Enhanced Matter Power Spectrum from Axion Kination after Big Bang Nucleosynthesis

TL;DR

This work investigates axion kination cosmologies with a post-BBN period of matter domination followed by kination, and assesses their impact on small-scale structure. By implementing two microphysical models—a log potential and a two-field axion—within perturbation theory and cosmological Boltzmann codes, the authors show that BBN and CMB data permit up to about a decade of modified expansion, producing distinct features in the matter power spectrum: a modest bump and suppression in the log model, and a broad, high-amplitude plateau in the two-field model on scales $1/\,\mathrm{Mpc}\lesssim k\lesssim 10^3/\mathrm{Mpc}$. These enhancements could accelerate early structure formation and potentially connect to JWST-era hints of early massive objects, while being constrained by Lyman-$\alpha$, dwarf galaxy, and spectral-distortion observations. The results highlight a concrete link between exotic post-BBN expansion histories and small-scale observables, offering new avenues to probe baryogenesis-related physics through cosmic structure formation.

Abstract

Despite stringent constraints from Big Bang Nucleosynthesis (BBN) and cosmic microwave background (CMB) observations, it is still possible for well-motivated particle physics models to substantially alter the cosmic expansion history between BBN and recombination. In this work we consider two different axion models that can realize a period of first matter domination, then kination, in this epoch. We perform fits to both primordial element abundances as well as CMB data and determine that up to a decade of late axion domination is allowed by these probes of the early universe. We establish the implications of late axion domination for the matter power spectrum on the scales $1/\mathrm{Mpc}\lesssim k \lesssim 10^3/$Mpc. Our 'log' model predicts a relatively modest bump-like feature together with a small suppression relative to the standard $Λ$CDM predictions on either side of the enhancement. Our 'two-field' model predicts a larger, plateau-like feature that realizes enhancements to the matter power spectrum of up to two orders of magnitude. These features have interesting implications for structure formation at the forefront of current detection capabilities.

Figures (14)

• Figure 1: Energy density as a function of (decreasing) temperature $T$, for radiation (red), the two-field model with $r_P=3$ (blue), and the log model (orange). The black line shows scalings of $\rho_\theta \propto T^3$ and $\rho_\theta \propto T^6$ at early and late times, respectively.
• Figure 2: Equation of state $w$ (solid line) and speed of sound squared $c_s^2$ (dashed line) as a function of scale factor $a$ normalized to the scale factor $a_{1/3}$ when $w=1/3$, for the two-field model with $r_P=3$ (blue) and the log model (orange). The equation of state corresponding to the black line in Fig. \ref{['fig:rho']} would be a unit step function centered at $a/a_{1/3}=1$.
• Figure 3: In both panels, blue (orange) curves are for two-field (log) model, while the black lines show the approximate scalings Co:2021lkc. Left: The temperature $T_{1/3}$ at which $w=1/3$ as a function of the radiation-to-matter domination transition temperature $T_{\rm RM}$ and the kination-to-radiation domination transition $T_{\rm KR}$ temperature. Right: Temperature ratios as functions of the ratio of the rotation energy density to that of radiation when $w=1/3$. The black lines show the scaling of the $T$ ratio $\propto (\rho_\theta / \rho_R |_{a = a_{1/3}})^{3/2}$ (solid) and $(\rho_\theta / \rho_R |_{a = a_{1/3}})^{1/2}$ (dashed).
• Figure 4: Left: Ratio of the matter power spectrum in the two-field model with $r_{\rm P} = 3$ to the prediction of the standard cosmology ($T(k) = P_{\rm kin}(k)/P_{\rm \Lambda CDM}(k)$) with fixed $T_{\rm KR} = 25 \ {\rm eV}$ and varying $T_{\rm RM}$; colors correspond to different total durations of axion domination as indicated in the legend. The black vertical line indicates $k_{\rm KR}$, while colored dotted vertical lines indicate $k_{\rm RM}$ for the correspondingly colored curve. Right: Same as left except for fixed $T_{\rm RM} = 5 \ {\rm keV}$ with varying $T_{\rm KR}$. Here the black line indicates $k_{\rm RM}$, while colored dotted lines indicate $k_{\rm KR}$.
• Figure 5: Evolution of conformal Newtonian gauge CDM perturbations $\delta_\chi$ (left) and metric perturbations $\phi$ (right) normalized to $\Phi_p$, the primordial curvature perturbation, with scale factor in a cosmology with axion domination in the two-field model (solid) and $\Lambda \rm{CDM}$ (dashed), for $T_{\rm KR} = 25 \ {\rm eV}$ and $T_{\rm RM} = 26.7~ T_{\rm KR} = 0.67$ keV. Colors indicate different comoving wavenumbers $k$ as shown in the legend.