‘Green Pea’ Galaxy Offers New Clues Into What Ended the Universe’s Dark Age

Soon after the Big Bang, the universe turned dark.

Not long after the Big Bang, the universe abruptly went dark. All of that early-days heat and light was subsumed into what cosmologists call the "dark ages."

For its first few hundred thousand years, the universe was a very bright place, an intense, opaque plasma of free electrons and naked, electron-less atomic nuclei. (That's what a plasma is: a bunch of atoms with their electron parts sheared away.) This state was enabled by the very high energies found within the early universe—electrons here didn't need atomic homes and could instead just haul ass around the new universe without needing to ever relax. Soon enough, however, the plasma became just a bunch of neutral atoms, and space became relatively "dark."

A few hundred million years later, the dark ages lightened up. Something forced a period of reionization, where once again the universe lit up with radiation. This something has largely remained a mystery. But a new paper in Nature from researchers at the University of Geneva offers an answer, in the form of what are known as green pea galaxies: relatively tiny galaxies that nonetheless emit loads of photons. These photons, the researchers believe, would have caused the reionization that ended the dark ages and led to the universe as we see it.


The Geneva team began its work by poring through data on some one million galaxies noted in the Sloan Digital Sky Survey, from which they distilled a set of around 5,000 galaxies appearing to meet the green pea criteria (small, lots of radiation). From these 5,000, the researchers picked just five for their experiment. Next, they turned to the Hubble telescope, which is capable of detecting UV radiation.

What they found is galaxy J0925, a small enough galaxy spewing photons with unprecedented intensity. "The galaxy is leaking ionizing radiation with an escape fraction of about 8 percent," the Geneva team reports. "The total number of photons emitted during the starburst phase is sufficient to ionize intergalactic medium material that is about 40 times as massive as the stellar mass of the galaxy."

The radiation of the reionization period is still around. The diffuse gas between galaxies known as the intergalactic medium (IGM) is almost completely ionized, and it's been this way since the event or chain of events that lit up the dark ages just after the Big Bang. This brightening was the last major phase transition in the universe and we'd really like to know more about it.

"We now know, from the scattering of cosmic microwave background photons by reionized electrons and from observations of the absorption by neutral hydrogen in the spectra of extremely distant quasars, that reionization was a gradual process, the midpoint of which occurred approximately 400 million years after the Big Bang," explains cosmologist Dawn K. Erb in a "news and views" article accompanying the study. "However, we have not yet observed the changing ionization state of the IGM directly," Erb writes, "and theoretical models struggle to explain how the known population of galaxies at this epoch could have produced enough radiation for reionization to occur."

To supply the radiation required for reionization to occur, a large number of faint galaxies would have been required. The catch is that for photons to escape these galaxies and go on to ionize atoms, they needed to avoid all of the neutral hydrogen atoms likely to be calling a star-forming galaxy home. These atoms are like photon insulation, absorbing much of the radiation that would otherwise make it out.

The fact that most of the radiation in question, what's known as Lyman continuum radiation, will never reach our telescopes, complicates the matter. The photons will mostly have been absorbed by the IGM, save for those originating within very nearby galaxies. It then becomes very difficult to discern between reionizing radiation and all of the other radiation we'd expect to be spewing from local galaxies.

The answer seems to be in the aforementioned green pea galaxies, which should be kicking out enough reionization radiation from a small enough place to register in data collected by the Hubble Space Telescope.

The work represents a crucial step toward understanding some of the more elusive mechanics of cosmic evolution. It's just a single galaxy, yes, but the James Webb Space Telescope, when it launches in 2018, will offer many more such revelations—and the role of green pea galaxies in brightening our universe may become clearer yet.