Physicist Hunts Galaxy-Sized Black Holes via CMB Shadows
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- Brian Lacki, an astronomer at Oxford's Breakthrough Listen project, proposed detecting SLABs by searching for temperature dips and bumps they would create in the cosmic microwave background
- The analysis found no evidence of SLABs and ruled out black holes larger than 100 quadrillion solar masses existing within the observable universe
- Bernard Carr at Queen Mary University London coined the term SLABs in 2020, proposing they formed as primordial black holes from rare density fluctuations shortly after the big bang
- Primordial black holes are linked to dark matter research — they remain a candidate for the mysterious substance that makes up 85% of all matter in the universe
- Dyson swarms and related SETI ideas shaped Lacki's thinking on SLABs — he theorized advanced alien civilizations might use galaxy-sized black holes as heat sinks for computation
- Earlier searches for SLABs, including gravitational effects on surrounding galaxies and radiation from infalling matter, have turned up nothing
Why it matters: For cosmologists studying dark matter and supermassive black hole origins, this search method adds a fresh observational window — and the negative result already constrains the maximum size of any black hole in the observable universe. Lacki notes a confirmed SLAB would point to unknown physics from shortly after the big bang.
