Limit on the energy density in the submillimetre background radiation

Abstract

Measurements of the microwave background radiation discovered1 at ∼7 cm have now been extended to ∼0.3 cm by ground-based observations2. As a cosmological explanation of the radiation background is generally accepted, one expects it to follow the blackbody law; all these observations are, however, at frequencies smaller than that corresponding to the peak intensity of the Planck distribution. But the initial observations of Shivanandan, Houck and Harwit3 and some subsequent experiments4,5 with rocket-borne detectors suggest the presence of a total flux of F_o=1.3 Ã-10âˆ'9 W cmâˆ'2 sterâˆ'1 in the range 0.13 cm to 0.04 cm. This flux would represent an energy density of ∼ 3.5 eV cmâˆ'3 which is more than 25 times the energy density expected in this band from a blackbody at 2.7 K. Measurements indicate that the radiation is isotropic to within 10% implying that this radiation may pervade a region as large as the Galaxy. In that case the intense radiation background must be in the form of lines because of the limitations imposed by upper limits on the excitation of low-lying states of CN and CH in the interstellar space6. The limits on intensities of narrow lines in the spectral band ∼0.16 to 0.07 cm determined using ground-based7 and airborne8 interferometers imply that the lines should be broad if the submillimetre flux is concentrated in this band.