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String Inflation After Planck 2013

C. P. Burgess, M. Cicoli, F. Quevedo

TL;DR

The paper assesses Planck 2013 constraints on string inflation, arguing that while string models are intrinsically multi-field, many reduce to effective single-field behavior at horizon exit and largely agree with the observed $n_s \approx 0.96$ and suppressed $r$. It surveys open and closed string inflation scenarios, detailing how D3/anti-D3, inflection-point, DBI, axion monodromy, and various moduli-based constructions fare in the $n_s$–$r$ plane and with non-Gaussianity bounds. It highlights that most string models predict a small tensor-to-scalar ratio and that non-Gaussianities are typically not observed, though certain mechanisms in multi-field setups can generate observable NG under specific conditions. The authors argue that Planck data favor simple effective descriptions but do not rule out the richer structure of string theory; they also stress that a future detection of primordial tensors would challenge a large portion of string-inflation models and help illuminate the underlying UV completion. Overall, the work provides a framework to interpret current constraints and to guide future model-building and observational tests within a quantum-gravity–consistent setting.

Abstract

We briefly summarize the impact of the recent Planck measurements for string inflationary models, and outline what might be expected to be learned in the near future from the expected improvement in sensitivity to the primordial tensor-to-scalar ratio. We comment on whether these models provide sufficient added value to compensate for their complexity, and ask how they fare in the face of the new constraints on non-gaussianity and dark radiation. We argue that as a group the predictions made before Planck agree well with what has been seen, and draw conclusions from this about what is likely to mean as sensitivity to primordial gravitational waves improves.

String Inflation After Planck 2013

TL;DR

The paper assesses Planck 2013 constraints on string inflation, arguing that while string models are intrinsically multi-field, many reduce to effective single-field behavior at horizon exit and largely agree with the observed and suppressed . It surveys open and closed string inflation scenarios, detailing how D3/anti-D3, inflection-point, DBI, axion monodromy, and various moduli-based constructions fare in the – plane and with non-Gaussianity bounds. It highlights that most string models predict a small tensor-to-scalar ratio and that non-Gaussianities are typically not observed, though certain mechanisms in multi-field setups can generate observable NG under specific conditions. The authors argue that Planck data favor simple effective descriptions but do not rule out the richer structure of string theory; they also stress that a future detection of primordial tensors would challenge a large portion of string-inflation models and help illuminate the underlying UV completion. Overall, the work provides a framework to interpret current constraints and to guide future model-building and observational tests within a quantum-gravity–consistent setting.

Abstract

We briefly summarize the impact of the recent Planck measurements for string inflationary models, and outline what might be expected to be learned in the near future from the expected improvement in sensitivity to the primordial tensor-to-scalar ratio. We comment on whether these models provide sufficient added value to compensate for their complexity, and ask how they fare in the face of the new constraints on non-gaussianity and dark radiation. We argue that as a group the predictions made before Planck agree well with what has been seen, and draw conclusions from this about what is likely to mean as sensitivity to primordial gravitational waves improves.

Paper Structure

This paper contains 13 sections, 7 equations, 1 figure.

Table of Contents

  1. Introduction
  2. Why consider such complicated models?
  3. Understanding acceleration
  4. A string-inflationary score card
  5. Open string inflationary models
  6. Closed string inflationary models
  7. Inflation using axions
  8. Inflation using moduli
  9. The $n_s$ vs $r$ plane
  10. Future prospects for measuring $r$
  11. Non-Gaussianity and emergent single-field models
  12. Late-epoch relics
  13. Summary

Figures (1)

  • Figure 1: Left panel: A comparison of WMAP constraints in the $n_s$-$r$ plane with several string models for $N_e\simeq 60$, taken from an ICHEP 2008 summary talk ICHEP2008. Right panel: The same comparison superimposed on the Planck constraints taken from Planck, with 'D3/$\overline{{\rm D3}}$ inflation' (yellow oval); 'D3/D7 inflation' (orange oval); 'closed-string inflation' (light green oval); 'Fibre inflation' (dark green oval) and 'Axion monodromy inflation' (cyan oval).