In both discoveries, the light he and his colleagues see comes from fluorescence, he says, “where the quasar shines at a range of energies onto a galaxy and the galaxy actually shines back.” Prochaska thinks that with the observing tools set to come online in the next five to 10 years, astronomers will find hundreds of quasars embedded in filaments and forming galaxy clusters.
To the early cosmos
The farther astronomers look from Earth, the less organization they see. That’s because they’re seeing the cosmos at an earlier stage, and modern-day structures — like spiral galaxies and dense clusters of thousands of galaxies — didn’t exist. The universe was not born complex. Instead, after the Big Bang, it was dense and hot, filled with electrons, protons, and light bouncing between those atomic pieces. The cosmos has been expanding since that moment 13.82 billion years ago.
Once it had expanded enough for the temperature throughout the universe to cool to about 4,900° F (3,000 kelvins), each proton grabbed a nearby electron to form a neutral hydrogen atom. With fewer particles floating around, the pinball game was over.
At that point, light was free to stream about the cosmos. That light has been traveling along the fabric of space-time ever since. Today, it bathes the sky in a cool microwave glow, its wavelength stretched by cosmic expansion.
This cosmic microwave background (CMB) looks nearly the same in every direction. It reveals to astronomers what the universe looked like just 380,000 years after the Big Bang: an almost featureless soup of hydrogen and helium.
The tiny differences in temperature it contains reflect tiny differences in density. Eventually, those denser areas grew into galaxies and galaxy clusters, while the least dense regions emptied.
“Understanding that transition, from a simple universe to something with interesting structure in it, is a crucial missing piece in astronomy,” says Steve Furlanetto of the University of California, Los Angeles.
This transformation happened in the astronomical Dark Ages when material wasn’t yet dense enough to form stars, which could light the way. When the first stars and the galaxies they congregated in formed, the overall mix developed into today’s cosmos. But the first galaxies, says Furlanetto, were up to a million times smaller than the Milky Way and lie so far away from us that telescopes cannot see them. Instead, astronomers search for these first objects by how they affected material around them.
Finding this intermediate range, which lies between the CMB (380,000 years into the universe’s history) and the quasars and galaxies that lived 1 billion years after the Big Bang, revolves around the most prevalent element in the cosmos: hydrogen. As stars and galaxies lit up, they spewed high-energy light. This radiation is powerful enough to knock away the one electron a hydrogen atom contains, creating a hydrogen ion.
Those light sources continued to emit energy, “and then bit by bit, the first sources carved cavities of ionized material,” says Saleem Zaroubi of the Kapteyn Astronomical Institute in the Netherlands. These cavities grew while more stars lit up, eventually ionizing all of the neutral hydrogen in their regions of space.
The key to spotting the transition is the prediction, from 1944, that neutral hydrogen can emit radio energy with a wavelength of 21 centimeters. Ionized hydrogen, however, doesn’t emit this radiation. Because neutral hydrogen filled the early cosmos, researchers expect there was enough of it to faintly glow as radio waves. This makes for a region nearer to us with no 21-centimeter signal and a stronger radiance farther away.