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How did COBE confirm that the cosmic microwave background has a temperature of 2.735 K?

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Oval map of the cosmic microwave background radiation from NASA's WMAP mission, showing a mottled pattern of blue, green, yellow, and red representing temperature variations. — Cosmic microwave background A full-sky map produced by the Wilkinson Microwave Anisotropy Probe (WMAP) showing cosmic background radiation, a very uniform glow of microwaves emitted by the infant universe more than 13 billion years ago. Color differences indicate tiny fluctuations in the intensity of the radiation, a result of tiny variations in the density of matter in the early universe. According to inflation theory, these irregularities were the “seeds” that became the galaxies. WMAP's data support the big bang and inflation models, and cosmic microwave background is at the farthest limits of the observable universe.

The Cosmic Background Explorer (COBE) satellite confirmed the temperature of the cosmic microwave background by utilizing its Far Infrared Absolute Spectrophotometer (FIRAS) instrument.1

While earlier measurements had been conducted using balloon-borne detectors, FIRAS allowed for significantly more accurate observations by measuring the spectrum of the radiation field from space. These precise measurements demonstrated that the radiation spectrum matched the theoretical predictions for a blackbody at a temperature of 2.735 K. By confirming this specific thermal signature, COBE provided critical evidence supporting the big bang theory, which posited that the early universe was hot and that its subsequent expansion would redshift that thermal radiation into the microwave range observed today.12