CMB Analysis Excludes Ultramassive Primordial Black Holes, X‑ray Microlensing Proposed for Tiny Ones

Overview

• The Johannes Gutenberg University Mainz study published Monday, November 9, 2026, finds that a large population of extremely massive primordial black holes would have left isocurvature imprints in the cosmic microwave background that conflict with Planck data.
• That exclusion covers objects from roughly 10^8 to 10^13 solar masses under the study’s assumptions that all PBHs share a single mass and are randomly distributed, and the authors caution different mass functions or spatial correlations need separate checks.
• Researchers at the Indian Institute of Science propose using brief microlensing brightening of X‑ray pulsars, notably SMC X‑1, to search for very low‑mass PBHs in the 10^-16 to 10^-11 solar mass range.
• Practical tests of the X‑ray method are stalled because existing NICER data are insufficient, NICER operations were suspended after an alignment fault with disposal planned, and the larger STROBE‑X mission remains an unselected concept.
• Both teams stress that many PBH mass windows remain open and that upcoming or proposed instruments such as LiteBIRD, the Simons Observatory, and targeted gravitational‑wave searches will be needed to further tighten or close those gaps.