Cosmic Microwave Background
The Cosmic Microwave Background (CMB) is electromagnetic radiation that fills the entire observable universe. It is detected as microwave radiation arriving from all directions of space with nearly uniform intensity. The CMB is a relic of the early universe, emitted approximately 380,000 years after the Big Bang during the recombination epoch, when the universe had cooled sufficiently for electrons and protons to combine into neutral hydrogen.
Origin and Recombination
Prior to recombination, the universe was a hot, dense plasma of protons, electrons, and photons. In this state, photons were constantly scattered by free electrons, preventing them from traveling freely. As the universe expanded and cooled to approximately 3,000 Kelvin, protons and electrons combined to form neutral hydrogen atoms. This process, known as recombination, drastically reduced the density of free electrons, allowing photons to decouple from matter and travel through space unimpeded. This “surface of last scattering” marks the point at which the universe became transparent to radiation.
Properties and Spectrum
The radiation released at the time of decoupling was originally in the visible and infrared spectrum. However, due to the expansion of the universe over the subsequent 13.8 billion years, the wavelength of this radiation has been stretched by a factor of approximately 1,100. This cosmological redshift has shifted the peak of the radiation into the microwave region of the electromagnetic spectrum. The CMB exhibits a nearly perfect blackbody spectrum with a current temperature of approximately 2.725 Kelvin.
Anisotropies and Cosmological Significance
While the CMB is highly isotropic, precise measurements reveal tiny temperature fluctuations, or anisotropies, on the order of one part in 100,000. These minute variations correspond to density fluctuations in the early universe, which served as the seeds for the formation of large-scale structures such as galaxies and galaxy clusters. The study of these anisotropies provides critical data for determining fundamental cosmological parameters, including the age, geometry, and composition of the universe, offering strong empirical support for the Big Bang theory.