Why the Coldest Place in the Universe Isn’t Space

Outer space is often imagined as the coldest environment possible: a dark, nearly empty expanse far from the warmth of stars and planets. Yet the lowest artificial temperature publicly recognized on record was not found in a distant nebula. It was created in an experiment on Earth, where researchers reduced the kinetic energy of a cloud of atoms to an extraordinary level corresponding to just 38 picokelvin above absolute zero.

The result was achieved by researchers associated with the QUANTUS collaboration using a Bose–Einstein condensate of rubidium-87 atoms and an advanced matter-wave lensing system. The experiment was conducted at the Bremen Drop Tower in Germany, and its findings were published in Physical Review Letters in 2021.

How Cold Is 38 Picokelvin?

The kelvin is the scientific unit of thermodynamic temperature. Its scale begins at absolute zero, written as 0 K, which is equivalent to −273.15°C or −459.67°F. A picokelvin is one trillionth of a kelvin, so 38 picokelvin can be written as:

At ordinary scales, the difference between this value and absolute zero is impossible to feel or observe directly. It becomes meaningful only through exceptionally sensitive measurements of atomic motion and expansion.

Temperature is related to the distribution of energy within matter. In a gas, warmer particles generally move more rapidly and in a wider range of directions. As a gas is cooled, its random thermal motion decreases. At picokelvin energy scales, a cloud of atoms expands so slowly that its movement must be measured using precision optical and magnetic instruments.

Absolute Zero Does Not Mean Perfect Stillness

Absolute zero is the lowest limit of the thermodynamic temperature scale. It is sometimes described as the point where atoms stop moving, but that explanation is incomplete.

Classical thermal motion would reach its minimum at 0 K. Quantum mechanics, however, predicts that particles can retain a small amount of unavoidable motion known as zero-point motion. The Heisenberg uncertainty principle prevents both a particle’s exact position and exact momentum from being fixed simultaneously.

Reaching exactly 0 K is also considered physically unattainable through a finite series of cooling operations. Scientists instead approach it ever more closely, measuring progressively smaller amounts of residual thermal or kinetic energy.

What the Bremen Experiment Actually Cooled

The phrase “coldest place on Earth” can create a misleading mental picture. Researchers did not refrigerate an entire laboratory, room, or building to 38 picokelvin. The record involved a carefully prepared cloud containing roughly one hundred thousand rubidium atoms.

Those atoms formed a Bose–Einstein condensate, an unusual state of matter produced when particles known as bosons are cooled until many of them occupy the same quantum state. Instead of behaving only as separate particles, the atoms begin displaying collective wave-like properties on a scale large enough to study experimentally.

The reported 38-picokelvin figure represented the condensate’s three-dimensional internal kinetic energy expressed as an equivalent temperature. Its measured uncertainty was approximately +6 and −7 picokelvin. This distinction matters because the value describes an extremely low expansion energy rather than the everyday equilibrium temperature of a solid object sitting inside a refrigerator.

How Matter-Wave Lensing Slowed the Atoms

Creating ultracold matter requires several stages. Researchers first prepare, trap, and cool atoms until a Bose–Einstein condensate forms. The Bremen team then used the interactions within the condensate and a carefully timed magnetic field to control how the atomic cloud expanded.

The method can be compared with focusing light through a lens, although the “lens” acts on matter waves rather than visible light. The researchers excited a controlled collective oscillation in the condensate and combined it with a magnetic lens. When timed correctly, the process reduced the velocities at which the atoms moved away from one another.

This technique is often described as matter-wave collimation or delta-kick cooling. It does not remove energy in exactly the same way as an ordinary refrigerator. Instead, it reshapes and slows the expansion of the atomic cloud, producing an exceptionally small kinetic-energy spread.

The experiment benefited from the Bremen Drop Tower’s microgravity conditions. During free fall, the atomic cloud could expand without resting on a container and with greatly reduced influence from gravity. That gave researchers more time and freedom to manipulate its matter-wave behavior.

Is Empty Space Really Colder?

Space does not have one universal temperature. A vacuum is defined largely by the absence of matter, so describing the vacuum itself as “hot” or “cold” can be misleading. Scientists generally discuss the temperatures of particles, dust, gas, radiation, spacecraft, and other objects within space.

Even the darkest regions between stars are filled with faint cosmic microwave background radiation—the afterglow of the early universe. Averaged across the sky, that radiation has a temperature of about 2.73 K.

Objects in space can be warmer or colder than this background depending on how they absorb, emit, and interact with radiation. Sunlit spacecraft may become extremely hot, while shielded instruments can be cooled to temperatures close to absolute zero.

The scale above is illustrative rather than proportional. Picokelvin values are so much smaller than one kelvin that a truly proportional bar would be nearly invisible.

The Boomerang Nebula: Nature’s Deep Freeze

Although human-made atomic experiments have reached lower effective temperatures, the Boomerang Nebula remains the best-known example of an extremely cold natural environment. Located about 5,000 light-years away in the constellation Centaurus, its inner gas has been measured at roughly 1 K.

The nebula became so cold because gas was expelled rapidly from its aging central star. As that gas expanded, it lost internal energy and cooled—similar in principle to the way an expanding gas can become colder on Earth. The expansion was powerful enough to cool parts of the nebula below the temperature of the cosmic microwave background.

Why Scientists Create Such Extreme Cold

Cooling atoms to picokelvin energy scales is not simply a competition to produce a smaller number. Thermal motion can blur delicate quantum effects. When random motion is reduced, researchers gain more time to observe and manipulate atomic wave behavior.

Ultracold atomic systems support research in several important areas:

• Atom interferometry: Atomic matter waves can be split and recombined to measure acceleration, rotation, gravity, and extremely small forces.
• Tests of fundamental physics: Long-lived atomic wave packets may help researchers test gravity, the equivalence principle, and possible deviations from established quantum theory.
• Quantum simulation: Controlled atomic gases can imitate complex materials and interactions that are difficult to calculate directly.
• Precision measurement: Slower atoms can remain observable for longer periods, improving the sensitivity of certain clocks, sensors, and interferometers.
• Space-based research: Microgravity environments allow ultracold clouds to expand for longer without falling out of the experimental region.

Extreme cooling is also important elsewhere in quantum technology, including some types of quantum computers. However, the Bremen experiment was primarily designed to improve matter-wave optics and precision interferometry—not to serve as a ready-made quantum processor.

Laboratory Cold and Everyday Cold Are Different

A sample can earn a temperature record while being extremely small and short-lived. That is very different from cooling a large object, a room, or an industrial system. The difficulty of cooling generally increases with the amount of material involved, the required duration, and the number of ways heat can enter the system.

For this reason, several “coldest” records can coexist:

• The lowest effective temperature achieved in a small atomic cloud
• The lowest temperature maintained in a large volume
• The coldest naturally occurring astronomical environment
• The coldest operational spacecraft instrument
• The lowest temperature reached by a particular material or degree of freedom

A headline claiming that something is “the coldest place in the universe” should therefore be read carefully. The answer depends on what was cooled, how temperature was defined, how much material was involved, and whether the result was natural or artificially produced.

The next time someone says that outer space must be the coldest place imaginable, the more accurate response is wonderfully surprising. Nature’s coldest known astronomical region is the Boomerang Nebula, while one of the lowest artificial kinetic-energy temperatures ever reported was created in a falling capsule inside a tower in Bremen, Germany.