Gravitational Waves in Axion Inflationary Models
Summary
Theories of the early Universe propose a phase of accelerated expansion known as inflation, frequently driven by a pseudoscalar inflaton field termed the axion. In axion inflationary models, a Chern–Simons interaction couples the axion to gauge fields, causing one polarisation of the gauge field to undergo tachyonic growth. This asymmetric amplification sources tensor perturbations that emerge as primordial gravitational waves. The resulting spectra can display unique features such as parity violation, scale-dependent peaks and non-Gaussian signatures. Model parameters—particularly the axion decay constant and coupling strength—govern the amplitude and frequency window of the gravitational-wave background, which may span from cosmic-microwave-background scales to pulsar-timing and ground-based interferometer bands. Detecting these signals would offer direct insights into high-energy physics, the dynamics of particle production during inflation and the origin of dark matter through mechanisms like primordial black-hole formation.
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Gravitational Waves in Axion Inflationary Models publication trend
The graph below shows the total number of articles in gravitational waves in axion inflationary models across all publications each year (not limited to Nature Index journals).
Technical terms
Axion: A hypothetical pseudoscalar particle originally proposed to resolve the strong-CP problem and often invoked as the inflaton.
Inflation: A brief epoch of exponential cosmic expansion that seeds primordial perturbations.
Chern–Simons coupling: An interaction term coupling an axion to gauge-field strengths, responsible for polarisation-dependent amplification.
Backreaction: The influence of amplified gauge or tensor modes on the background inflaton dynamics.
Primordial gravitational waves: Tensor fluctuations generated during inflation that carry information about ultra-high-energy processes.
Non-Gaussianity: Departures from a Gaussian statistical distribution in cosmological perturbations, signalling non-linear generation mechanisms.
References
- Inflation from Multiple Pseudo-scalar Fields: Primordial Black Hole Dark Matter and Gravitational Waves. The Astrophysical Journal (2023).
- Lattice simulations of axion-U(1) inflation. Physical Review D (2023).
- Axion-gauge dynamics during inflation as the origin of pulsar timing array signals and primordial black holes. Physics Letters B (2024).
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