A map of the cosmic background radiation
The entire Universe is saturated with what is known as microwave background radiation, a remnant of the Big Bang. Microwaves are electromagnetic waves – just like visible light. However, microwaves have wavelengths between one metre and one millimetre (= one thousandth of a metre), while the wavelength of light lies between 380 and 780 nanometres (= one thousandth of one millionth of a metre).
After the Big Bang, the formation of matter, space and time out of virtually nothing, the prevailing temperatures were at first almost inconceivably high. However, as the Universe expanded the temperature sank – to approximately minus 270 degrees Celsius, the temperature that it is today. The expansion of space also lengthened the wavelength of the electromagnetic radiation until it entered the microwave range of between one metre and one millimetre. Today, this radiation can be measured reaching us evenly from all directions of space – thus the term 'background radiation'. It would 'heat up' any colder object to the space temperature of minus 270 degrees Celsius.
The microwave background radiation ensures an even 'space climate'
Can the Universe still be colder in some places? Yes, when they are shielded and actively cooled. This is because absolute zero, the lowest temperature theoretically possible, is minus 273 degrees Celsius – approximately a further three degrees lower than the temperature in space. This lowest temperature is reached to within a few millionths of a degree in the laboratories of low-temperature physicists, where matter is cooled using complex processes. For this reason, the coldest point in the Universe is on the Earth – providing there is no extra-terrestrial civilisation in whose laboratories temperatures are attained that are even closer to absolute zero.