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Gravitational waves at interferometer scales and primordial black holes in axion inflation (1610.03763v3)

Published 12 Oct 2016 in astro-ph.CO, gr-qc, hep-ph, and hep-th

Abstract: We study the prospects of detection at terrestrial and space interferometers, as well as at pulsar timing array experiments, of a stochastic gravitational wave background which can be produced in models of axion inflation. This potential signal, and the development of these experiments, open a new window on inflation on scales much smaller than those currently probed with Cosmic Microwave Background and Large Scale Structure measurements. The sourced signal generated in axion inflation is an ideal candidate for such searches, since it naturally grows at small scales, and it has specific properties (chirality and non-gaussianity) that can distinguish it from an astrophysical background. We study under which conditions such a signal can be produced at an observable level, without the simultaneous overproduction of scalar perturbations in excess of what is allowed by the primordial black hole limits. We also explore the possibility that scalar perturbations generated in a modified version of this model may provide a distribution of primordial black holes compatible with the current bounds, that can act as a seeds of the present black holes in the universe.

Citations (227)

Summary

  • The paper demonstrates that axion inflation models can produce detectable gravitational wave signals with unique chirality and non-Gaussian features.
  • It employs pseudo-scalar couplings to amplify gauge fields, mapping the parameter space for interferometer-scale observations.
  • The study connects gravitational wave detection prospects with primordial black hole constraints, guiding future observational strategies.

Overview of Gravitational Waves at Interferometer Scales and Primordial Black Holes in Axion Inflation

The paper under consideration presents an in-depth analysis of the potential for detecting a stochastic gravitational wave (GW) background generated by axion inflation models and investigates its implications for primordial black hole (PBH) formation. The paper is anchored on the prospects of detecting such a GW signal at terrestrial and space interferometers (e.g., Advanced LIGO, Virgo, KAGRA, LISA) and through Pulsar Timing Array (PTA) experiments. This work provides a comprehensive examination of the distinct properties of the originated signals, emphasizing their potential to infer aspects of inflationary physics that are not accessible via cosmic microwave background (CMB) or large-scale structure (LSS) data.

In exploring the axion inflationary models, the authors focus on the amplification of gauge fields through pseudo-scalar couplings. These amplified fields produce distinctive GW signatures characterized by chirality and non-Gaussian features. The paper explores the potentiality and conditions under which these GW signals might be detected without breaching existing PBH abundance constraints, which provide an essential touchstone for preventing the overproduction of scalar perturbations.

Detection Prospects and Model Parameterization

The analysis begins with a synopsis of inflationary constraints observable through a range of experimental platforms, mapping out the parameter space primarily as a function of the number of e-folds before the end of inflation. The research articulates that current experimental setups only explore a limited portion of the inflationary period, urging the necessity to probe smaller scales where the proposed GW signals might reside. It suggests that the unique observational windows presented by these smaller scales motivate further exploration of inflationary models capable of producing observable GWs.

Focusing on models that augment the axion-inflaton interaction, the paper outlines the mechanics by which these interactions—and associated gauge field amplifications—translate into detectable GWs. Specific emphasis is placed on characterizing the resulting spectral profile of both scalar and tensor modes, offering numerical evaluations for fields beyond standard CMB/LSS observational scales.

Numerical Results and Theoretical Implications

The analysis explores variants of axion inflationary scenarios, accounting for different coupling strengths and considering the associated constraints from non-Gaussian scalar perturbations that influence PBH formation limits. It is evidenced that a substantive GW signal can emerge within the range accessible by future LISA observatories. However, for particular parametric conditions, these signals may not manifest strongly at the sensitivity afforded by Advanced LIGO.

Additionally, the paper entertains a scenario involving multiple gauge fields, suggesting that their parallel amplification could swath inflationary dynamics sufficiently to enhance the detectable GW amplitude while maintaining compliance with current PBH constraints.

Complex Interactions and Alternative Scenarios

Another intriguing dimension considered is the role of a secondary field distinct from the inflaton in generating local enhancements in GW production, as exemplified by the model employing a rolling pseudo-scalar field. Here, the authors engage a mechanism that could yield prominent tensor signals amidst localized bursts of scalar perturbations. This model, originally postulated to derive high tensor-to-scalar ratios at CMB scales, now extends its potential to GW production observable across interferometer scales.

Conclusions and Future Directions

The conclusions underscore the nuanced challenge of observing GWs of cosmological origin and outline the unique signatures of the signal that could distinguish it from astrophysical backgrounds—namely chirality and significant non-Gaussianity. These features, alongside PBH constraints, frame the broader implications of the research in validating inflationary physics beyond the realms explored by traditional CMB analysis.

Future investigations might fruitfully delve into the domains of numerical simulations to refine scalar perturbation predictions and assess potential observational correlations between GW signals and PBH distributions within cosmic volumes, enhancing our understanding of post-inflationary anisotropies.

The discussion outlined serves as a potential blueprint for interpreting upcoming GW observations in light of theoretical models that entertain non-minimal inflation dynamics. This integration holds profound opportunities for unveiling new facets of fundamental physics and solidifying our grasp of the early universe's structural blueprint.