COBE's Blackbody Spectrum (1990): The Big Bang's Perfect Glow

The COBE blackbody measurement, published by John Mather and the COBE team in 1990, showed that the cosmic microwave background has the spectrum of an almost perfect blackbody at 2.735 K. That is exactly what a hot, dense early universe predicts, and it came from just 9 minutes of observations.

The question before the paper

Since its discovery in 1965, the cosmic microwave background had been read as the cooled afterglow of the Big Bang. If so, its brightness at different wavelengths should follow the smooth curve of a blackbody, the radiation from a body in thermal equilibrium. For several years cosmologists had puzzled over reports of a large excess in the spectrum at wavelengths shorter than 1 millimeter, which would have meant the simple picture was missing something. A clean test needed to be done from space.

What the paper found

• A blackbody at 2.735 ± 0.06 K. The spectrum matched a single-temperature blackbody curve.
• Deviations below 1%. Any departure from the ideal curve was smaller than 1% of the peak intensity.
• From a tiny data set. The result used only 9 minutes of observations near the north Galactic pole.

How they did it

NASA’s Cosmic Background Explorer, COBE, launched on November 18, 1989. One of its instruments, the Far Infrared Absolute Spectrophotometer (FIRAS), is an interferometer that compared the sky with an internal reference blackbody and was calibrated against an external blackbody with an emissivity better than 0.9999. Measuring the difference across a range of wavelengths let FIRAS check the shape of the sky’s spectrum directly. The team first presented the curve at the January 1990 meeting of the American Astronomical Society.

Why it mattered

A perfect blackbody is the natural outcome of a hot early universe in thermal equilibrium, and it is something rival ideas struggled to produce. The FIRAS curve settled the nature of the cosmic microwave background and remains one of the most precise blackbody measurements in nature. Together with the COBE detection of temperature ripples two years later, it earned John Mather and George Smoot the 2006 Nobel Prize in Physics for the discovery of the blackbody form and anisotropy of the cosmic microwave background.

What we know now

Later missions built on COBE’s foundation. WMAP and Planck mapped the radiation’s small variations across the sky in fine detail and used them to measure the age of the universe and its composition. The blackbody shape that FIRAS confirmed is the starting assumption of all of that cosmology. Research on the cosmic microwave background continues at institutes such as the Max Planck Institute for Astrophysics.

Who wrote it

John Mather was the lead scientist behind FIRAS and first author of the paper, written with the COBE science team. When he showed the spectrum at the January 1990 AAS meeting, according to Physics Today, the packed lecture hall “burst into sustained applause.” The measurement also put to rest the reported short-wavelength excess.

Read the original

Mather, J. C., Cheng, E. S., Eplee, R. E., Jr., et al. (1990). “A preliminary measurement of the cosmic microwave background spectrum by the Cosmic Background Explorer (COBE) satellite.” The Astrophysical Journal 354, L37. doi:10.1086/185717. Free scanned PDF at the NASA Astrophysics Data System.