During the first billion years of the Universe's history, the gradual cooling from the Big Bang was reversed, and much of the gas was re-heated and ionized. This event, known as reionization, happened in the epoch where the first stars and galaxies formed. Largely due to the challenges of observing galaxies that far back in time, much is still mysterious about it. Most of our understanding of reionization comes from theoretical models.
One new model, proposed by Anastasia Fialkov, Rennan Barkana, and Eli Visbal, suggests that energetic X-rays could have heated the primoridal gas to the point that reionization happened relatively rapidly. That's in contrast with other hypotheses, which predict a more gradual reionization process. The X-rays in the new model were emitted by systems that include neutron stars or black holes. The nicest feature of the new proposal is that it predicts a unique pattern in light emission from the primordial gas, which could conceivably be measured by current radio telescopes.
Roughly 380,000 years after the Big Bang, the Universe became transparent. Before that time, the ordinary matter—mostly protons and electrons, with a smattering of helium nuclei—was a hot plasma mixed with photons. The transition to transparency occurred when the temperature of the plasma dropped low enough that electrons could join protons to make hydrogen, freeing the photons that we now observe as the cosmic microwave background.
Some time after stable atoms formed, something reionized much of the gas in the Universe. Reionization happened somewhere between 150 million and one billion years after the Big Bang, the same era that saw the formation of the first stars and galaxies. However, this epoch is challenging to observe: astronomers have yet to see examples of the first generation of stars, and the great distance means that only a few of the earliest galaxies have been seen. Those were only visible thanks to magnification by gravitational lensing.
The culprits behind the reionization likely included energetic ultraviolet radiation from newborn stars in the first galaxies as well as X-ray light from the remnants of the first generation of stars to die. According to various models, high-mass stars formed in large numbers early in the Universe's history, but something that massive explodes in a supernova after a relatively short time. Many of the remains—neutron stars and black holes—would form binary systems with stars or other remnants, generating huge numbers of X-ray photons as they fed on stray matter.
Typical models predicted that these X-ray binaries would heat the primordial gas rapidly, creating bubbles of plasma surrounded by neutral material. However, the new model shows that the kind of high-energy X-ray photons would be very inefficient at transferring energy to the gas and would therefore heat it far more slowly. That means reionization occurred toward the end of the epoch rather than spreading out through the entire period. Additionally, reheating would be more uniform in this model, creating a smoother mix of plasma than the bubbles previous ideas had suggested.
The new predictions in this model have immediate observational consequences. For instance, the relatively late occurrence of reionization means the neutral gas was a lot cooler than in other models, leading to increased absorption that could be measured. While this signal would be very small—a few thousandths of a degree—several current and proposed radio telescopes could reach that level of sensitivity in the next few years. If this signature of neutral gas is measured, it would be a strong hint that X-ray binaries were responsible for a lot of reionization.
With the advent of observatories designed to study the epoch of reionization, it's more important than ever to get the theoretical foundation for that era right. After all, the best models help us guide and interpret observations. Without astronomical data, models were free to make assumptions that may or may not be justified. It's always better to have a variety of predictions available when the data starts coming in.