We refer to this radiation as a background
because we see it no matter where we look. It clearly doesn't come from any
nearby objects, such as stars or clouds within our Galaxy, or even from external
galaxies. It is clearly a distant, "background" source of radiation. You can
think of the whole Universe as being filled with this background of microwave
We refer to it as "cosmic" because the only known source of this radiation is the early universe. It can now be firmly concluded that the CMB is the cooled remnant of the hot Big Bang itself.
All of the forms of electromagnetic radiation travel at the same speed, the speed of light, which is 300,000 km/s. E-m radiation of different wavelengths will interact with matter in different ways. For example, radio waves are picked up by a radio receiver, your eye detects visible light, infra-red radiation warms your skin, x-rays penetrate your body, gamma-rays can give you radiation damage. Microwaves are the name given to radiation between the infra-red and radio region, with wavelengths typically in the 1mm to 10cm range. Some specific wavelengths of microwaves can be used to excite the molecules in foodstuff, so that you can use them to cook. It turns out that if you had a sensitive microwave telescope in your house you would detect a faint signal leaking out of your microwave oven, and from various other man-made sources, but also a faint signal soming from all directions that you pointed. This is the Cosmic Microwave Background.
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