The largest known black holes are around 40 billion solar masses — ultramassive objects at the centers of the most massive galaxies, grown through billions of years of gas accretion and galaxy mergers. But the astrophysical processes that build them don't obviously stop at 40 billion. If black holes exist with masses above a trillion solar masses — stupendously large black holes, or SLABs — they would be too large to have formed through any known stellar or galactic pathway. They would require primordial origin: direct collapse in the early universe, or growth from primordial density fluctuations, or some other mechanism that doesn't begin with stars.
Lacki (arXiv 2602.22587, February 2026) proposes using the cosmic microwave background as a backlight to detect or constrain them.
A black hole between us and the last scattering surface absorbs CMB photons. It creates a cold spot — a shadow — whose angular size is set by the black hole's Schwarzschild radius projected on the sky. For a trillion-solar-mass black hole, this shadow is tiny but potentially detectable with current microwave surveys. The surprising result is that the shadows become easier to detect at higher redshifts. A SLAB at redshift 5 subtends a larger angle than the same mass at redshift 1, because the angular diameter distance peaks around redshift 1.6 and decreases at higher redshifts — objects beyond this turnover appear larger, not smaller, on the sky.
This inverts the usual observational problem. Most astronomical objects are harder to detect at higher redshift because they become fainter and smaller. SLABs detected through CMB shadows become more prominent at higher redshift, at least in angular size. The signal strength also benefits: the CMB is the same temperature in every direction to one part in a hundred thousand, providing a uniform backlight against which any absorber stands out. The search becomes a hunt for negative point sources in microwave maps.
The non-detection in existing surveys already constrains the population. No SLAB above 10^17 solar masses exists within the observable universe, or we would have seen its shadow. For masses between 10^15 and 10^18 solar masses, the density parameter — the fraction of the critical density contributed by SLABs — is below 10^-5. These are not tight constraints on individual objects, but they rule out scenarios where SLABs are common.
The method is clean because it requires no assumptions about the black hole's environment. An isolated black hole in intergalactic space, accreting nothing, emitting nothing, completely invisible to every electromagnetic survey, would still cast a shadow on the CMB. The backlight is guaranteed. The only requirement is that the black hole exists and that photons from the last scattering surface pass near enough to be absorbed. The technique detects objects through their absence — not through what they emit but through what they block.