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'''Absolute zero''' is the minimum [[temperature]], defined as 0 [[Kelvin]] on the [[absolute temperature scale]]s (the zero on the Kelvin scale has had to adjust, slightly, as measurements of absolute zero are refined). This corresponds to -273.15 degrees [[Celsius]] and -459.67 degrees [[Fahrenheit]]. At absolute zero, all motion (i.e. the movement of [[molecules]] in a [[gas]], [[atom]]s in a [[crystal]]) would cease.
No place in our universe is at absolute zero, but there are places that get close. [[Outer space|Space]] is approximately 2.73 kelvin. This is not because heat from [[star]]s in the universe keep the temperature up, but because of cosmic background radiation. The [[Cosmic Background Explorer]] (CBE) measured this value. The [[Big Bang theory]] predicts a leftover background radiation and that such radiation would have a relative uniformity, which is reinforced by the evidence presented by the CBE's observations. However, these observations cannot account for the [[horizon problem]].
== Reaching Towards Absolute Zero ==
It is considered theoretically impossible to achieve absolute zero, but scientists have come quite close. The first example of this occurred in 1908 when [[Kammerlingh Onnes]] liquefied [[Helium]]. The temperature he reached was approximately four kelvin. Lower temperatures have been reached with different [[isotope]]s of Helium. A temperature of 0.3 kelvin has been reached with <sup>3</sup>He. The lowest temperature ever reached by any substance was 280 picokelvins.
== Sources ==
[http://www.ph.rhul.ac.uk/schools/ZeroT/Absolute.html http://www.ph.rhul.ac.uk/schools/ZeroT/Absolute.html]
[[category:physics]]