Pierre-Marie Robitaille
Department of Radiology, The Ohio State University, 395 W. 12th Ave, Suite 302, Columbus, Ohio 43210, USA
E-mail: robitaille.1@osu.edu
The COBE Far Infrared Absolute Spectrophotometer (FIRAS) operated from _30 to _3,000 GHz (1–95 cm1) and monitored, from polar orbit (_900 km), the _3 K microwave background. Data released from FIRAS has been met with nearly universal admiration. However, a thorough review of the literature reveals significant problems with this instrument. FIRAS was designed to function as a diferential radiometer, wherein the sky signal could be nulled by the reference horn, Ical. The null point occurred at an Ical temperature of 2.759 K. This was 34 mK above the reported sky temperature, 2.725_0.001 K, a value where the null should ideally have formed. In addition, na 18 mK error existed between the thermometers in Ical, along with a drift in temperature of _3 mK. A 5 mK error could be attributed to Xcal; while a 4 mK error was found in the frequency scale. A direct treatment of all these systematic errors would lead to a _64 mK error bar in the microwave background temperature. The FIRAS team reported _1 mK, despite the presence of such systematic errors. But a 1 mK error does not properly reflect the experimental state of this spectrophotometer. In the end, all errors were essentially transferred into the calibration files, giving the appearance of better performance than actually obtained. The use of calibration procedures resulted in calculated Ical emissivities exceeding 1.3 at the higher frequencies, whereas an emissivity of 1 constitutes the theoretical limit. While data from 30–60 GHz was once presented, these critical points are later dropped, without appropriate discussion, presumably because they reflect too much microwave power. Data obtained while the Earth was directly illuminating the sky antenna, was also discarded. From 300–660 GHz, initial FIRAS data had systematically growing residuals as frequencies increased. This suggested that the signal was falling too quickly in the Wien region of the spectrum. In later data releases, the residual errors no longer displayed such trends, as the systematic variations had now been absorbed in the calibration files. The FIRAS team also cited insu_cient bolometer sensitivity, primarily attributed to detector noise, from 600–3,000 GHz. The FIRAS optical transfer function demonstrates that the instrument was not optimally functional beyond 1,200 GHz. The FIRAS team did not adequately characterize the FIRAS horn. Established practical antenna techniques strongly suggest that such a device cannot operate correctly over the frequency range proposed. Insufficient measurements were conducted on the ground to document antenna gain and Field patterns as a full function of frequency and thereby determine performance. The effects of signal diffraction into FIRAS, while considering the Sun/Earth/RF shield, were neither measured nor appropriately computed. Attempts to establish antenna side lobe performance in space, at 1,500 GHz, are well outside the frequency range of interest for the microwave background (<600 GHz). Neglecting to fully evaluate FIRAS prior to the mission, the FIRAS team attempts to do so, on the ground, in highly limited fashion, with a duplicate Xcal, nearly 10 years after launch. All of these findings indicate that the satellite was not sufficiently tested and could be detecting signals from our planet. Diffraction of earthly signals into the FIRAS horn could explain the spectral frequency dependence first observed by the FIRAS team: namely, too much signal in the Jeans-Rayleigh region and not enough in the Wien region. Despite popular belief to the contrary, COBE has not proven that the microwave background originates from the universe and represents the remnants of creation.
October, 2009 PROGRESS IN PHYSICS Volume 4 p. 17-42
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