Temperature measures the average kinetic energy of particles within a substance. When a material is hot, its atoms and molecules move quickly, possessing high kinetic energy. As a material cools, particle movement slows down, reducing kinetic energy. The search for the universe’s lowest temperature is a search for the point where this particle motion is at its absolute minimum. This quest spans from theoretical physics to deep space and cutting-edge laboratories on Earth.
The Limit of Cold: Absolute Zero
The theoretical minimum temperature for any physical system is Absolute Zero, defined as 0 Kelvin. This is equivalent to approximately -273.15 degrees Celsius or -459.67 degrees Fahrenheit. At this temperature, particles possess the minimum possible energy, and all thermal motion effectively ceases.
This concept is formalized by the Third Law of Thermodynamics. The law states that as a system approaches Absolute Zero, its entropy, or molecular disorder, approaches a constant minimum value. The law also implies that reaching 0 Kelvin is physically impossible through any finite number of steps.
Removing the final thermal energy becomes progressively harder as the temperature drops. It would theoretically require an infinite amount of work to extract all remaining energy, making 0 Kelvin an unattainable theoretical boundary. Absolute Zero serves as the baseline for all scientific temperature measurements.
The Coldest Natural Environment
The vast emptiness of space is permeated by a faint, uniform warmth set by the Cosmic Microwave Background (CMB). The CMB is relic radiation left over from the Big Bang, representing the universe’s background temperature.
The temperature of this universal background is approximately 2.7 Kelvin (-270.4 degrees Celsius). This radiation has cooled over the 13.8 billion years since the universe began and acts as a permanent thermal bath for everything not actively heated by a star. The CMB represents the average minimum temperature of the cosmos.
One known exception to this universal floor is the Boomerang Nebula, located about 5,000 light-years away. This dying star ejects gas at high speed, causing it to expand rapidly. This rapid expansion cools the gas dramatically through adiabatic expansion. This effect has cooled portions of the nebula to approximately 1 Kelvin (-272 degrees Celsius), making the Boomerang Nebula the coldest known natural object in the universe.
Surpassing Cosmic Cold in the Laboratory
Scientists on Earth have created temperatures far colder than the Boomerang Nebula using advanced cooling techniques. Researchers cool matter to fractions of a degree above Absolute Zero by manipulating atoms in controlled laboratory environments. These experiments primarily study the quantum mechanical phenomena that occur when particle motion is nearly stopped.
Cooling Techniques
One primary method combines laser cooling and magnetic trapping. Laser light slows the movement of atoms, dramatically reducing their kinetic energy. A magnetic field then traps these supercooled atoms, allowing further cooling through radiofrequency evaporation. This process can produce a fifth state of matter, known as a Bose-Einstein Condensate, where individual atoms behave as a single quantum entity.
The Coldest Record
The current record belongs to a team in Germany that cooled a cloud of rubidium atoms to just 38 picokelvin, or 38 trillionths of a degree above Absolute Zero. To maintain these ultracold conditions, the experiment utilized a 120-meter drop tower. This allowed the atoms to be suspended in near-weightlessness during the final cooling phase. Such extreme cold allows physicists to observe fundamental quantum effects usually masked by thermal noise.