Maybe the most striking example of a black body spectrum was provided by the radiation emitted after the "big bang," at which the universe began, when all matter was contained in a very dense and very hot "primordial fireball." The behavior of radiation in an expanding universe is a bit like that of a gas in an expanding volume. In your home air conditioner, gas is compressed outside the house (or room), then piped inside and allowed to expand, and that expansion cools it markedly (the cooled gas then passes a radiator, over which air is blown by a fan).. The radiation emitted after the big bang has "cooled" too, which means its photon distribution, which started out very energetic, has by now had slumped to the black-body spectrum of an object just 3 degrees above absolute zero, with wavelengths in the microwave range.
According to currently accepted models, that radiation started when the universe was about 380,000 years old. Before that (excluding very early stages) the universe consisted of a very hot "fireball" (3000 deg or more) whose atoms, photons and positive particles exchanged energy furiously, leading to an equilibrium energy distribution like Planck's, but at very high temperature. At that time the universe had grown big enough and cool enough (same energy, filling more space) that electrons started to recombine and form neutral atoms, allowing the electro-magnetic radiation tos expand with little interference.
It has been expanding ever since, as has the universe. The volume of the universe is much larger now, meaning the energy density is much lower, and since the total energy stays the same, the wavelength of the electromagnetic radiation has shifted down from light to microwave radiation. However, the "Planck spectrum" of wavelength distribution remains, inherited from the time it was first formed.
The existence of this "primordial microwave background" was first confirmed in 1965 by Arno Penzias and Robert Wilson of the Bell Labs facility in New Jersey, work which earned them the 1978 Nobel prize. The energy distribution of the primordial microwaves was measured in 1990 by COBE (Cosmic Background Explorer), a specially designed NASA spacecraft. COBE's microwave detector was cooled and shielded with the help of liquid helium--necessary because anything on the spacecraft warmer than 3 degrees above absolute zero emits itself too many interfering microwaves.
COBE established that the spectrum of the radiation fit a black body spectrum at a temperature of 2.73 degrees above absolute zero. When the results were first shown (figure above), the audience was amazed to see how well observations (squares) matched the expected black-body spectrum (curve), and informally dubbed the graph "fingerprint of God."
It was the strongest proof yet for the big-bang theory of the origin of our universe. On 3 October 2006 it was announced that the Nobel prize in physics was to be awarded jointly to two leaders of the COBE team--John Mather of NASA's Goddard Space Flight Center, who led the effort to design and fly COBE, and George Smoot of the U. of California, Berkeley, who headed the data analysis effort.
Max Planck himself was awarded the 1918 Nobel Prize for his discovery, but he also suffered personal blows: his wife died in 1906, one son was lost in World War I while another was executed by Nazi Germany for participating in a plot to assassinate Hitler. Planck stayed in Germany throughout World War II. After the war, in 1948, when the central German scientific society was re-established--it was first founded in 1911 bearing the name of Kaiser Wilhelm II--it was renamed the Max Planck Society for the Advancement of Science, and it still maintains many "Max Planck Institutes" in diverse scientific fields.
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