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Inflationary paradigm in trouble after Planck2013

Anna Ijjas, Paul J. Steinhardt, Abraham Loeb

TL;DR

This work argues that Planck2013 data, while compatible with a narrow class of single-field plateau-like inflation models, simultaneously impose three serious challenges to the inflationary paradigm: a problematic initial-condition requirement, a worsened multiverse/unpredictability issue, and a new unlikeliness problem making plateau inflation statistically unlikely within the inflationary framework. The authors show that plateau-like potentials, though favored by observational data, demand fine-tuning and exacerbate starting conditions, and they discuss potential escapes that fail to resolve core issues. They also explore implications of LHC findings on the Higgs sector, which could further complicate the inflationary picture. The paper concludes that forthcoming observations of B-modes, non-Gaussianity, and new particles will be decisive in assessing the viability of inflation in its current form.

Abstract

Recent results from the Planck satellite combined with earlier observations from WMAP, ACT, SPT and other experiments eliminate a wide spectrum of more complex inflationary models and favor models with a single scalar field, as reported by the Planck Collaboration. More important, though, is that all the simplest inflaton models are disfavored statistically relative to those with plateau-like potentials. We discuss how a restriction to plateau-like models has three independent serious drawbacks: it exacerbates both the initial conditions problem and the multiverse-unpredictability problem and it creates a new difficulty that we call the inflationary "unlikeliness problem." Finally, we comment on problems reconciling inflation with a standard model Higgs, as suggested by recent LHC results. In sum, we find that recent experimental data disfavors all the best-motivated inflationary scenarios and introduces new, serious difficulties that cut to the core of the inflationary paradigm. Forthcoming searches for B-modes, non-Gaussianity and new particles should be decisive.

Inflationary paradigm in trouble after Planck2013

TL;DR

This work argues that Planck2013 data, while compatible with a narrow class of single-field plateau-like inflation models, simultaneously impose three serious challenges to the inflationary paradigm: a problematic initial-condition requirement, a worsened multiverse/unpredictability issue, and a new unlikeliness problem making plateau inflation statistically unlikely within the inflationary framework. The authors show that plateau-like potentials, though favored by observational data, demand fine-tuning and exacerbate starting conditions, and they discuss potential escapes that fail to resolve core issues. They also explore implications of LHC findings on the Higgs sector, which could further complicate the inflationary picture. The paper concludes that forthcoming observations of B-modes, non-Gaussianity, and new particles will be decisive in assessing the viability of inflation in its current form.

Abstract

Recent results from the Planck satellite combined with earlier observations from WMAP, ACT, SPT and other experiments eliminate a wide spectrum of more complex inflationary models and favor models with a single scalar field, as reported by the Planck Collaboration. More important, though, is that all the simplest inflaton models are disfavored statistically relative to those with plateau-like potentials. We discuss how a restriction to plateau-like models has three independent serious drawbacks: it exacerbates both the initial conditions problem and the multiverse-unpredictability problem and it creates a new difficulty that we call the inflationary "unlikeliness problem." Finally, we comment on problems reconciling inflation with a standard model Higgs, as suggested by recent LHC results. In sum, we find that recent experimental data disfavors all the best-motivated inflationary scenarios and introduces new, serious difficulties that cut to the core of the inflationary paradigm. Forthcoming searches for B-modes, non-Gaussianity and new particles should be decisive.

Paper Structure

This paper contains 7 sections, 6 equations, 1 figure.

Table of Contents

  1. Which inflationary models survive after Planck2013?
  2. How do plateau-like inflationary models affect the initial conditions problem?
  3. Is a plateau-like potential likely according to the inflationary paradigm?
  4. Is Planck2013 data compatible with the multiverse?
  5. Is there any escape from these new problems?
  6. More trouble for inflation from the LHC?
  7. Discussion

Figures (1)

  • Figure 1: Plateau-like models favored by Planck2013 data: (a) Higgs-like potential $V$ with standard Einstein gravity that has both plateau at $\phi \ll \phi_0$ (solid red) and power-law behavior at $\phi \gg \phi_0$ (dashed blue), where $N_{\text{max}}$ is the maximum number of e-folds of inflation possible for the maximal range $\Delta \phi$; (b) unique plateau-like model (solid red) for semi-infinite range of $\phi$ if perfectly tuned compared to continuum of power-law inflation models (dashed blue) without tuning; (c) periodic (axion-like) plateau potential (solid red) for $\phi$ plus typical power-law inflation potential (dashed blue) for second field $\psi$; (d) designed inflationary potential with power-law inflation segment or false vacuum segment (dotted green) grafted onto a plateau model (solid red).