Some theoretical models for the early Universe predict a spike-type enhancement in the primordial power spectrum on a small scale, which would result in forming early-formed dark matter halos (EFHs). Some recent studies have claimed to have placed limits on such small scales, which, however, involve uncertainties, such as the physics of substructures and the halo-galaxy relations. In this work, we study the cosmic microwave background (CMB) lensing effect, considering the existence of EFHs, and investigate the potential to probe the EFHs and the primordial perturbations on scales smaller than 1 Mpc, complementing these previous studies. We numerically calculate the angular power spectrum of the lensing potential and the lensed CMB anisotropy of temperature, E-mode, and B-mode polarization, including the nonlinear effects of EFHs. We find the possibility that the lensed CMB temperature anisotropy is significantly enhanced on small scales, $\ell >1000$, and could be tested by component decomposition of observed signals through multifrequency observations. Through the calculation with different models of the spiky-type power spectrum, we demonstrate that the accurate measurements of the CMB lensing effect would provide insight into the abundance of EFHs within the limited mass range around ${10}^{11}{M}_{\odot }$ and the primordial power spectrum on the limited scales around $k\sim 1{\mathrm{Mpc}}^{-1}$. In particular, we find that the existence of such EFHs can amplify the lensed anisotropy of CMB B-mode polarization even on large scales, $\ell <100$, as the overall enhancement by $\sim 5\%$ level compared to the standard structure formation model without EFHs. Therefore, future CMB measurements, such as the LiteBIRD satellite, can probe the existence of the EFHs and the spike-type primordial power spectrum through the precise measurement of the large-scale CMB B-mode polarization.

• Received 8 January 2024
• Accepted 22 April 2024

DOI:https://doi.org/10.1103/PhysRevD.109.103524