[Paper Review] Gravitational waves from inflation

Maria Chiara Guzzetti, Nicola Bartolo+2 more|arXiv (Cornell University)|May 5, 2016

Cosmology and Gravitation Theories348 references173 citations

TL;DR

A comprehensive review of primordial gravitational waves produced during inflation, including vacuum and non-vacuum sources, reheating, modified gravity scenarios, and observational prospects."

ABSTRACT

The production of a stochastic background of gravitational waves is a fundamental prediction of any cosmological inflationary model. The features of such a signal encode unique information about the physics of the Early Universe and beyond, thus representing an exciting, powerful window on the origin and evolution of the Universe. We review the main mechanisms of gravitational-wave production, ranging from quantum fluctuations of the gravitational field to other mechanisms that can take place during or after inflation. These include e.g. gravitational waves generated as a consequence of extra particle production during inflation, or during the (p)reheating phase. Gravitational waves produced in inflation scenarios based on modified gravity theories and second-order gravitational waves are also considered. For each analyzed case, the expected power-spectrum is given. We discuss the discriminating power among different models, associated with the validity/violation of the standard consistency relation between tensor-to-scalar ratio $r$ and tensor spectral index $n_{ m T}$. In light of the prospects for (directly/indirectly) detecting primordial gravitational waves, we give the expected present-day gravitational radiation spectral energy-density, highlighting the main characteristics imprinted by the cosmic thermal history, and we outline the signatures left by gravitational waves on the Cosmic Microwave Background and some imprints in the Large-Scale Structure of the Universe. Finally, current bounds and prospects of detection for inflationary gravitational waves are summarized.

Motivation & Objective

• Summarize how inflation generates a stochastic background of gravitational waves and why it is a key probe of Early Universe physics.
• Outline the main mechanisms producing GWs during or after inflation, including quantum vacuum fluctuations, particle production, and reheating.
• Discuss how GW signals encode information about reheating, modified gravity, and the thermal history of the Universe, and how they imprint on CMB and LSS.
• Review current observational bounds on primordial GWs and prospects for future detections.
• Highlight the role of the tensor-to-scalar ratio r and the consistency relation with the tensor spectral index n_T as model discriminants.

Proposed method

• Present the standard single-field slow-roll inflation framework and derive the evolution equation for tensor perturbations and their power spectrum (2.3.1).
• Discuss the consistency relation r = -8 n_T and its implications for inflationary models (section 2.3.2).
• Catalogue second-order and post-inflationary (reheating) sources of GWs (sections 3–4).
• Survey GW production in modified gravity theories and generalized G-inflation (section 5).
• Link GW spectra to thermal history and CMB/LSS signatures (sections 9–10).
• Summarize current bounds and future observational prospects (section 11).

Experimental results

Research questions

• RQ1 What are the predicted primordial GW spectra in standard single-field slow-roll inflation and how can they be distinguished observationally?
• RQ2 What are the alternative mechanisms (second-order, particle production, reheating, modified gravity) that can generate or enhance primordial GWs during or after inflation, and how do their spectra differ?
• RQ3 How do reheating and the thermal history imprint on the present-day GW energy density and CMB/LSS observables?
• RQ4 Under what conditions can the standard consistency relation r = -8 n_T be violated or altered in non-standard inflationary scenarios?
• RQ5 What are the current observational bounds on r and primordial GWs, and what are the prospects for detection with upcoming experiments?

Key findings

• The standard single-field slow-roll inflation predicts a nearly scale-invariant tensor spectrum with r constrained by current data (r < 0.07 at 95% C.L. for k* = 0.05 Mpc^-1).
• Excluding temperature data, a bound r < 0.09 at 95% C.L. yields a present-day Omega_GW ~ 10^-15 at f ~ 10^-17 Hz.
• Additional GW backgrounds arise from extra particle production during inflation or during reheating, and from modified gravity scenarios.
• Second-order GWs sourced by scalar perturbations can contribute to the stochastic background, with signatures dependent on the sourcing mechanism (curvaton, spectator fields, etc.).
• Reheating/preheating can generate significant GW signals, with features tied to the specific preheating mechanism (parametric resonance, tachyonic preheating).
• Observational imprints include B-mode polarization in the CMB and potential direct detection prospects for certain spectra in future experiments.