Short answers, pulled from the story.
Temperature is a quantitative expression of the average kinetic energy of the constituent particles of matter. It measures the frantic, invisible dance of atoms vibrating, colliding, and spinning within every object. This concept was established through the development of kinetic theory in the nineteenth century.
The lowest temperature ever achieved in a macroscopic system was 20 nanokelvins, recorded in 1995 at the National Institute of Standards and Technology. Scientists have approached absolute zero with astonishing precision, reaching temperatures as low as 38 picokelvins, but the third law of thermodynamics dictates that absolute zero can never be actually reached.
The definition of the kelvin scale shifted to rely on the Boltzmann constant on the 20th of May 2019. Before this date, the kelvin was defined by the triple point of water, which was set at exactly 273.16 Kelvin. This change links the macroscopic world to the microscopic behavior of particles.
William Thomson, later known as Lord Kelvin, published his seminal paper on absolute temperature in 1848. His work was based on the earlier ideas of Sadi Carnot regarding heat engines and provided the blueprint for an absolute temperature scale independent of the properties of any particular substance.
Systems with negative temperature on the thermodynamic scale are hotter than any positive temperature and will immediately transfer energy to any positive temperature system. This phenomenon occurs in systems with a limited number of energy states, such as a collection of particle spins in a magnetic field. Such negative temperature systems are unstable and effectively represent the hottest possible state.
The cosmic microwave background, the afterglow of the Big Bang, has a temperature of approximately 2.7 Kelvin. This remnant represents the hot, dense state of the early universe and serves as a baseline for measuring temperatures in interstellar space.