Black Holes Warmed Universe Later Than Previously Believed

Black holes warmed early universe later than previously believed

According to a group of scientists working at Tel Aviv University, black holes that served as companions to some of the earliest stars may have taken longer to warm the ancient universe than previously conjectured. The latest study was published in the Feb. 5 issue of the journal Nature.

The scientists established that black holes, spawned from the very first stars, heated the gases throughout space later than previously considered and also “imprinted” a signature in radio waves that astronomers are now able to look for.

According to Professor Rennan Barkana, who works at Tel Aviv University’s School of Physics and Astronomy, the universe was filled with hydrogen atoms during the formation of the first stars. In light of this, Barkana maintains that the most suitable method for witnessing this era of ancient stars involves “… measuring the emission of hydrogen using radio waves.”

A few hundred years following the Big Bang, two major milestones occurred – ubiquitous hydrogen gas was heated and, resultantly, made transparent. The hydrogen dominated the universe, in the aftermath of the Big Bang, when protons and neutrons joined together. During a period in time dubbed the “Dark Ages” – within the first 100 million years after the Big Bang – the universe was shrouded in darkness. Billions of years after the first stars and galaxies had formed, reionization took place, along with the clearance of the afore-mentioned hydrogen gas.

Astronomers’ principal ambitions involve study of the cosmological past, spanning billions of years back in time. Astronomers are able to directly observe these historical events by using light transmitted from distant objects. Since this light takes a long time to reach astronomers back on Earth, they can observe the state of objects, as they were in the early universe, when the light was initially emitted. Thus, since cosmic heating is believed to have transpired much earlier than originally suspected, astronomers don’t need to look quite as far, and it should be easier to observe this cosmic milestone.

The researchers believe that cosmic heating holds the key to unlocking the mysteries of the earliest black holes. Just as with the stars of today, those of the early universe were often associated with companions. When one of these binary stars exploded, and created a black hole, an X-ray binary was subsequently formed. X-ray binaries produce X-rays, which result from the matter of the intact donor star falling towards the black hole and becoming accreted.

In the nascent universe, these highly energetic X-rays helped to heat the hydrogen gas of space. Although scientists previously believed that this heating was caused by low-energy X-rays, recent models suggest that X-rays from X-ray binary systems were actually responsible. Anastasia Fialkov, of Tel Aviv University, and Eli Visbal, of Columbia University, employed these new models to determine the length of time needed to increase the temperature of the hydrogen within the universe. Intriguingly, the researchers claim the high-energy X-rays actually took longer to raise the temperature than lower energy X-rays.

Speaking to, Barkana indicates that high-energy X-rays typically travel vast distances, over a long time period, before their energy heats the gas:

“Eventually, all their energy is deposited, but ‘eventually’ is too late in the early universe, when galaxy and star formation are ramping up.”

Since the hydrogen has caused the early universe to remain relatively opaque, scientists find it difficult to observe the events that occurred within the first billion years after, the Big Bang. If the cosmic heating was caused by low-energy X-rays, the reionization process would have finished much earlier, and would no longer be observable. Therefore, according to the latest research, high energy X-rays must have dominated the ancient universe.

To detect radio waves that were emitted from hydrogen of the early universe, a number of international teams have already constructed a number of telescope arrays, such as the Precision Array for Probing the Epoch of Reionization in South Africa. However, these telescopes were designed at a time when researchers mistakenly believed that early cosmic heating could not be detected. However, scientists now believe that soon-to-be radio telescopes, including the Square Kilometer Array (SKA) could be used to detect heating by early black holes, if designers considered the latest research.

By James Fenner

Tel Aviv University
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