Absolute Radiometer for Cosmology, Astrophysics,
and Diffuse Emission (ARCADE) is a program
which utilizes high-altitude balloon instrument
package intended to measure the heating of
the universe by the first stars and galaxies
after the big bang and search for the signal
of relic decay or annihilation.
In July 2006 a strong residual radio source
was found using the radiometer, approximately
six times what is predicted by theory.
This phenomenon is known as "space roar" and
remains an unsolved problem in astrophysics.
ARCADE has been funded by the NASA's Science
Mission Directorate under the Astronomy and
Physics Research and Analysis Suborbital Investigation
program.
The program is composed of a team led by Alan
Kogut of NASA's Goddard Space Flight Center.
ARCADE was launched from NASA's Columbia Scientific
Balloon Facility in Palestine, Texas, conducted
under the auspices of the Balloon Program
Office at Wallops Flight Facility.
The balloon flew to an altitude of 120,000
feet (37 km), viewing about 7% of the sky
during its observations.The instrument is
designed to detect radiation at centimeter
wavelengths.
The craft contained seven radiometers which
were cooled to 2.7 K (−270.45 °C; −454.81
°F) using liquid helium, with the intent
to measure temperature differences as small
as 1/1000 of a degree against a background
which is only 3 K (−270.15 °C; −454.27
°F).
The optics in the instrument package were
placed near the top of the dewar flask which
cooled them in order to prevent the instruments
from seeing the walls of the container, thereby
simplifying the processing of the observational
data.
This design choice necessitated the use of
superfluid pumps in order to drench the radiometers
in liquid helium.
The design also utilized heaters in order
to create a cloud of helium gas, in place
of using a (relatively warm) window, which
also simplified processing of the observational
data.
== Residual emission ==
In 2011, the ARCADE 2 researchers reported,
"Correcting for instrumental systematic errors
in measurements such as ARCADE 2 is always
a primary concern.
We emphasize that we detect residual emission
at 3 GHz with the ARCADE 2 data, but the result
is also independently detected by a combination
of low-frequency data and FIRAS."
== 
See also ==
List of cosmic microwave background experiments
Cosmic microwave background radiation
