One of the exciting frontiers in astronomy is the era of the formation
of the first stars. 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 characteristic emission of hydrogen at
a wavelength of 21 cm , which falls in the range of radio waves.
Our new study reveals that black holes that formed from the first
stars in the universe heated the gas throughout space later than
previously thought, also imprinting a clear signature in these radio
waves, which astronomers can now search for.
Astronomers explore our distant past, billions of years back in
time. Unlike archaeologists on Earth, however, who can only study
remnants of the past, astronomers can see the past directly. It takes
the light from distant objects a long time to reach us, and we see
these objects as they were back when they emitted their light. 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 look out as far, and it will be
easier to see this cosmic milestone.
Cosmic heating may offer a way to directly probe the earliest black
holes, since it was likely driven by stellar 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 novelty in the
new research is the delay of this heating.
The cosmic radio show
It was previously believed that the heating occurred very early, but
we discovered that this standard picture depends sensitively 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 about 400
million years old, compared to its current age of 14 billion years) at
which the sky was uniformly filled with radio waves emitted by the
hydrogen gas.
Several large international groups have built and begun operating new
arrays of radio telescopes, in order to detect the expected radio
waves from hydrogen in the early universe. These arrays have been
designed under the assumption that cosmic heating occurred too early
to see, so that 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 between galaxies ("cosmic reionization"). 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.
More details and related images can be found here.
Links in Hebrew
Israel radio (Reshet Bet) news magazine (HaBoker HaZe), Feb. 7'th at 6:45 AM: audio ,
text
Israel radio (Reshet Bet) hourly news, Feb. 6'th at 10 AM: audio ,
text
