What’s the Hottest Possible Temperature in the Universe?

It was recorded by a weather observation post, and the temperature was just one degree less than what the thermometer was capable of measuring. 

Need additional context for how hot that is? The human body starts to experience a rapid change in mental status, a lack of sweating, and faintness above 104 degrees Fahrenheit, according to the National Institutes of Health.

So it was deadly hot. 

The universe, however, can get a lot hotter — hotter than the hottest day on Earth, hotter than the Earth’s core, and far hotter than the Sun…or any known star, for that matter. 

‍What’s hot and cold? ‍

As you may already know, heat is the amount of energy that molecules have due to their movement. The faster the molecules move, the more energy they have and the more heat they produce. 

Absolute zero is the point when all molecular motion stops, so there’s no longer any heat being generated. It’s reached at -459.67 degrees Fahrenheit (-273.15 degrees Celsius). Scientists think it’s impossible ever to reach absolute zero. However, one strange and mysterious place in the universe has gotten really close. The Boomerang Nebula clocks an average of -458 degrees Fahrenheit  (-272 degrees Celsius). The icy cloud of gas maintains temperatures colder than empty space itself. 

How? Well, we aren’t exactly sure. 

However, researchers think the Boomerang nebula may be so cold because a red giant star sucked a smaller star into its heart. This event may have caused most of the red giant's matter to be rapidly ejected as an ultra-cold outflow of gas and dust. Lead researcher Raghvendra Sahai, an astronomer at NASA's Jet Propulsion Laboratory, explained in a statement, "most of the stellar envelope from the massive red giant star has been blasted out into space at speeds far beyond the capabilities of a single, red giant star.”

Sahai clarified that a pair of stars appeared to be the only viable explanation. "The only way to eject so much mass and at such extreme speeds is from the gravitational energy of two interacting stars, which would explain the puzzling properties of the ultra-cold outflow," he continued.

On the other extreme, absolute hot is the point when molecular motion can’t produce any more heat. It’s called the Planck temperature, after the German physicist Max Planck. It’s the highest temperature in conventional physics, and it’s reached at roughly 2,556,000,000,000,000,000,000,000,000,000,000 degrees Fahrenheit (1,410,000,000,000,000,000,000,000,000,000,000 degrees Celsius). For readers who prefer shorter numbers, that’s 2.55x10³²  degrees Fahrenheit (1.42x10³² degrees Celsius) 

But no matter what way to write it, it’s an absurdly large number. 

Above this temperature, physics as we know it starts to break down. The gravitational force becomes equally as strong as the other fundamental forces (electromagnetism, and the strong and weak nuclear forces), and space and time cease to have any real meaning. To put it simply, inexplicable things that we can’t fathom happen. So, until we have a quantum theory of gravity, the Planck temperature will likely remain the highest temperature in conventional physics. 

Of course, particles begin to break down much sooner than the fundamental forces. The Hagedorn temperature, first conceived by Rolf Hagedorn in the 1960s, represents the maximum temperature in particle physics. The temperature sits around 1.7×10¹²K. At this point, hadronic matter (the ordinary matter in the universe) becomes unstable, and physicists believe it either A) Evaporates or B) Transitions into quark matter, which can then be further heated. 

However, quark matter is only a hypothetical phase of matter, and we aren't sure if it actually exists, meaning that further heating beyond the Hagedorn temperature is uncertain. So, until we have evidence that it does exist, the Hagedorn temperature will remain the highest temperature in particle physics. 

‍Comparing the hottest things in the known universe

Luckily, nothing in the universe is remotely as hot as absolute hot. Such temperatures would be very bad for life as we know it. However, if we look back at the history of the universe, things get a little close. Scientists think the hottest temperature ever reached occurred a tiny fraction of a second after the Big Bang  – 10^-43 seconds after, to be precise. At this time, the universe was 1.8x10³² degrees Fahrenheit (10³² degrees Celsius). 

Since then, the hottest temperature we’ve encountered was in the lab. Yes, in a lab. Specifically, it was in the Large Hadron Collider in 2012. Scientists smashed lead ions together at 99 percent the speed of light to create a quark-gluon plasma, a state of matter that’s thought to have filled the universe right after the Big Bang.

In the process, they reached temperatures as high as 9.9 trillion degrees Fahrenheit (5.5 trillion degrees Celsius). 

Outside of the lab, celestial objects and phenomena can get really hot, too. 

The hottest object in our solar system is, obviously, the Sun. While the surface of the Sun is a cool 10,000 degrees Fahrenheit (5,538 degrees Celsius), the core is a cauldron at 27 million degrees Fahrenheit (15 million degrees Celsius). 

Next up, the hottest planet in our solar system is Venus. You might be thinking that it should have been Mercury, and you’re not silly for thinking this. Mercury is far closer to the Sun, but Venus’ thick atmosphere is full of the greenhouse gas carbon dioxide (hits a bit too close to home, right?). The atmosphere traps heat from the Sun like a furnace, so average surface temperatures fluctuate around 867 degrees Fahrenheit (464 degrees Celsius), though daily temperatures can soar above 900 degrees Fahrenheit (475 degrees Celsius).

Now, let’s move beyond our own little solar system. When a star that’s eight times more massive than our Sun dies, it explodes in a powerful, luminous display known as a “supernova.” On the outside, the star swells into a red supergiant. On the inside, the core shrinks. As it shrinks more and more, it grows hotter and denser. Then, before it explodes, temperatures in the core soar up to 180 billion degrees Fahrenheit (100 billion degrees Celsius). That’s 6,000 times hotter than the Sun’s core. 

But things in the cosmos can get even hotter. When supermassive black holes are consuming matter, they blast excess gas into space and create massive beams of blistering radiation called quasars. Scientists found that the core of one particular quasar, 3C 273, was hotter than 18 trillion degrees Fahrenheit (10 trillion degrees Celsius). They’re still working out the details and exact numbers, though.  

When scientists search for the coldest and hottest things in the universe, they find some of the most fascinating phenomena. I guess that’s what happens when you push things to their limits. And so, in the future, we may find evidence of new events and objects that increase the hottest known temperatures. If that happens, we'll be sure to update this handy article.