Rocket measurement of the cosmic-background-radiation mm-wave spectrum

Abstract

We report here the most precise constraint to date on the spectrum of the cosmic background radiation (CBR), obtained from measurements made with a liquid-helium-cooled spectrometer carried above the atmosphere on a rocket. The spectrum is very well fitted by a Planck function of temperature 7=2.736 K. The scatter of the equivalent temperature in the band 3-16 cm -1 is ±10 mK, about j% of the mean whereas the estimated overall accuracy of the mean temperature is ± 17 mK. These results are inconsistent with a previously reported excess ir intensity in the CBR but are in good agreement with COBE results.PACS numbers: 98.70.Vc, 98.80.Es The cosmic background radiation (CBR), discovered in 1965, 1 is a very important observable for the field of cosmology, being one of the oldest entities accessible to measurement. In the simple big-bang scenario, 2 " 4 it is identified as highly redshifted radiation which was in equilibrium with matter just as the temperature of the Universe fell to a value permitting the formation of neutral hydrogen. In this standard model the spectrum of the CBR would be strictly Planckian (or thermal), this form remaining invariant under expansion of the Universe; early measurements were consistent with a temperature of about 2.7 K. 5 For values of the redshift parameter z less than about 10 7 , however, radiation-matter interactions are insufficient to completely thermalize the radiation spectrum in case it is perturbed. Thus, energy-releasing processes occurring since that time can be expected to have left their imprint on the CBR, and accurate measurements of deviations from a Planckian spectrum can give important information about such processes. 6 In the range 10 7^z^l 0 4 conditions would be such that kinetic equilibrium, but not thermodynamic equilibrium, is established between electrons and photons, resulting in a Bose-Einstein spectrum characterized by a chemical potential /z, a measure of the deposition of energy relative to the radiative energy density. For z;Sl0 4 , even kinetic equilibrium cannot be established, but Compton scattering occurs resulting in a typical spectrum characterized by a "Comptonization" parameter y, a measure of the discrepancy between electron and radiative temperatures. The results reported here set close constraints on both ju and y.There are several difficulties in making detailed checks on the thermal nature of the spectrum over a wide spectral range. First, since the Earth's atmosphere is opaque at wavelengths where the CBR is expected to be most intense (A,;S2 mm), measurements in this important spectral range need to be made from high altitude. Second, the low temperature of the source necessitates the use of cryogenic techniques to minimize contaminating radiation of the measuring instrument itself. And third, since the radiation is of such low intensity, one needs to avoid sources such as the Earth, the Sun, or dust in the plane of our galaxy.Based on experience gained with earlier attempts to measure the CBR spectrum from rockets, 7-9 a new instrum...

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