Absolute zero is defined as exactly minus 273.15 degrees Celsius, which represents zero on the Kelvin thermodynamic temperature scale. Because this is a matter of definition and standard scientific measurement, no citation is needed.
The claim states the exact numerical value of absolute zero in Celsius, which is a matter of definition by international agreement (specifically defining the Kelvin scale relative to the Celsius scale). According to the prompt instructions, a claim that is trivially true by definition or direct everyday observation is classified as COMMON_KNOWLEDGE, requiring no citation.