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  • Open Access

Gravitational waves from spectator scalar fields

Marcos A. G. Garcia*

Sarunas Verner†

  • Institute for Fundamental Theory, Physics Department, University of Florida, Gainesville, Florida 32611, USA

  • *Contact author: marcos.garcia@fisica.unam.mx
  • †Contact author: verner.s@ufl.edu
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Phys. Rev. D 114, 023551 – Published 23 July, 2026

DOI: https://doi.org/10.1103/dxht-n25w

Abstract

We propose a mechanism for gravitational wave (GW) production sourced by spectator scalar fields during inflation. These fields, while not driving cosmic expansion, generate blue-tilted isocurvature fluctuations that naturally satisfy current cosmic microwave background (CMB) constraints at large scales while producing enhanced power spectra at smaller scales accessible to GW detectors. The resulting GW spectrum spans an exceptionally broad frequency range from 10−20 to 1 Hz, with amplitudes ranging from ΩGWh2∼10−19 to 10−14 depending on the reheating temperature and spectator field mass. Observable signals across multiple detector bands are possible in a restricted region of parameter space, mχ/HI within the isocurvature- and ΔNeff-allowed window and Treh, such that the spectator either provides all of the dark matter (stable case) or decays near matter-radiation equality (unstable case). Our analysis reveals multiple complementary constraints on spectator field parameters. For a high reheating temperature of Treh>1010  GeV, GW-induced limits on the effective number of relativistic species (ΔNeff) require mχ≳0.59HI, stronger than CMB isocurvature bounds alone (mχ≳0.54HI). The nonobservation of primordial B-modes by Planck provides stronger constraints, mχ≳0.64HI, with projected LiteBIRD sensitivity potentially reaching mχ≳0.67HI. This mechanism enables a unique multimessenger probe of beyond the standard model physics during inflation, providing simultaneous constraints on inflationary dynamics, dark matter production, and reheating through current and next-generation GW experiments.

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Phys. Rev. D 114, 023551– Published 23 July, 2026
  • Received 19 June 2025
  • Accepted 29 June 2026
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