Cosmic Hot Tap: How Primordial Black Holes Rewrite the Speed of the Universe ✛

‹›

The universe cooled like a giant bathtub after a hot start. At 400 000 years after the Big Bang, hydrogen was ready to recombine — protons capturing electrons to become neutral atoms. But somewhere in this dark water, a tap kept dripping. The role of drops was played by primordial black holes — microscopic objects with masses of order 10⁻¹⁸ solar mass, left over from the first moments of the cosmos. These holes slowly evaporated, emitting energy through the quantum process predicted by Stephen Hawking, and each such drop added heat to the cooling gas. Even rare drops noticeably delayed the moment when the universe became transparent to light — the very moment imprinted in the cosmic microwave background.

This delay in recombination could resolve a long-standing mystery — the Hubble tension. The two most precise methods for measuring the expansion rate of the universe give different numbers. Adam Riess and his team, observing Cepheids and supernovae in nearby galaxies, get about 73 km/s/Mpc. The Planck satellite, on the other hand, looks at ancient light and derives 67,4 km/s/Mpc. The discrepancy is nearly 9% — too much for random error. But if primordial black holes heated the gas, the surface of last scattering, where the cosmic microwave background comes from, shifts. And with it, the calculated value of H₀. The heat from black holes does not push galaxies apart faster — it changes the calibration of our cosmic thermometer.

Astrophysicists Batten and Mould tested the idea by modifying the recfast code, which calculates the thermal history of the universe. They added heating from evaporating PBHs in the mass range 10⁻²⁰–10⁻¹⁷.⁵ M☉, when Hawking radiation is most energetic. The mechanism is simple: hard Gamma radiation scatters off electrons, quickly transferring energy to the gas. Losses to neutrinos, which carry away up to half the energy, are accounted for. Lighter holes (<10⁻¹⁹ M☉) evaporate before the epoch of nucleosynthesis and are constrained by its observations. The efficiency is almost independent of photon energy, so even a modest fraction of holes in dark matter has a noticeable effect. If just 0,01% of dark matter consists of PBHs, the surface of last scattering shifts by Δz ≈ 13 — increasing H₀ by 1,8%. And a fraction of 0,1% with a certain mass function shifts z by 64 and raises H₀ by 8,9%, fully eliminating the discrepancy. However, too high a concentration of holes — more than 1% of dark matter — would have reionized all hydrogen, which is not observed. More massive PBHs that survive to the epoch of the first stars are ruled out by absorption lines in intergalactic gas.

This whole story turns the problem on its head: perhaps the resolution of the Hubble tension lies not in new physics of dark energy, but in the composition of dark matter. If primordial black holes indeed make up even a small fraction, then the inference of H₀ from the cosmic microwave background loses its calibration reliability. Local measurements at z ≲ 1 are cleaner — they do not depend on the non-gravitational properties of the dark component. The early universe's bathtub turns out to be sensitive to every extra drop. Ahead: refining gray-body factors that weaken the flux, and a full-scale analysis with varying cosmology using Monte Carlo methods. Searching for the gamma-ray background and the positron annihilation signal in the Galaxy will either strengthen the hypothesis or close it. Thus, small black holes from the universe's infancy remind us of themselves today — as drops that shifted the moment of transparency of the cosmic bath.