Firas

FIRAS, or the Far-Infrared Absolute Spectrophotometer, was an instrument aboard NASA's Cosmic Background Explorer (COBE) satellite. Launched in 1989, its mission was to measure the spectrum of the cosmic microwave background radiation with high precision across far-infrared to submillimeter wavelengths, in order to test predictions of the Big Bang model.

FIRAS employed a cryogenically cooled Fourier transform spectrometer that compared the sky signal to a reference blackbody calibrator. The optical system and detectors were designed to minimize systematic errors and thermal noise, enabling accurate reconstruction of the CMB spectrum. The instrument was part of COBE's scientific payload along with the Differential Microwave Radiometers (DMR) and the Diffuse Infrared Background Experiment (DIRBE).

Results and impact: FIRAS produced one of the defining results of modern cosmology, showing that the cosmic microwave background has a nearly perfect blackbody spectrum with a temperature of about 2.725 kelvin. The data placed stringent upper limits on spectral distortions from energy release in the early universe, effectively ruling out large deviations from a blackbody and constraining models of decaying particles and other non-standard processes. These findings provided strong support for the hot Big Bang paradigm and established a precise baseline for subsequent CMB studies.

Legacy: The FIRAS measurements, together with results from DIRBE and DMR, helped shape the standard cosmological model and set benchmarks for later experiments aiming to detect subtle spectral distortions in the CMB. The instrument was led by John C. Mather at NASA Goddard Space Flight Center.