The universe was born dark and cold, like a room with the lights off. Then someone started turning on the bulbs — the first stars. Around each such star, gas heated up and lost electrons, becoming transparent to light. These glowing regions grew like bubbles in boiling water and gradually merged until the entire cosmos became transparent.
Catching such a bubble directly is nearly impossible: it doesn't shine in the usual sense. But the neutral hydrogen around it gives it away. Hydrogen emits radio waves at a wavelength of 21 centimeters, and when an ionized bubble appears nearby, the signal at that spot vanishes, like a hole in the radio map of the sky.
Scientists have figured out how to quickly find such holes in the data from the future SKA radio telescope. Instead of sifting through all possible combinations of signals, they use a clever mathematical trick that saves hours of computation. It turns out that just a hundred hours of observations would be enough to see the bubbles with clarity as if they were photographed point-blank.
The new method not only finds bubbles but also measures their size, position, and how strongly the gas around them is still not ionized. It's like determining the shape and size of an object from its shadow on the wall. This way, astronomers hope to reconstruct the picture of how the first stars lit up the universe.
While light from the first stars traveled to us, it stretched by a factor of 8–9: the wavelength of the hydrogen radio signal increased from 21 centimeters to one and a half to two meters.
In Carl Sagan's novel "Contact," the characters receive a radio signal from a distant civilization — much like astronomers hope to hear the whisper of the first stars.