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This theoretical study examines whether quantum fluctuations of axion fields during cosmic inflation could produce the observed dark matter abundance instead of the conventional "misalignment mechanism." The researchers find that for standard axions, fluctuation-dominated scenarios violate observational constraints on cosmic microwave background isocurvature perturbations by eight orders of magnitude, but modified axion models with blue-tilted spectra could satisfy these constraints if the axion mass exceeds specific thresholds (ranging from 0.7 eV to 200 GeV depending on inflationary energy scale and model parameters). For the QCD axion specifically, they conclude that the misalignment mechanism remains the dominant production mechanism.
Why it matters
This work refines our understanding of how axion dark matter could have been produced in the early universe and identifies specific parameter ranges where alternative production mechanisms might be viable. The findings help constrain theoretical models of both dark matter and cosmic inflation by connecting axion properties to observable cosmological signatures.
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⚠️ Preprint – Noch nicht peer-reviewed
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Abstract: The relic abundance of light stable axion dark matter is usually set by misalignment. We study whether the inflationary fluctuations of the axion can dominate instead, and we follow the modes analytically through an instantaneous transition from de Sitter expansion to radiation domination, neglecting the axion mass during inflation and keeping it afterward. Since the energy density is quadratic in the field, a fluctuation-dominated abundance carries an isocurvature perturbation even when the mean displacement vanishes. For a canonical minimally coupled axion the spectrum is scale invariant, and fluctuation-dominated dark matter exceeds the Planck isocurvature limit by about eight orders of magnitude. An inflaton-dependent kinetic function or a nonminimal coupling to the Ricci scalar tilts the spectrum blue, moving the fluctuation power from CMB scales to small scales. Imposing the Planck limit at the pivot scale, we find that fluctuation-dominated dark matter with a constant kinetic slope requires the spectral tilt $n_eta-1gtrsim0.27$ and the axion mass $m_{eta} gtrsim200 rm{GeV}$ at $H_I=10^{10}rm{GeV}$, and $n_eta-1gtrsim0.34$ and $m_etagtrsim4.5,$keV at $H_I=10^{13}rm{GeV}$. A kinetic function with a running slope lowers these masses to $1.4~rm{GeV}$ and $0.7,$eV, respectively. For the QCD axion, whose oscillations start late, we estimate that any single power-law spectrum blue enough to satisfy the pivot limit gives too little fluctuation abundance, and misalignment dominates.
Source: Can inflationary axion fluctuations dominate over misalignment?