Israeli astronomers adjust date of initial cosmic heating

The story of the early universe is only now being fully understood, and a number of Israeli astrophysicists are leading the way.

According to Professor Rennan Barkana of Tel Aviv University’s School of Physics and Astronomy, black holes, products of the first stars in our universe, warmed the gases throughout space later than previously believed. Moreover, these black holes imprinted a clear radio wave signature that astronomers can now search for, which is a major finding about the origins of the universe.

“One of the exciting frontiers in astronomy is the era of the formation of the first stars,” Barkana explained. “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.”

The study, published in the interdisciplinary journal Nature, was co-authored by Barkana, Dr. Anastasia Fialkov of TAU and the École Normale Supérieure in Paris, and Dr. Eli Visbal of the American universities Columbia and Harvard.

Because light from distant objects in the universe takes enormous lengths of time to be visible from Earth, astronomers can observe these objects in their condition when that light was originally emitted, even the first stars. Therefore, observers do not have to search as far to determine the validity of the cosmic heating theory.

Cosmic heating may offer a method to directly investigate the earliest black holes, likely driven by star systems called “black-hole binaries,” pairs of stars in which the larger star terminated in a supernova explosion that left a black-hole remnant in its place. In the wake of this phenomenon, the smaller, companion star is dragged toward the black hole, subsequently torn apart in gravitational energies of the black hole. The byproduct of this process is emitted as a high-energy X-ray radiation, distributed across vast distances, and believed to have re-heated the cosmic gas, after it had cooled after the original cosmic expansion.

“It was previously believed that the heating occurred very early,” said Barkana, “but we discovered that this standard picture delicately depends on the precise energy with which the X-rays come out. 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.”

The new discovery overturns the commonly held assumption, which had prompted several international organizations to construct a new array of radio telescopes, that cosmic heating occurred at a point too early to see.

But it also coincides with a related theory by Abraham Loeb, chair of the Astronomy department at Harvard University, that says that conditions shortly after the Big Bang, more than 13 billion years ago, could have been conducive to the rise of life elsewhere in the universe.

Terrestrial life as we know it is thought to have evolved on Earth around 3.5 billion years ago.

Around 15 million years after the Big Bang, a blink of an eye in cosmic terms, the endemic background cosmic radiation of the universe, which was super-heated after the Big Bang, would have, as the universe expanded, cooled to an approximate Earth-like range, he notes.

This is important because scientists believe that life can arise only under certain conditions, one of which is the right temperature. Much of the current analysis of exo-planets — planets orbiting other stars, of which hundreds have been discovered in recent years — revolves around the search for a “Goldilocks” planet, one where the conditions are “just right” for the support of life.

Loeb said that this period of Earth-like temperature would have lasted around seven million years, giving rise to the possibility that some form of life could have evolved during this time.

Current theory indicates that planetary systems had not formed into their current configurations so soon after the Big Bang, but Loeb’s work could still be applied to free-floating rocky bodies or other masses, which during that brief window could have had Earth-like temperatures, and water, even if they were not in orbit around a sun.

Edwin Turner, an astrophysicist at Princeton University, told Science.com, which published an analysis of Loeb’s paper last week, that the new theory was “very original, stimulating and thought-provoking.”