The standard explanation is that it is residual heat from the very hot quark-gluon plasma that filled the early universe. The observed radiation fits very well with a model that posits the Big Bang happened about 13 billion years ago.
Cosmic Microwave Background (CMB) Radiation.
The source of cosmic background radiation filled the entire universe.
I sent a form of thermal energy throughout the universe.
Where? Everywhere. This is the residual radiation, from when the Universe was very, very hot.
The cosmic background radiation is believed to be the remains of the radiation emitted by the Universe when it started to get transparent - when it had cooled down to a temperature of about 3000 kelvin.
cosmic microwave background radiation.
Cosmic Microwave Background (CMB) Radiation.
The source of cosmic background radiation filled the entire universe.
I sent a form of thermal energy throughout the universe.
Where? Everywhere. This is the residual radiation, from when the Universe was very, very hot.
The cosmic background radiation is an observation of the effects of the Big bang [See related question] it does not effect the Universe changing in anyway.
The cosmic background radiation is believed to be the remains of the radiation emitted by the Universe when it started to get transparent - when it had cooled down to a temperature of about 3000 kelvin.
I am not entirely sure about the wavelength; however, the features of the background radiation agree very closely to what would be expected from the Big Bang theory.
Cosmic Background Radiation
The redshift of the cosmic microwave background radiation
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I didn't check the year... But the cosmic background radiation is said to support the big bang theory, because it agrees with the radiation that is expected from an expanding Universe.