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Why is the cosmic microwave background only about 2.7 kelvins today?

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The cosmic microwave background (CMB) is a relic of the big bang, the primordial explosion that occurred 13.8 billion years ago. At that time, the universe was in a state of extremely high temperature and density.12

As the universe has expanded over the last 13.8 billion years, this initial thermal radiation has cooled significantly. The expansion of the universe causes the wavelength of each photon to stretch, a process known as redshift. Because the energy of these photons is inversely proportional to their wavelength, this cosmological expansion results in a continuous decrease in the radiation's temperature.134

By the present epoch, this cooling process has reduced the temperature of the background radiation to approximately 2.7 kelvins above absolute zero. This low-temperature, isotropic radiation now fills the entire universe, serving as a fundamental piece of evidence for the big-bang model.2351