Michael: The universe is filled with plenty of hot things, from volcanoes to supernovas, but some things are hotter than others — and something has to hold the record for “hottest thing ever”, right? It turns out that it’s sort of hard to figure out what the hottest thing ever is, though. Because depending on what you mean by “ever,” it could be an experiment done in 2012, the stuff inside galaxy clusters, or even the whole universe, just a moment after it began.

Let’s start with the hottest thing in recorded history: an experiment from the Large Hadron Collider, or LHC, back in 2012. To learn what particles are made of and how they interact, the LHC accelerates ions or protons close to the speed of light before smashing them into each other. These collisions normally create explosions, but this one was a lot bigger than usual.

In August 2012, scientists were working on an experiment with a very descriptive, detailed name: A Large Ion Collider Experiment, or ALICE. The goal of the experiment was to learn about a substance from the dawn of time called quark-gluon plasma.

Quarks and gluons are elementary particles that make up bigger particles like protons and neutrons. We’ve never seen a quark or gluon by itself, but the early universe was filled with a super hot, soupy liquid called quark-gluon plasma, where quarks and gluons zoomed around almost independently. They permanently clumped together into larger particles when the plasma cooled, but we aren’t exactly sure how it happened.

And by creating the plasma themselves and watching it cool down, scientists are hoping to find out. But to create quark-gluon plasma, you need a huge explosion. And there’s no better way to create an explosion than by smashing things together.

In the ALICE experiment, the LHC accelerated heavy lead ions close to the speed of light, giving them enough energy that when they crashed together, they created a lot of pressure and heat. The result was a fireball so hot that, for a split second, it melted the ions into a quark-gluon plasma. That fireball was a whopping 5.5 trillion degrees Celsius.

Particle accelerators have created other hot explosions before, but this one was particularly hot because it used heavy lead ions instead of lighter particles. We don’t have any official results yet about quarks and gluons, but the Large Hadron Collider is our best and most powerful resource for recreating the conditions of early universe – and creating a lot of heat.

So, ALICE led to the hottest temperature in the entire universe, but only for a fraction of a second. To find the hottest thing on a normal day in the cosmos, you’ll have to travel a lot farther from Earth. Throughout the universe, galaxies are sometimes clumped together in neighborhoods called galaxy clusters.

But between the galaxies is another kind of super-heated plasma, called the intracluster medium, which can be up to 300 million degrees Celsius! We still aren’t exactly sure where this medium came from, but we’re starting to understand why it’s so hot.

Before galaxy clusters formed, they were just huge, swirling balls of matter. Eventually, that matter condensed, forming stars and galaxies and planets, and that process gave off a huge amount of energy, which was ultimately converted into heat. The more massive the galaxies, the more energy was given off, which made the intracluster medium hotter. But galaxy clusters formed billions of years ago, so you’d think the medium would have cooled down by now.

It hasn’t, though, because it’s being kept warm by black holes! After a black hole’s gravity sucks in matter, it gives off energy that’s turned into heat. And since a lot of galaxies have black holes at their centers, there are plenty of heat factories to keep the medium at the toasty temperatures created when the cluster formed.

But there was one event that was truly the hottest thing in the whole universe, for all of history. And that was the moment right after the Big Bang. When the universe began, it was an infinitely small, infinitely dense point that expanded into everything we know.

Our laws of physics don’t have an answer for what happened at the exact moment the universe was born, but we do have some guesses about what happened one-ten-billion-trillion-trillion-trillionth seconds after the Big Bang. And in that tiny fraction of a second, the universe would have been almost a billion-trillion-trillion degrees Celsius - that’s 1 followed by 32 zeroes!

It was probably so hot because all the matter and energy in the entire universe was crammed into one spot, meaning everything was under an incredible amount of pressure. And all that pressure caused a ton of heat. In fact, it might have been as hot as something could ever get: what’s called absolute hot.

Absolute hot is the opposite of absolute zero, the coldest something could ever be: -273.15 degrees Celsius, or 0 Kelvin. Some physicists believe absolute hot is 10^32 degrees Celsius, but it might also be a bit lower, like 10^30 or 10^17 degrees. Either way, it’s much hotter than anything we could imagine.

As the universe expanded, it cooled, which allowed quarks and gluons to come together, atoms to form, and everything you know and love to come into existence! So even though the universe at absolute hot was definitely weird and probably awesome, maybe it’s for the best that it cooled down eventually.

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