A new study from Tel Aviv University reveals that black
holes, formed from the first stars in our universe, heated the gas throughout
space later than previously thought. They also imprinted a clear signature in
radio waves which astronomers can now search for. The work is a major new
finding about the origins of the universe.
"One of the exciting frontiers in astronomy is the
era of the formation of the first stars," explains Prof. 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."
The study, just published in the journal Nature, was co-authored by
Dr. Anastasia Fialkov of TAU and the École Normale Supérieure in Paris and Dr. Eli
Visbal of Columbia and
Harvard Universities.
Cosmic archaeology
Astronomers explore our distant past, billions of years
back in time. Unlike Earth-bound archaeologists, 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.
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.
The cosmic radio show
"It was previously believed that the heating
occurred very early," says Prof. 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."
In order 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. 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.