THE Universe lit up just 200 million years after it was born, according to evidence gathered by MAP. That’s a surprise for astrophysicists, who had thought that the cosmos was pitch black for half a billion years.
The darkness began when the fires of the big bang subsided and the Universe was filled with cool gases that emitted little radiation. Until now, no one knew when the dark age ended because any light reaching us now from the earliest objects would be too faint for our telescopes to detect. MAP has not seen this first light, but a subtle side effect.
The probe has found that the microwave background radiation is slightly polarised. This polarisation happened when microwaves scattered off free electrons. So something must have been pumping out ultraviolet radiation – enough to ionise hydrogen gas and release its electrons.
Advertisement
On small scales, the polarisation is blurred. That’s because microwaves from nearby points in the sky can travel to the same point and then scatter in our direction, so they overlap. But on larger scales, patches of sky bigger than 200 million light years across, MAP researchers saw a strong polarisation signal. That means that microwaves from such distant points had not had time to travel far enough to overlap with each other when they were scattered. Since microwaves travel at light speed, the Universe must have been just 200 million years old when polarisation happened, the team concludes.
So what created the Universe’s first light? It was probably a generation of giant stars. According to recent computer models, these would have been up to a few hundred times the mass of the Sun, and many millions of times brighter. But the same models predict that they shouldn’t have started to form for at least half a billion years after the big bang. So MAP may be telling us that the physics in these models is wrong.
Or maybe the light didn’t come from stars at all. Massive black holes born in the big bang or shortly afterwards may have begun to consume the gas around them, turning into brilliant sources of radiation.