Early Universe Got Heated By Black Holes Much Later Than Previously Thought

The warming up of the ancient universe by the black holes that were formed from the early stars took place much later than previously thought, according to a new study, seen as a major finding about the origins of the universe.

The new study by the researchers from Tel Aviv University, or TAU, in Israel said that the black holes acting as companions to the first stars took longer to heat the gas throughout space, which means that astronomers could detect signs of the heating process, which was previously thought to be off limits.

“One of the exciting frontiers in astronomy is the era of the formation of the first stars,” Rennan Barkana of TAU's School of Physics and Astronomy and an author of the study said in a statement. “Since the universe was filled with hydrogen atoms at that time, the most promising method for observing the epoch of the first stars is by measuring the emission of hydrogen using radio waves.”

According to the study, published in the journal Nature, cosmic heating could help directly investigate the earliest black holes as it was likely driven by star systems called “black-hole binaries” -- pairs of stars in which the larger one ended its life with a supernova explosion that left a black-hole remnant in its place.

When gas from the companion star is pulled in towards the black hole, it gets ripped apart in the strong gravity and emits high-energy X-ray radiation. This radiation reaches large distances, and is believed to have re-heated the cosmic gas, after it had cooled down as a result of the original cosmic expansion. The discovery in the new research is the delay of this heating, according to the researchers.

“It was previously believed that the heating occurred very early, but we discovered that this standard picture delicately depends on the precise energy with which the X-rays come out,” Barkana said. “Taking into account up-to-date observations of nearby black-hole binaries changes the expectations for the history of cosmic heating. It results in a new prediction of an early time (when the universe was only 400 million years old) at which the sky was uniformly filled with radio waves emitted by the hydrogen gas.”

Several radio telescopes have been developed to observe the first stars and galaxies, under the assumption that cosmic heating occurred too early to see and the new arrays of telescopes can only search for a later cosmic event. But, the new discovery suggests that these instruments can also detect signs of cosmic heating by early black holes once thought to be out of reach.