Early universe ‘heated up’ sooner than previously believed, says new study

Tel Aviv University researchers suggest cosmic heating may offer a way to directly investigate the earliest black holes.

By JUDY SIEGEL-ITZKOVICH
space 311 (photo credit: REUTERS)
space 311
(photo credit: REUTERS)
Tel Aviv University astrophysicists who spend their waking hours thinking about the Big Bang and the origin of the universe have developed a theory they believe will be proven true soon by using radio telescopes and antennas to measure the emission of hydrogen.
They published their work on Wednesday in the prestigious journal Nature.
Black holes – formed from the first stars in our universe – heated the gas throughout space later than previously thought, Prof. Rennan Barkana of TAU’s School of Physics and Astronomy, his former doctoral student Dr. Anastasia Fialkov of TAU and the École Normale Supérieure in Paris, and Dr. Eli Visbal of Columbia and Harvard universities wrote.
Astronomers explore our distant past billions of years back in time. Unlike Earth-bound archeologists, however, who can only study remnants of the past, astronomers can see the past directly. The light from distant objects takes a long time to reach the Earth, and astronomers can see these objects as they were back when that light was emitted. This means that if astronomers look out far enough, they can see the first stars as they actually were in the early universe.
Thus, 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.
Their work, described as a major finding about the origins of the universe and the target of much international interest, implies that cosmic heating imprinted a clear signature in radio waves that astronomers can now search for. “One of the exciting frontiers in astronomy is the era of the formation of the first stars,” explained the authors. “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.”
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 blackhole 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, said Barkana.
“It was previously believed that the heating occurred very early, but we discovered that this standard picture definitely depends on the precise energy with which the x-rays come out. Taking into account up-todate 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.”
Until now, he told The Jerusalem Post, “people knew that the universe started out very hot and dense, that it expanded and became less dense and hot. But at some point, it stopped cooling when stars and black holes formed, because these produced radiation that heated the universe.
Black-hole binaries emit x-ray radiation, which is high energy radiation. The x-rays hit cosmic hydrogen gas in the space between galaxies and heat it up.
“The hydrogen gas itself emits radio radiation that we can see, and this emission carries a signature of the heating of the gas,” Barkana explained.
To detect the expected radio waves from hydrogen in the early universe, several large international groups have built and begun operating new arrays of radio telescopes, he continued. “These arrays were designed under the assumption that cosmic heating occurred too early to see, so instead the arrays can only search for a later cosmic event, in which radiation from stars broke up the hydrogen atoms out in the space in-between galaxies. 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.”
This is similar to archeology, which is interested in our distant past, said Barkana. “Today, we have a universe of stars, planets and life. But the formation of the first stars, when the universe was in its initial state, is a very different period in the history of the universe. The conditions were different, and galaxies were much smaller, only one-millionths as big as the Milky Way. When we succeed in observing this early period, we hope to find interesting surprises, such as different types of stars than exist today, and new hints about the dark matter that is the dominant form of matter in the universe,” he said.