ARCADE 2 MEASUREMENT OF THE ABSOLUTE SKY BRIGHTNESS AT 3-90 GHz

TLDR

The ARCADE 2 balloon instrument measured absolute sky temperature from 3 to 90 GHz using an open‑aperture cryogenic system with no emissive windows, an in‑situ blackbody calibrator, differential radiometers cooled to CMB‑like temperatures, and a linear model that compares radiometer outputs to onboard thermometers while correcting for residual emission from the flight train, balloon, atmosphere, and Galactic foregrounds. ARCADE 2 data alone reveal a 54 ± 6 mK excess at 3.3 GHz above the CMB temperature of 2.731 ± 0.004 K, and when combined with literature the spectrum follows a power law T = 24.1 ± 2.1 K (ν/ν0)^–2.599 ± 0.036 from 22 MHz to 10 GHz, with a CMB temperature of 2.725 ± 0.001 K.

Abstract

The ARCADE 2 instrument has measured the absolute temperature of the sky at frequencies 3, 8, 10, 30, and 90 GHz, using an open-aperture cryogenic instrument observing at balloon altitudes with no emissive windows between the beam-forming optics and the sky. An external blackbody calibrator provides an in situ reference. Systematic errors were greatly reduced by using differential radiometers and cooling all critical components to physical temperatures approximating the cosmic microwave background (CMB) temperature. A linear model is used to compare the output of each radiometer to a set of thermometers on the instrument. Small corrections are made for the residual emission from the flight train, balloon, atmosphere, and foreground Galactic emission. The ARCADE 2 data alone show an excess radio rise of 54 ± 6 mK at 3.3 GHz in addition to a CMB temperature of 2.731 ± 0.004 K. Combining the ARCADE 2 data with data from the literature shows an excess power-law spectrum of T = 24.1 ± 2.1 (K) (ν/ν0)−2.599 ± 0.036 from 22 MHz to 10 GHz (ν0 = 310 MHz) in addition to a CMB temperature of 2.725 ± 0.001 K.