The Observational Search for Hawking Radiation: Evaluating Constraints on Primordial Black Holes

Primordial Black Holes (PBHs) are a class of black holes which are thought to have formed in the early universe due to extreme density fluctuations. They are not created by the collapse of stars, which makes them different from stellar black holes. These black holes can span a wide range of masses, which makes them a strong candidate for explaining dark matter. In 1974, Stephen Hawking proposed that black holes emit thermal radiation and slowly lose mass over time through quantum evaporation. Consequently, black holes that were created with an initial mass around 5 × 10¹¹ kg should be reaching the end of their lives in the present epoch. This review examines the thermodynamics of how black holes lose mass and evaluates modern observational efforts to detect signatures of this process. Researchers have used space-based and ground-based observatories such as Fermi-LAT, INTEGRAL, and Voyager 1 to search for transient bursts of energy or diffuse background emissions. Even though observations have not yet identified any definitive signals of Hawking radiation, these null results establish stringent upper bounds on PBH abundance. Current constraints demonstrate that low-mass primordial black holes cannot exist in large abundances, meaning that PBHs smaller than 10¹⁴ kg cannot constitute a significant fraction of dark matter, whereas higher mass black holes in the asteroid mass window remain a viable possibility.