Exploring the viability of a pseudo-Nambu-Goldstone boson as ultralight dark matter in a mass range relevant for strong gravity applications

We study a simple extension of the Standard Model featuring a dark sector with an ultralight pseudo-Nambu-Goldstone boson as a dark matter candidate. We focus on the mass range <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mrow><a:mi mathvariant="script">O</a:mi><a:mo stretchy="false">(</a:mo><a:msup><a:mrow><a:mn>10</a:mn></a:mrow><a:mrow><a:mo>−</a:mo><a:mn>20</a:mn></a:mrow></a:msup><a:mi>–</a:mi><a:msup><a:mrow><a:mn>10</a:mn></a:mrow><a:mrow><a:mo>−</a:mo><a:mn>10</a:mn></a:mrow></a:msup><a:mo stretchy="false">)</a:mo><a:mtext> </a:mtext><a:mtext> </a:mtext><a:mi>eV</a:mi></a:mrow></a:math>, relevant for strong gravity applications, and explore its production and evolution in the early Universe. The model is formulated in such a way that dark matter does not couple directly to photons or other Standard Model particles avoiding some of the most stringent cosmological bounds related to axionlike particles. In this work, two different scenarios are considered depending on whether dark matter is produced in a preinflationary or postinflationary regime. We also discuss the effect from emergent topological defects such as cosmic strings and domain walls and estimate the spectrum of stochastic gravitational waves produced by their decay, enabling us to test the model at current and future gravitational-wave experiments. Published by the American Physical Society 2024