10,000
years100,000
years1 million
years10 million
years100 million
yearsCosmic
microwave
background
380,000 yearsIONIZATIONBIG
BANGEPOCH OF
DARK AGES REIONIZATIONThree
minutesFirst stars
and
galaxies formClouds
of neutral
hydrogenLogarithmic scalebillion light-years
13 13.25GRB 090423
(farthest
gamma-ray
burst)
13.10 billion
light-yearsEGSY8p7
(farthest
galaxy)
13.14 billion
light-yearsULAS J1120+0641 (farthest quasar)
12.96 billion light-years60 ASTRONOMY • DECEMBER 2015along a filament that contains the quasar. Its light
has been traveling for nearly 11 billion years.
Second, the researchers recently found four qua-
sars close to one another and embedded in an enor-
mous cloud of hydrogen. “It’s a structure that will
evolve into something like [the] Virgo [Cluster]
today,” says Prochaska.
In both discoveries, the light he and his col-
leagues see comes from f luorescence, he says,
“where the quasar shines at a range of energies onto
a galaxy and the galaxy actually shines back.”
Prochaska thinks that with the observing tools set
to come online in the next five to 10 years, astrono-
mers will find hundreds of quasars embedded in
filaments and forming galaxy clusters.To the early cosmos
The farther astronomers look from Earth, the less
organization they see. That’s because they’re see-
ing the cosmos at an earlier stage, and modern-day
structures — like spiral galaxies and dense clusters
of thousands of galaxies — didn’t exist. The uni-
verse was not born complex. Instead, after the Big
Bang, it was dense and hot, filled with electrons,
protons, and light bouncing between those atomicpieces. The cosmos has been expanding since that
moment 13.82 billion years ago.
Once it had expanded enough for the tempera-
ture throughout the universe to cool to about
4,900° F (3,000 kelvins), each proton grabbed a
nearby electron to form a neutral hydrogen atom.
With fewer particles f loating around, the pinball
game was over.
At that point, light was free to stream about the
cosmos. That light has been traveling along the fab-
ric of space-time ever since. Today, it bathes the sky
in a cool microwave glow, its wavelength stretched
by cosmic expansion.
This cosmic microwave background (CMB)
looks nearly the same in every direction. It reveals
to astronomers what the universe looked like just
380,000 years after the Big Bang: an almost feature-
less soup of hydrogen and helium.
The tiny differences in temperature it contains
ref lect tiny differences in density. Eventually, those
denser areas grew into galaxies and galaxy clusters,
while the least dense regions emptied.
“Understanding that transition, from a simple
universe to something with interesting structure in
it, is a crucial missing piece in astronomy,” says
Steve Furlanetto of the
University of California, Los
Angeles.
This transformation hap-
pened in the astronomical DarkVOIDS AREN’T
EMPTYThe spaces between galax-
ies are not empty. Huge
reservoirs of hot gas —
invisible to human eyes but
glowing in X-rays — fill the
gaps. And even the large-
scale voids in the cosmic
web structure aren’t
vacant. The Cosmic
Infrared Background
Experiment (CIBER), with its
onboard Wide Field Imager
(WFI) took four 7-minute
rides on sounding rockets
between 2009 and 2013.
WFI collects near-infrared
radiation with two 4.4-inch
telescopes. During two of
those rides, WFI found a
brighter glow in the uni-
verse than expected. The
scientists involved with
CIBER think there are far
more stars wandering
through the cosmic voids,
likely flung from their
home galaxies by gravita-
tional interactions. — L. K.
Astronomers think a span of time that lasted hundreds of millions of years called the
reionization era began when the first stars and galaxies emitted radiation that turned hydrogen
atoms (one proton, one electron) into hydrogen ions (a proton without an electron). ASTRONOMY: ROEN KELLYHow it all formed
ESO/UKIDSS/SDSS (ULAS J1120+0641); I. LABBÉ (LEIDEN UNIVERSIT Y)/NASA/ESA/JPL-CALTECH (EGSY8P7); ESA/PLANCK COLLABORATION (CMB)