When the universe was less than a billion years old, bright points already blazed within it: quasars. Some of them had plasma jets traveling at nearly the speed of light. If such a jet points straight at Earth, we see it as extremely bright. It is like a flashlight: it dazzles only when aimed at your eyes. The most distant such flashlight, the blazar VLASS J0410−0139, began to flicker.
Astronomers caught radio waves from this source twice using a telescope in India. Only 16 days passed between observations, as measured by the blazar's own clock, yet its light had changed. This means the jet is alive and rapidly evolving. From the rate of change, scientists deduced that the jet is boosted by at least a factor of 19. In other words, the source is intrinsically faint; we are simply lucky to be looking straight at it.
If there are many such faint quasars with jets, and we only see those aimed at us, then the early universe must have contained thousands of them. To find the rest, we need new radio telescopes that can detect much fainter objects. Then we will understand how many black holes ejected jets during the first billion years.
Doppler boosting δ>19.3 means that if the jet of J0410−0139 were not pointed at us but at a random angle, its radio flux would be thousands of times weaker, and we would simply not see it. In effect, we are looking down the barrel of a cosmic gun.
In Stanisław Lem's novel "Fiasco," the heroes try to make contact with a civilization hidden behind powerful natural radio sources resembling blazars. The variability of J0410−0139 makes it like a lighthouse that flashes and fades, as if someone were steering the beam.