide probability does not wipe out predictions for perturbations from the last observable inflationary stage. Ijjas et al. regard adding potential structure, special histories and a measure as an erosion of generic predictive power. Neither paper establishes a practical single-observation falsifier of the entire paradigm.

Useful for the practical question; do not confuse Guth et al.’s position with a demonstration that every variant is independently predictive.