Primordial Black Holes: How Hawking Radiation Delays Recombination and Eases the Hubble Tension ✛
Context
Somewhere in the early universe, 400,000 years after the Big Bang, a cooling cloud of hydrogen suddenly heats up again—an invisible speck of dark matter evaporates, blasting out hard radiation. This speck—a primordial black hole—acts like a tiny immersion heater, preventing hydrogen from recombining. A black hole with a mass of 10⁻¹⁸ M☉ (roughly that of a small asteroid) evaporates over hundreds of thousands of years. This isn't science fiction; it's a possible key to the 'Hubble tension'—a 5σ discrepancy between measurements of the universe's expansion rate. Adam Riess and colleagues, using Cepheids and supernovae, get around 73 km/s/Mpc, while the Planck satellite, analyzing the cosmic microwave background, yields 67.4 km/s/Mpc, following Hubble's law. Evaporating through Hawking radiation predicted by Stephen Hawking, primordial black holes might have warmed the gas, like a dripping tap in a cooling bath. Studying this scenario not only seeks to solve the puzzle but also places constraints on dark matter itself.
Methods
To test the hypothesis, the researchers modified the recfast code, adding the energy from evaporating primordial black holes (PBHs) into the thermal balance equations. They considered a mass range of 10⁻²⁰–10⁻¹⁷·⁵ M☉, where black holes actively lose mass during recombination. The main heating channel is secondary ionization: hard Gamma radiation scatters off electrons and produces pairs, rapidly transferring energy to the gas. Efficiency hardly depends on photon energy. The team varied the PBH fraction in dark matter (from 10⁻⁶ to 10⁻²), the shape of the initial mass function (power-law spectra M⁻¹ and M⁻²), and the maximum mass. For realism, they accounted for losses to neutrinos, which carry away up to 50% of the energy, and greybody factors that weaken the flux. Lighter PBHs (<10⁻¹⁹ M☉) evaporate before nucleosynthesis and are constrained by primordial nucleosynthesis.
Results
Simulations showed that even a modest PBH fraction in dark matter—10⁻⁴ Ω_C—shifts the surface of last scattering by Δz ≈ 13, raising H₀ by 1.8%. At a fraction of 10⁻³ Ω_C with an M⁻¹ mass function, the shift reaches Δz ≈ 64, and H₀ increases by 8.9%. The discrepancy vanishes. A steeper M⁻² mass function amplifies the effect by a factor of 6.4. However, at fractions above 10⁻² Ω_C, black holes cause complete re Reionization, which contradicts observations. PBHs with masses >10⁻¹⁷·⁵ M☉ survive to z < 100 and are ruled out by Lyman-alpha forest data. Computing the cosmic microwave background spectrum with CAMB showed that the polarization differences between models are only 0.6%. Here, expansion of the universe acts as an indirect probe: we see its speed through the lens of thermal history.
Implications
The work shows that a hypothetical population of evaporating PBHs can tune the H₀ value without new physics. This changes the nature of the problem: it's now a question for dark matter. If even a small fraction of dark matter consists of PBHs, the H₀ inferred from the cosmic microwave background loses its calibration reliability. Consequently, local measurements of the Hubble constant at z ≲ 1, based on Hubble's law, turn out to be more direct and less dependent on the non-gravitational properties of dark matter.
Future development
Next, greybody factors need refinement via the GrayHawk code—they could weaken the heating by two orders of magnitude. Detailed joint analyses of varying cosmology and PBHs using Markov Chain Monte Carlo are needed. Another interesting avenue is searching for the 511 keV signal from positron annihilation in the Galaxy. If data rule out PBHs as the dominant form of dark matter, the set of models to explain the tension will narrow.
Impact
Will impact cosmology (H₀ measurement), dark matter physics, and early universe models.
Next steps
Carry out a joint analysis with varying cosmology. Include detailed gas cooling. Test predictions against the gamma-ray background.
Key open problems
The article directly addresses the nature of dark matter and the origin of primordial black holes. If PBHs explain the Hubble tension, it strengthens the hypothesis that not all dark components are elementary particles. In the bathtub of the early universe, even a faint drip from evaporating black holes can noticeably delay the moment when the water becomes transparent to photons. The mechanism of PBH formation during inflation remains an open question.
The knowledge machine's view
This section is addressed to the author of the paper and their colleagues. Neighbouring works were found by meaning-based search across our archive, not by word overlap. We do not review: we show the field around the work.
What the work does
This study examines how evaporating primordial black holes (PBHs) affect the thermal history of recombination. The recfast code was modified to include heating from secondary ionization by hard radiation, and the mass and abundance of PBHs in dark matter were varied. It is shown that a PBH fraction of 10⁻³ Ω_C with a mass function of M⁻¹ increases H₀ by 8.9%, alleviating the Hubble tension.
Where its strength is
The strength of this work lies in directly calculating the impact of PBH evaporation on the shift of the last scattering surface and H₀, accounting for realistic neutrino losses and gray-body factors. This provides a quantitative tool for joint analysis with CMB data.
Where it could go next
- It might be interesting to see how PBH heating affects the formation of the first stars: there is a related work on dark stars, where massive black holes are born in the centers of dark matter halos. Your additional heating mechanism could alter the collapse conditions.based on: Dark Stars: Cradle of Giant Black Holes arXiv:2511.08578
- In our archive, there is a study where PBHs leave a trace in the gamma-ray background. It is worth comparing your constraints on the PBH fraction with their gamma-ray predictions—this could narrow the parameter space.based on: Invisible Black Holes Leave a Mark in Gamma-Ray Light arXiv:2606.13788
- A work on long-lived black holes shows how slow evaporation is audible in the radio signal from the dark ages. Your approach to heating could be extended to later epochs to check whether your models contradict 21-cm line data.based on: Long-lived black holes: how their slow boiling is heard in a radio signal from the past arXiv:2512.06941
Worth trying
- Investigate how additional PBH heating changes the halo mass threshold for the formation of the first stars, using a dark star model.
- Compare the predicted gamma-ray flux from evaporating PBHs in your scenario with the observational background to constrain the allowed abundance.
- Calculate the 21-cm signal for your heating models to see whether PBH effects are distinguishable from standard reionization.
Why this may matter
If PBH heating indeed shifts recombination and alters H₀, it links early Universe physics to local measurements and also affects the formation of the first structures. Joint analysis with gamma-ray and radio observations could turn PBHs from a hypothetical solution into a testable component of dark matter.
Nearby works in our archive
- The Gentle Boiling of the Cosmossimilarity 0.69
- Invisible Black Holes Leave a Mark in Gamma-Ray Lightsimilarity 0.69
- Black Holes from the Universe's Infancysimilarity 0.69
- How the First Black Holes Are Bornsimilarity 0.67
- Long-lived black holes: how their slow boiling is heard in a radio signal from the pastsimilarity 0.67
- Dark Stars: Cradle of Giant Black Holessimilarity 0.67
The closest work in the archive sits at 0.69 by meaning — the usual level for our corpus.
Original abstract
as written by the authors arXiv:2607.18528v1
Primordial black holes (PBHs) evaporate through Hawking radiation, emitting high-energy photons and ionising their surrounding environment. As contributors to the density of dark matter, Omega_C, if 10^-18 solar mass PBHs are present in the Universe they delay recombination and move the surface of last scattering of the cosmic microwave background (CMB). We perform recombination simulations using the software Recfast, and calculate the PBH fraction of dark matter required to resolve the Hubble tension. We find that nominally a cosmic PBH energy density of Omega_PBH ~ 10^-3 Omega_C would cause an 8.9% increase in the value of H_0, enough to entirely reduce the tension between the early- and late-time observations. This PBH fraction is modified by Gray Body Factors affecting Hawking radiation. Furthermore, also fitting the CMB leaves the Hubble tension not fully relieved with our present PBH prescription. Until relevant non-gravitational properties of the dominant dark matter species are ruled out, we suggest that the hypothesis that the ionisation history of the universe matches the thermal history of the standard LCDM cosmology is too precarious to hang the expansion rate on, and that it is better to measure H_0 locally at z <~ 1.
The gas temperature change around an evaporated PBH doesn't depend on its initial mass: a more massive hole is simply surrounded by proportionally more gas. It's like a large log imparting the same heat per unit mass to a liter of water as a small splinter, if both burn completely.
The idea of primordial black holes as an energy source is played out in Greg Egan's novel 'The Eternal Flame,' where evaporating black holes are used as weapons. In Stephen Baxter's story 'The Moment,' the explosion of a primordial black hole threatens Earth. In this scenario, though, such black holes act as cosmic heaters, influencing the entire universe.
Key numbers
- Hubble constant (Planck): 67.37 km/s/Mpc
- Hubble constant (SH0ES): 73.2 km/s/Mpc
- required PBH fraction to resolve the tension: ~10⁻³ Ω_C
- redshift shift: Δz ≈ 64
- upper limit on PBH fraction: 10⁻² Ω_C