The blackbody radiation is seen as a remnant of the transparency point at which the expanding universe dropped below about 3000K so that radiation could escape.
3K Background Radiation
A uniform background radiation in the microwave region of the spectrum is observed in all directions in the sky. It shows the wavelength dependence of a "blackbody" radiator at about 3 Kelvins temperature. It is considered to be the remnant of the radiation emitted at the time the expanding universe became transparent at about 3000 K temperature. The discovery of the 3K microwave background radiation was one of the crucial steps leading to the calculation of the standard "Big Bang" model of cosmology, its role being that of providing estimates of relative populations of particles and photons. Recent research using the Far Infrared Absolute Spectrophotometer (FIRAS) onboard the COBE satellite have given a temperature of 2.725 +/- 0.002 K. Previous experiments had shown some anisotropy of the background radiation due to the motion of the solar system, but COBE collected data showing fluctuations in the background. Some fluctuations in the background are necessary in big bang cosmology to give enough non-uniformity for galaxies to form. The apparent uniformity of the background radiation is the basis for the "galaxy formation problem" in big bang cosmology. The more recent WMAP mission gave a much higher resolution picture of the anisotropies in the cosmic background radiation.
The data for the round figure of 109 photons per nuclear particle is the "most important quantitative conclusion to be drawn from the measurements of the microwave radiation background ..."(Weinberg p66-70). This allowed the conclusion that galaxies and stars could not have started forming until the temperature dropped below 3000K. Then atoms could form and remove the opacity of the expanding universe; light could get out and relieve the radiation pressure. Star and galaxy formation could not occur until the gravitational attraction could overcome the outward radiation pressure, and at 109 photons/baryon a critical "Jean's mass" of about a million times that of a large galaxy would be required. With atom formation and a transparent universe, the Jeans mass dropped to about 10-6 the mass of a galaxy, allowing gravitational clumping.
Role of 3K in Cosmology
The 3K background implies about 5.5 x 105 photons/liter. The range of estimates for baryon density is from twice critical density at 6 x 10-3/liter to the low end estimate of the visible galaxy, 3 x 10-5/liter. This gives a range of 1 x 108 to 2 x 1010 photons/baryon. It is this estimate of the number of photons per baryon which was crucial in calculations of the big bang. In the modeling of nucleosynthesis in the big bang, including the hydrogen/helium ratio, the relative population of baryons and photons agreed with observations.
When the trace quantities of D, 3He, and 7Li are examined and made a part of the big bang model, the ratio of baryons to photons is constrained more tightly. The Particle Data Group gives the baryon/photon ratio η as
2.6 x 10-10 < η < 6.3 x 10-10 baryons/photon
Since the conservation of baryon number is a strong conservation principle, it is inferred that the ratio of photons to baryons is constant throughout the process of expansion. No known process in nature changes the number of baryons.
Anisotropy of 3K Background
An anisotropy of about 0.1% exists in the cosmic microwave background radiation which is attributed to a Doppler shift caused by the motion of the solar system through the radiation. The Particle Data Group reports the asymmetry as mostly dipole in nature with a magnitude of 1.23 x 10-3. This value is used to calculate a velocity of about 600 m/s for the Earth compared to an observer keeping track with the general expansion.
The COBE satellite has discovered fluctuations in the cosmic microwave background radiation with the use of a differential microwave radiometer. The size of the fluctuations are ΔT/T = 6x10-6. This is just above the level at which the big bang cosmological calculations would have been in trouble. The scale of the fluctuations is larger than the horizon at the time the background radiation was emitted, indicating that the fluctuations are primordial, dating from a time before the separation of radiation and matter, the transparency point. The "horizon" is the distance within which there can be causal connections, i.e., within light transit time of each other.
NASA's Cosmic Background Explorer satellite (COBE) was launched to explore
the cosmic microwave background radiation. Data points are shown superimposed on
the theoretical blackbody curve.
The fit of the Planck radiation formula is so precise that it provides a powerful confirmation of the idea that it is a remnant of big bang expansion.
This data was adapted from Mather, J. C., et al., Astro. Jour. 354, L37 (1990).
The data from COBE have been so precise that it has discovered fluctuations in that radiation which are important to big bang cosmological calculations. It carried three main instruments, a Differential Microwave Radiometer, a Far-Infrared Absolute Spectrophotometer (cooled to 1.6 K by liquid helium) , and the Diffuse Infrared Background Experiment, also at 1.6K. The infrared instrument will measure infrared spectra of the background which are presumed to be uniform, but any unexpected variations might indicate the presence of energy sources which might have driven turbulence to trigger galaxy formation. The infrared instruments sensitivity is 100 times greater than that achievable from the Earth's surface. The Infrared Background Experiment will look at distant primordial galaxies and other celestial objects that formed after the big bang.
John C. Mather of the NASA Goddard Spaceflight Center and George F. Smoot of the Lawrence Berkeley National Laboratory were awarded the 2006 Nobel Prize in Physics for work associated with the COBE satellite.
The picture has been further clarified by the more recent WMAP mission which provided a higher resolution picture of temperature fluctuations.