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We give updated constraints on hypothetical light bosons with a two-photoncoupling such as axions or axion-like particles (ALPs). We focus on masses andlifetimes where decays happen near big bang nucleosynthesis (BBN), thusaltering the baryon-to-photon ratio and number of relativistic degrees offreedom between the BBN epoch and the cosmic microwave background (CMB) lastscattering epoch, in particular such that $N_{\rm eff}^{\rm CMB} < N_{\rmeff}^{\rm BBN}$ and $\eta^{\rm CMB} < \eta^{\rm BBN}$. New constraintspresented here come from Planck measurements of the CMB power spectrum combinedwith the latest inferences of primordial $^4$He and D/H abundances. We findthat a previously allowed region in parameter space near $m=1\,\rm MeV$ and$\tau=100\,\rm ms$, consistent with a QCD axion arising from a symmetrybreaking near the electroweak scale, is now ruled out at $>3\sigma$ by thecombination of CMB+D/H measurements if only ALPs and three thermalized neutrinospecies contribute to $N_{\rm eff}$. The bound relaxes if there are additionallight degrees of freedom present which, in this scenario, have theircontribution limited to $\Delta N_{\rm eff}=1.1\pm0.3$. We give forecastsshowing that a number of experiments are expected to reach the sensitivityneeded to further test this region, such as Stage-IV CMB and SUPER-KEKB, thelatter a direct test insensitive to any extra degrees of freedom.

New Bounds for Axions and Axion-Like Particles with keV-GeV Masses