Scientists from Tel Aviv University also imprinted a clear signature in radio waves which astronomers can now search for.
"One of the exciting frontiers in astronomy is the era of the formation of the first stars," said Professor Rennan Barkana of TAU's School of Physics and Astronomy, an author of the study.
"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," Barkana.
The new finding that cosmic heating occurred later than previously thought means that observers do not have to search as far, and it will be easier to see this cosmic milestone.
Cosmic heating may offer a way to directly investigate the earliest black holes, since it was likely driven by star systems called "black-hole binaries."
These are pairs of stars in which the larger star ended its life with a supernova explosion that left a black-hole remnant in its place.
Gas from the companion star is pulled in towards the black hole, 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.
"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," said Barkana.
"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," Barkana said.
The new discovery overturns the common view and implies that these radio telescopes may also detect the tell-tale signs of cosmic heating by the earliest black holes.
The study was published in the journal Nature.