Why is it so important to study the cosmic microwave background? These photons fill the Universe today (there are roughly 400 in every cubic centimetre of space) and create a background glow that can be detected by far-infrared and radio telescopes. Due to the expansion of space, the wavelengths of the photons have grown (they have been ‘redshifted’) to roughly 1 millimetre and thus their effective temperature has decreased to just 2.7 Kelvin, or around -270✬, just above absolute zero. Over the intervening billions of years, the Universe has expanded and cooled greatly.
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In the absence of free electrons, the photons were able to move unhindered through the Universe: it became transparent. After about 380,000 years, it had cooled to around 3000 Kelvin (approximately 2700✬) and at this point, electrons were able to combine with protons to form hydrogen atoms, and the temperature was too low to separate them again. However, the Universe was expanding and as it expanded, it cooled, as the fixed amount of energy within it was able to spread out over larger volumes. That means that the early Universe was opaque, like being in fog.
In particular, for roughly the first 380,000 years, the photons were constantly interacting with free electrons, meaning that they could not travel long distances. When the Universe was born, nearly 14 billion years ago, it was filled with hot plasma of particles (mostly protons, neutrons, and electrons) and photons (light). The cosmic microwave background (or CMB) fills the entire Universe and is leftover radiation from the Big Bang. Cosmic microwave background seen by Planck
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