The cosmic microwave background radiation (CMB) is electromagnetic radiation in the microwave region that reaches the Earth from all directions in space. It was discovered by accident in 1965 by Penzias and Wilson.Its spectrum is almost exactly that of a black body at a temperature of about 2.7 K. It is also almost perfectly isotropic: its intensity is the same in every direction to about 1 part in 100 000.Origin: about 380 000 years after the Big Bang, the universe had cooled to about 3000 K. At this temperature, electrons and nuclei could combine to form neutral atoms. Before this, photons were constantly scattered by free charged particles, so the universe was opaque. Once atoms had formed, the photons could travel freely and the universe became transparent. Those photons are the CMB.At that time the radiation was mostly visible and infrared (peak wavelength about 1 μm). Since then the universe has expanded about 1000 times, and the wavelengths have been stretched by the same factor. This is why the radiation is now in the microwave region and corresponds to such a low temperature.Worked example: use Wien's law to find the peak wavelength of the CMB.
λmax = 2.9 × 10−3 / 2.7 = 1.1 × 10−3 m (about 1 mm, a microwave)The CMB is strong evidence for the Big Bang: it is exactly what a hot, dense early universe that has since expanded and cooled would leave behind. Tiny variations in its temperature show where matter was slightly denser, which later formed galaxies.
λmax = 2.9 × 10−3 / 2.7 = 1.1 × 10−3 m (about 1 mm, a microwave)The CMB is strong evidence for the Big Bang: it is exactly what a hot, dense early universe that has since expanded and cooled would leave behind. Tiny variations in its temperature show where matter was slightly denser, which later formed galaxies.