Absolute zero materials at temperatures near absolute zero, describing superconductivity and superfluids for the first time. The average temperature of the universe today is approximately , based on measurements of cosmic microwave background radiation. Absolute zero cannot be achieved, although it is possible to reach temperatures close to it through the use of cryocoolers, dilution refrigerators, and nuclear adiabatic demagnetization. The use of laser cooling has produced temperatures less than a billionth of a kelvin. At very low temperatures in the vicinity of absolute zero, matter exhibits many unusual properties, including superconductivity, superfluidity, and Bose–Einstein condensation. To study such phenomena, scientists have
Scale of temperature below as M. Assume the set M has the cardinality of c, then one can construct an injective function ƒ: M → R , by which every thermal system will have a number associated with it such that when and only when two thermal systems have the same such value, they will be in thermal equilibrium. This is clearly the property of temperature, and the specific way of assigning numerical values as temperature is called a "scale of temperature". In practical terms, a temperature scale is always based on usually a single physical property of a simple thermodynamic system, called