Invisible Black Holes Explain the Expansion of the Universe ✛
The universe cooled like a giant bathtub after a hot start. 400,000 years after the Big Bang, hydrogen was ready to turn neutral — protons were catching electrons. But in this dark water, a tap kept dripping. The drops were primordial black holes — microscopic black holes left over from the universe’s first moments. Their mass is that of a small asteroid, crushed into a point. These holes slowly evaporated through the process discovered by Stephen Hawking (Hawking radiation), adding heat to the gas. Each drop delayed the moment the universe became transparent to the cosmic microwave background — the ancient light reaching us from that era.
This lag helps solve the mystery of the expansion of the universe rate. Two precision methods give different numbers. Adam Riess and colleagues, observing stars in nearby galaxies, get a speed of about 73 km/s per megaparsec. The Planck satellite, studying the cosmic microwave background, infers 67 km/s per megaparsec. The difference is nearly 9%—too much to be an error. But if primordial black holes were heating the gas, then the surface from which the light came shifted. This changes the rate calculation, as if a thermometer in the tub showed the wrong temperature because of the drip heating. The heat from the holes doesn’t push galaxies apart faster: it throws off the calibration of our cosmic ruler.
Scientists tested the idea by adding heating into calculations of the universe’s thermal history. Gamma radiation from the holes quickly transferred energy to the surrounding gas, while some energy was carried away by neutrinos — light particles that barely interact with matter. It turned out that if just 0,1% of the invisible dark matter consists of such holes, the discrepancy in the rate vanishes. However, the holes can’t be too light: they would have evaporated before the era of nucleosynthesis. And they can’t be too numerous — otherwise they would cause Reionization of all the hydrogen, and we don’t see that. So these droplets from the universe’s childhood might be the key to a precise map of the cosmos.
The warming from an evaporating black hole doesn’t depend on its mass: a large one heats a larger volume, just as a thick log and a splinter heat water equally.
In science fiction, such as Greg Egan’s novel “The Eternal Flame,” evaporating black holes serve as energy sources. Here, they act as cosmic heaters.