The CMB is a snapshot of the oldest light in the universe, imprinted on the sky when the universe was just a baby in cosmic terms.
It is essentially the afterglow radiation from the hot, dense state of the universe just 380,000 years after the Big Bang, a time known as the recombination era. At this point, the universe had cooled enough for protons and electrons to combine into neutral hydrogen atoms, allowing photons (light particles) to travel freely for the first time without being constantly scattered by charged particles.
These photons have been stretching and cooling as the universe expands, shifting from visible and ultraviolet light into the microwave part of the electromagnetic spectrum, hence the name Cosmic Microwave Background.
The CMB is of critical importance in cosmology for several reasons:
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Uniformity and Fluctuations: The CMB is remarkably uniform in all directions, with a temperature of about 2.725 Kelvin (-270.425°C or -454.765°F). However, it contains tiny fluctuations or anisotropies in temperature and density. These slight variations are incredibly significant because they represent the seeds from which all the structures in the universe (like galaxies and clusters of galaxies) grew.
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Evidence for the Big Bang: The discovery and detailed measurement of the CMB are among the strongest pieces of evidence for the Big Bang theory. The existence of the CMB was predicted by theoretical models of the Big Bang before it was observed, and its discovery in 1965 by Arno Penzias and Robert Wilson provided a major confirmation of the Big Bang model of cosmology.
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Cosmological Parameters: Detailed observations of the CMB, such as those provided by the COBE (Cosmic Background Explorer), WMAP (Wilkinson Microwave Anisotropy Probe), and Planck satellite missions, have been used to determine many important cosmological parameters. These include the universe’s age, the rate of its expansion (the Hubble constant), the density of matter and energy in the universe, and the geometry of the universe (whether it is flat, open, or closed).
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Insight into Dark Matter and Dark Energy: Analysis of the CMB also provides indirect evidence and constraints on the nature of dark matter and dark energy, which together comprise about 95% of the total mass-energy content of the universe but are not directly observable.