Shadows of giants: constraints on stupendously large black holes from negative sources against the cosmic microwave background

Journal article

Stupendously large astrophysical black holes (SLABs) are hypothetical black holes with masses of more than a trillion Suns. Because observable consequences of their existence have only recently been seriously considered, there have been relatively few constraints on their abundance. This work motivates a simple yet powerful constraint on SLABs: their huge shadows are visible against the cosmic microwave background. SLABs could thus appear as negative sources in microwave data. In fact, the shadow of a SLAB with a fixed mass becomes easier to detect with increasing redshifts past 1.6 where the angular diameter distance stars falling. The limits are powerful enough to rule out SLABs of mass ≳ 1017 M⊙ within the last scattering surface, and imply ΩBH ≲ 10-5 for masses 1015–1018 M⊙. I also discuss the effects of accretion and their implications for the limits: SLAB growth, positive accretion luminosity, and obscuring material.

Authors: Lacki, BC

• Publication status: Published
• Peer review status: Peer reviewed
• Publisher: IOP Publishing
• Journal: Journal of Cosmology and Astroparticle Physics
• Volume: 2026
• Issue: 06
• Pages: 049
• Article number: 049
• Publication date: 2026-06-17
• Acceptance date: 2026-05-01
• DOI: 10.1088/1475-7516/2026/06/049
• EISSN: 1475-7516
• ISSN: 1475-7516
• Language: English
• Keywords: primordial black holes; GR black holes; CMBR theory; dark matter theory