The Most Extreme Temperatures in the Universe: From Absolute Zero to the Planck Limit
Photo: Muhammad Nabeel / Pexels
In August 2018, a canister of rubidium gas was dropped 120 meters down a tower in Bremen, Germany. In the seconds of free fall, magnetic fields were switched on and off in carefully timed pulses, and the gas cooled to 38 picokelvin — 38 trillionths of a degree above the theoretical floor of all temperature. That record, published in Physical Review Letters in 2021, sits at one end of the most extreme physical scale the universe offers. At the other end, the ALICE experiment at CERN smashed lead nuclei together in 2012 to produce quark-gluon plasma at 5.5 trillion kelvin. Between these two points lies everything: every star, every cell, every chemical reaction that has ever occurred.
What Temperature Actually Measures
Temperature is not a substance and not a fluid — it is a measure of the average kinetic energy of particles in a system. The faster particles move, vibrate, or oscillate, the higher the temperature. In a solid, temperature reflects the vigor of atomic lattice vibrations. In a gas, it reflects the mean speed of molecules bouncing off each other and any containing walls.
Kinetic energy — the energy a particle has by virtue of its motion. Temperature is directly proportional to mean kinetic energy per particle in an ideal gas: the relationship is T = (2/3) × (KE_avg / k_B), where k_B is Boltzmann's constant (1.380649 × 10⁻²³ joules per kelvin).
This definition immediately implies that temperature cannot go below zero on the kelvin scale, because kinetic energy cannot be negative. At absolute zero (0 K, which equals −273.15 °C or −459.67 °F), particles would have their minimum possible energy. The word "minimum" here is doing significant work: quantum mechanics guarantees that even at 0 K, particles retain zero-point energy — a residual quantum jitter that can never be removed. This is why absolute zero is approached but never reached.
Zero-point energy — the lowest possible energy that a quantum mechanical system can have, arising from Heisenberg's uncertainty principle. A perfectly stationary particle would have a precisely known momentum of zero, violating the uncertainty principle; so particles are always jittering even at 0 K.
Source: NIST (2019), "The International System of Units (SI)," 9th edition — defines the kelvin as the SI unit of thermodynamic temperature, anchored to the Boltzmann constant.
Why Absolute Zero Is Unreachable
The third law of thermodynamics is one of the most fundamental constraints in physics: it states that the entropy of a perfect crystal at absolute zero is exactly zero, and as a consequence, no process can bring any system to exactly 0 K in a finite number of steps.
Think of it like trying to halve a distance to a wall. Each step takes you halfway closer. After ten steps you are very close. After a thousand steps you are extraordinarily close. But you never arrive. Each cooling technique, applied in sequence, removes a fraction of the remaining thermal energy — but each successive step becomes less efficient precisely because there is less energy left to remove. The wall recedes as fast as you approach it.
The practical consequence: experimentalists have gotten astonishingly close to absolute zero, but the record always carries a nonzero number. That number has been getting smaller for over a century, and the current minimum is 38 picokelvin. The gap between that and 0 K is smaller than any gap that came before, but it is not zero.
The third law of thermodynamics — formulated by Walther Nernst between 1906 and 1912, it states that the entropy of a system at absolute zero is a well-defined constant (taken as zero for perfect crystals), and that reaching absolute zero requires infinite work.
Source: Nernst, W. (1906), "Ueber die Berechnung chemischer Gleichgewichte aus thermischen Messungen," Nachrichten von der Gesellschaft der Wissenschaften zu Göttingen — the foundational paper establishing what became the third law.
The Coldest Temperature Ever Achieved: 38 Picokelvin
The record was set by the QUANTUS collaboration based at the University of Bremen and published in Physical Review Letters in October 2021. The team used the ZARM Drop Tower — a 146-meter concrete tower designed to create microgravity conditions for up to 9.2 seconds by dropping experiments in free fall.
The process started with 100,000 rubidium-87 atoms trapped in a magnetic field and laser-cooled to form a Bose-Einstein condensate (BEC) — a state of matter in which atoms are cooled so close to absolute zero that they collectively occupy the lowest quantum energy state and behave as a single quantum object rather than a collection of individuals. Think of it as the difference between a crowd of individuals each dancing to their own rhythm and an entire crowd that suddenly moves as one synchronized wave.
Bose-Einstein condensate (BEC) — a phase of matter predicted by Satyendra Nath Bose and Albert Einstein in 1924-25 and first experimentally achieved in 1995 at JILA/NIST by Eric Cornell and Carl Wieman, who shared the 2001 Nobel Prize in Physics for the work. In a BEC, atoms lose individual quantum identities and merge into a single macroscopic quantum state.
Once the rubidium BEC was formed, it was released into the drop tower's free-fall shaft. During the fall, the magnetic trap was switched on and off in rapid pulses. In normal gravity, the BEC tends to expand under gravity even while cooling, which limits how cold it can get — the expansion adds a small amount of kinetic energy. In microgravity, gravity's distorting influence is removed. The BEC could expand in three dimensions symmetrically, decelerating the expansion to near zero. The temperature corresponding to that near-zero expansion rate: 38 picokelvin.
For scale: the temperature of the cosmic microwave background radiation — the faint afterglow of the Big Bang, pervading all of space — is 2.725 K. The interior of a BEC at 38 picokelvin is more than 71 billion times colder than the universe's background temperature.
Source: Deppner, C. et al. (2021), "Collective-Mode Enhanced Matter-Wave Optics," Physical Review Letters, Vol. 127, 100401 — primary publication reporting the 38 picokelvin result from the QUANTUS Drop Tower experiment.
Source: Guinness World Records (2021), "Lowest Artificial Temperature" — independent verification of the 38 picokelvin record as the lowest temperature ever achieved by human apparatus.
The Hottest Temperature Ever Made: 5.5 Trillion Kelvin
In August 2012, the ALICE (A Large Ion Collider Experiment) collaboration at CERN's Large Hadron Collider announced results from lead-ion collisions that produced matter at approximately 5.5 trillion kelvin (5.5 × 10¹² K), equivalent to an energy of about 470 MeV per particle. The result was announced at the Quark Matter 2012 conference in Washington, D.C., and represented a 38% increase over the previous record of roughly 4 trillion kelvin achieved at Brookhaven National Laboratory's Relativistic Heavy Ion Collider (RHIC) in 2010.
At this temperature, ordinary protons and neutrons dissolve. Matter exists not as atoms, not as nuclei, but as a quark-gluon plasma (QGP) — a soup of the fundamental constituents that are normally confined inside protons and neutrons. In everyday matter, quarks are permanently bound together by the strong nuclear force, mediated by particles called gluons. The binding is so strong that no force exerted by any conventional means can separate a single quark from its partners. But at trillions of degrees, thermal energy exceeds the binding energy, and quarks and gluons roam freely.
Quark-gluon plasma (QGP) — the state of matter that existed in the first microsecond after the Big Bang, when the universe was too hot and dense for quarks to be confined inside hadrons. The LHC recreates this state for a fraction of a second in lead-ion collisions, allowing physicists to study the properties of matter at its most fundamental.
The QGP created at ALICE behaves as an almost perfect liquid with extremely low viscosity — described by physicists as a "nearly perfect fluid." It persists for about 10⁻²³ seconds before cooling and allowing quarks to recombine into ordinary protons and neutrons. In that tiny window, the experiment records the particle shower as the plasma cools, reading the properties of the QGP backward from the products of its decay.
Source: ALICE Collaboration (2012), "Quark Matter 2012 conference proceedings," CERN — announcement of the 5.5 trillion kelvin quark-gluon plasma temperature record.
Source: Guinness World Records (2012), "Highest Man-Made Temperature" — independent recognition of the ALICE result as the hottest temperature ever measured.
Natural Extremes: Stars, Supernovae, and Neutron Star Mergers
Laboratory records exist on a very narrow slice of the temperature scale. Nature operates across a far wider range.
| Event / Object | Temperature | Notes |
|---|---|---|
| Surface of the Sun | ~5,778 K | Photospheric temperature; varies by latitude |
| Solar corona | 1–3 million K | Paradoxically hotter than the surface |
| Core of the Sun | ~15 million K | Nuclear fusion of hydrogen to helium |
| Core of a massive star pre-collapse | ~3–7 billion K | Silicon burning before core collapse |
| Core of a Type II supernova | ~100 billion K (~10¹¹ K) | Seconds after core collapse |
| Neutron star merger (kilonova) | ~10¹¹–10¹² K | r-process nucleosynthesis of heavy elements |
| Big Bang (1 second after) | ~10¹⁰ K | Neutrino decoupling era |
| Big Bang (10⁻⁶ seconds after) | ~10¹³ K | Quark-gluon plasma era |
| Planck epoch (10⁻⁴³ seconds after) | ~1.416 × 10³² K | Planck temperature; physics as known breaks down |
The core of the Sun sits at roughly 15 million K — hot enough for hydrogen nuclei to overcome their mutual electromagnetic repulsion and fuse into helium via the proton-proton chain. This process releases energy at a rate of about 276 watts per cubic meter in the core, which sounds modest but adds up across the Sun's enormous volume to a luminosity of 3.828 × 10²⁶ watts.
Supernovae are a different category entirely. In the seconds after a massive star's iron core collapses, the temperature in the collapsing core peaks around 10¹¹ K. At these temperatures, iron nuclei are photodisintegrated — broken back apart into protons and neutrons by high-energy photons — and a flood of neutrinos carries away approximately 99% of the explosion's energy. The neutrino luminosity of a core-collapse supernova in its first ten seconds exceeds the combined light output of all the stars in the observable universe.
Source: Woosley, S.E. and Janka, T. (2005), "The Physics of Core-Collapse Supernovae," Nature Physics, Vol. 1 — comprehensive review of temperatures and neutrino emission during stellar core collapse.
Source: Abbott, B.P. et al. (2017), "GW170817: Observation of Gravitational Waves from a Binary Neutron Star Inspiral," Physical Review Letters, Vol. 119 — the neutron star merger event that produced observed r-process kilonova temperatures.
The Planck Temperature: The Wall at the Top
The Planck temperature is 1.416808 × 10³² kelvin, derived from three fundamental constants: the speed of light (c), the gravitational constant (G), and the reduced Planck constant (ħ). The formula is T_P = √(ħc⁵/Gk_B²), where k_B is Boltzmann's constant.
This is not merely a very hot temperature — it is the temperature at which the physics used to describe it breaks down. At the Planck temperature, the thermal energy of a single particle equals the Planck energy, and the corresponding wavelength (the de Broglie wavelength) equals the Planck length — the scale at which quantum gravitational effects become dominant and neither general relativity nor quantum mechanics, taken individually, can correctly describe what happens.
Think of it like trying to read a sign while running at it — above a certain speed, the sign blurs beyond legibility. At the Planck temperature, the universe becomes illegible to current physics. The Big Bang is thought to have begun at approximately the Planck temperature, meaning the very first instant of the universe's existence is cut off from direct theoretical description by our current best tools.
Planck units — a system of natural units defined by Max Planck in 1899, in which fundamental constants including c, G, and ħ are all set to 1. The Planck scale represents the boundary where quantum mechanical effects and gravitational effects are of equal importance — a regime where a theory of quantum gravity would be required.
Source: Planck, M. (1899), "Über irreversible Strahlungsvorgänge," Sitzungsberichte der Königlich-Preußischen Akademie der Wissenschaften — Planck's original paper introducing the natural unit system that defines the Planck temperature.
The Coldest Natural Temperatures: Interstellar Space and Boomerang Nebula
Absolute zero is unreachable, but parts of the universe get very close. The cosmic microwave background (CMB) fills all of space at 2.725 K. This is the temperature of the universe's "room" — the residual thermal radiation left over from the recombination era about 380,000 years after the Big Bang, when the universe cooled enough for electrons to bind to protons and form neutral hydrogen.
Some parts of space are colder than the CMB average. The Boomerang Nebula, located about 5,000 light-years away in the constellation Centaurus, has been measured at approximately 1 K — colder than the CMB. It achieves this because gas ejected from the central star expands rapidly, and adiabatic expansion (like gas leaving a pressurized can) cools the gas below the background temperature. It is the coldest known natural object in the universe.
Adiabatic cooling — the drop in temperature that occurs when a gas expands without exchanging heat with its surroundings. The expanding gas does work on its surroundings, which reduces its internal energy and therefore its temperature. It is the same principle that makes your fingers cold when you discharge a compressed air can.
Source: Sahai, R. and Nyman, L.A. (1997), "The Boomerang Nebula: The Coldest Region of the Universe?," The Astrophysical Journal Letters, Vol. 487 — observations showing the Boomerang Nebula at ~1 K, below the CMB temperature.
How the Temperature Extremes Rank: A Full Spectrum
| Temperature (K) | What It Is | Source |
|---|---|---|
| 0 | Absolute zero | Theoretical; unreachable |
| 3.8 × 10⁻¹¹ | Coldest lab temperature | Bremen QUANTUS, 2021 |
| ~1 | Boomerang Nebula (coldest known natural object) | ESO observations |
| 2.725 | Cosmic microwave background | Planck satellite measurement |
| 300 | Typical room temperature | — |
| 5,778 | Solar photosphere | NASA solar data |
| 1.5 × 10⁷ | Solar core | NASA/ESA helioseismology |
| ~4 × 10⁹ | Brookhaven RHIC quark-gluon plasma | Brookhaven NL, 2010 |
| 5.5 × 10¹² | LHC ALICE quark-gluon plasma (hottest man-made) | CERN ALICE, 2012 |
| ~10¹¹ | Type II supernova core | Stellar physics models |
| ~10¹²–10¹³ | Neutron star merger core | GW170817 kilonova models |
| 1.416 × 10³² | Planck temperature | Theoretical physics limit |
Source: Fixsen, D.J. (2009), "The Temperature of the Cosmic Microwave Background," The Astrophysical Journal, Vol. 707 — precise measurement of the CMB temperature at 2.7255 ± 0.0006 K.
What Happens Near Absolute Zero: The Quantum World Opens Up
The cold end of the scale is not a scientific curiosity — it is where some of the most technologically consequential physics happens. Near absolute zero, quantum effects that are normally masked by thermal noise become dominant.
Superconductivity — in many metals, cooling below a critical temperature (which varies by material, but is typically between 1 K and 30 K for conventional superconductors) causes electrical resistance to drop to exactly zero. Electrons pair up into Cooper pairs and flow through the lattice without scattering. High-temperature superconductors (those operating above 77 K, the boiling point of liquid nitrogen) are a major area of condensed-matter research because superconducting magnets drive MRI machines, particle accelerators, and may eventually enable lossless power transmission.
Superfluidity — liquid helium-4 cooled below 2.17 K becomes a superfluid: it flows without viscosity, climbs walls of its own accord, and conducts heat almost perfectly. A superfluid placed in a rotating bucket develops quantized vortices — a direct macroscopic demonstration of quantum mechanics.
The proximity of absolute zero is where BEC physics, quantum computing, and precision metrology converge. The most precise atomic clocks operate near absolute zero. Quantum computers based on superconducting qubits (including those built by IBM and Google) operate at around 15 millikelvin — colder than outer space by a factor of roughly 180.
Source: Bardeen, J., Cooper, L.N., and Schrieffer, J.R. (1957), "Theory of Superconductivity," Physical Review, Vol. 108 — the BCS theory of superconductivity, explaining Cooper pair formation near absolute zero.
Frequently Asked Questions
Q: Why can't we reach absolute zero?
A: The third law of thermodynamics — formulated by Walther Nernst in 1906 — states that reaching absolute zero would require removing every last joule of thermal energy, which takes an infinite number of steps. Each cooling stage becomes less efficient as temperature falls, because there is less energy left to extract. You approach the limit asymptotically, like repeatedly halving a distance, but you never arrive.
Q: Is the Planck temperature really the hottest possible temperature?
A: It is the hottest temperature that current physics can meaningfully describe. Above 1.416 × 10³² K, the thermal de Broglie wavelength of particles becomes smaller than the Planck length, and both general relativity and quantum mechanics break down simultaneously. Whether something can exist at a higher temperature is unknown — it would require a working theory of quantum gravity to answer.
Q: How does the LHC create temperatures hotter than the Big Bang?
A: The LHC does not recreate the entire Big Bang — it recreates a tiny volume of matter in the same thermal state that existed microseconds after the Big Bang. In lead-ion collisions, two nuclei smash together at 99.9% of the speed of light, converting enormous kinetic energy into thermal energy over a volume roughly the size of an atomic nucleus. The resulting quark-gluon plasma is hotter than the universe was during most of its first second of existence.
Q: What is a Bose-Einstein condensate and why does it get so cold?
A: A BEC forms when atoms are cooled so close to absolute zero that they all collapse into the same lowest-energy quantum state and behave as a single quantum object. The coldness comes from successive cooling stages — laser cooling, evaporative cooling, and then magnetic manipulation — that progressively strip away the most energetic atoms. In free fall, the BEC can expand without gravitational distortion, allowing its "temperature" (measured by how slowly it expands) to reach the picokelvin range.
Q: What is the coldest natural place in the universe?
A: The Boomerang Nebula, about 5,000 light-years away in Centaurus, is measured at approximately 1 K — colder than the cosmic microwave background (2.725 K). The nebula achieves this through rapid adiabatic expansion of gas ejected from a dying star, which cools the gas below the background radiation temperature. It is the coldest known object outside a laboratory.
Q: How hot is a neutron star merger compared to the LHC?
A: Neutron star mergers produce temperatures in the range of 10¹¹ to 10¹² K — comparable to the LHC's quark-gluon plasma temperatures and potentially higher in the core during the collision. These extreme conditions drive the rapid neutron-capture process (r-process), which forges heavy elements including gold, platinum, and uranium. The gravitational wave event GW170817 in 2017 was the first direct observation of a neutron star merger and its associated kilonova.
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SciHub101 is an independent science and technology publication founded and edited by Buğra Sözeri, a software engineer and app developer based in Türkiye. AI tools assist with drafting and research. Source documentation in the existing archive is uneven, and articles are being reviewed and corrected. The Editorial Team byline identifies editorial responsibility; it does not certify specialist or independent human review. Where we compare products, anything built by our own team is labelled as such — see the editorial guidelines and About page for how corrections and disclosures work.
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