284
The temperature of the CMB today is
a chilly 2.7 K. The thermal spectrum
at that temperature contains no
visible light, which is why space looks
black to human eyes. However, the
spectrum has redshifted (stretched)
over time as the universe has
expanded. Extrapolating back to
the moment the CMB was emitted
gives an original temperature
of about 3,000 K. The color of
radiation at this temperature is
orange, so the CMB started out
as a flash of orange light that shone
out from every point in space.
Smooth signal
The early observations of the CMB
suggested that it was isotropic,
which means that its spectrum is
the same everywhere. In cosmology,
the terms density, energy, and
temperature are somewhat
synonymous when discussing
the early universe. So the isotropic
nature of the CMB suggested that,
in those early days, space had a
uniform density, or spread of energy.
However, this did not tally with the
developing theories of the Big Bang,
which demanded that matter and
energy were not evenly spread
through the young universe, but
had been concentrated together
in places. These denser areas, or
anisotropies, were where the stars
and galaxies formed. COBE was
sent into space to take a close look
at the CMB to see if it could find
any anisotropies, to find out whether
the CMB changed, however slightly,
depending on where it looked.COBE’s mission
A mission to study the CMB from
space had been in the planning
stages since the mid-1970s.
Construction of COBE began in 1981.
It was initially designed to enter
polar orbit (its orbit passing over both
poles). However, the Challenger
disaster of 1986 grounded the shuttle
fleet, and the COBE team had to look
for another launch system. In 1989,
the satellite was launched using
a Delta rocket, and it was placed
in a sun-synchronous geocentric
orbit—orbiting in a way that saw
it pass over each place on Earth at
the same time of day. This worked
just as well as a polar orbit in that
it allowed COBE to point away from
Earth and scan the entire celestial
sphere, strip by strip.
The spacecraft carried three
instruments, all protected from
the sun’s heat and light by a cone-
shaped shield, and chilled toOBSERVING THE CMB
2 K (colder than space itself) using
100 gallons (650 liters) of liquid
helium. George Smoot ran the
Differential Microwave Radiometer
(DMR), which mapped the precise
wavelengths of the CMB, while
John Mather was in charge of
FIRAS, the Far-InfraRed Absolute
Spectrophotometer, which collected
data on the spectra of the CMB.
These two experiments were
looking for anisotropies. The third
detector on COBE had a slightly
different goal. The Diffuse Infrared
Background Experiment, run
by Mike Hauser, found galaxies
that were so ancient and far
away that they are only visible by
their heat radiation (or infrared).
COBE’s instruments created
the most accurate map of the
CMB to date. However, it was not
a simple surveying job. Smoot and
Mather were interested in primary
anisotropies—that is, the density
differences that were present at
the time the CMB formed. To find
these, they needed to filter out the
secondary fluctuations caused by
obstacles that lay between COBEThe full-sky map produced
by WMAP in 2011 showed many
fine details of the isotropy of the
CMB. Colder spots are blue, while
hotter spots are yellow and red.[COBE has made] the
greatest discovery of the
century, if not of all time.
Stephen Hawking