What's The Hottest Object In The Universe?

Illustration: Benjamin Currie, Gizmodo

One hundred degrees might seem hot, when you’re sweating through your shirt in July—but on a cosmic scale it barely registers. The Sun itself is over 15 million degrees; and in a hottest-object contest, the Sun wouldn’t even rank. Scientists, in fact, have produced temperatures many times that, right here on Earth (in terms of kinetic energy in microscopic places). We reached out to a number of scientists for this week’s Giz Asks—astronomers and physicists—to find out what the hottest object in the universe actually is.

Abraham Loeb

Professor, Astronomy, Harvard University

The hottest object in the Universe, literally speaking, is the Big Bang. If we go back in time, the Universe gets denser and hotter without a limit. The Big Bang singularity marks the breakdown of Einstein’s theory of gravity, where the density and temperature of matter and radiation diverge to infinite values. To treat the Big Bang properly, we need a to modify Einstein’s equations by incorporating quantum mechanics. Unfortunately, we do not have a reliable theory of quantum gravity that can tell us what happened at and before the Big Bang.

In a recent paper we wrote with Xingang Chen and Zhong-Zhi Xianyu, we showed that one could observe what may have happened before the Big Bang in the cosmic microwave background. The other singularities we find in the Universe are black holes, and our hope is that the divergences there will also be cured by the same theories.

James Beacham

Postdoctoral Researcher Physics, Duke University and particle physicist at CERN

I think the hottest known objects in the universe are the collision points created by heavy ion collisions like those at RHIC, at Brookhaven on Long Island, and here at the Large Hadron Collider at CERN. These are hotter than what we think the temperatures of supernovae are. That said, we can’t rule out the possibility that another civilisation somewhere in the universe has a more advanced particle physics program than ours and achieves heavy ion collision energies higher than those of the LHC.

There’s an interesting corollary to this. When we collide heavy ions at about 5 tera electron Volts in the Large Hadron Collider, the LHC is both the coldest extended object in the universe—because the 27-km of magnets used to bend and steer the beam in the LHC tunnel are bathed in liquid helium at 1.9 K, colder than the 2.7 K of outer space—and is simultaneously creating the places, the collision points, with the hottest temperatures in the universe. A truly unique object.

Eileen T. Myer

Assistant Professor, Physics, University of Maryland, Baltimore County

I’ll be that scientist and say “what do you mean by hot?” Most people would say “duh, the highest temperature.” But “temperature” is actually a more loaded concept than people might realise.

The most common way we use it implies that you have some “stuff” (water, the air inside your oven, etc) which is in something called “thermal equilibrium.” What this means, colloquially speaking, is that any individual unit of your stuff has an internal energy which is pretty similar to all the rest. If you made a histogram, they’d form something that looks a bit like a bell curve: most of the stuff is close to the average in energy. When that is the case, then temperature has a well-defined meaning. So, giving the caveat that I’m talking about these kinds of “thermal” sources, and sticking with things that actually last a long time at these extremes, the answer is, weirdly, kind of a tie: on the one hand, the centres of stars, where the plasma is so dense that elements are being fused together (up to 200 million degrees for the biggest fastest-burning stars); on the other, the incredibly thin gas that stretches between galaxies in galaxy clusters, which can also reach hundreds of millions of degrees, as in the enormous cluster we call “El Gordo”. How exactly that gas is heated is still being studied, but most people think it has to do with the eruptions from monster black holes at the centres of galaxies in the cluster.

Of course, there is one place that has even these guys beat, if only for a very brief moment, and that is massive stars at the instant they go supernova. Those bad boys can get up to billions of degrees.

Assuming that “hottest” might also be liberally interpreted to mean “super energetic,” I’ll also put in a good word for the objects that I study, which are these enormous jets of plasma that are powered by supermassive black holes at the centres of galaxies (jets that are probably heating up the gas around their host galaxies). These sources are what we call “relativistic.” The plasma in them is fully ionized, and the electrons in that plasma are moving with ridiculously high speeds—we’re talking 99.9999% of the speed of light. Science fiction and space travel fans probably know that while it obviously takes more and more energy to make something go faster and faster, as you approach the speed of light the energy required increase exponentially. In fact it takes infinite amounts of energy to accelerate any massive object up to the speed of light, so reaching 99% or more requires a TON of energy. The total kinetic energy in these jets from black holes is enormous. By some measures, they’re the most energetic events in the Universe, thanks to their long lifetimes of hundreds of thousands of years or more.

Spencer Klein

Senior Scientist at Lawrence Berkeley National Laboratory and Research Physicist at UC Berkeley

The answer depends on the definition of ‘object.’ Temperature is a measure of average energy per particle, but only with a large number particles that have thermalized (reached similar average energies). We’d never discuss the temperature of a single atom, just like we wouldn’t call a single molecule a liquid or a gas. Nor would we use the term temperature if the individual particles have vastly disparate energies.

Normally, we’d like to have 100-1,000 particles to talk about a thermalized system. With this limit, the hottest terrestrial objects are the quark gluon plasmas (QGPs) produced when CERN’s Large Hadron Collider (LHC) collides two lead nuclei. These objects contain thousands of quarks and gluons produced when the kinetic energy of the lead nuclei converts into nuclear matter. The quarks and gluons expand and cool, eventually combining to form protons and other hadrons. The highest temperature reached depends when we can first call this a Quark Gluon Plasma, which depends on how long it takes for the quarks and gluons to interact enough to thermalize. This is not well understood. So, it is not easy to define a time when the QGP first exists as a thermalized object. One calculated initial time corresponds to a temperature of about 5.5 trillion degrees Kelvin. It’s the same temperature in Celsius; the 273 degree difference doesn’t matter, or about 10 trillion degrees Fahrenheit. With a different initial time, the temperature might be 50% higher or lower.

If the question has no constraints on space and time, the Big Bang was much hotter than this. As with the QGP, deciding when it first becomes a thermalized object depends on the definition of ‘object,’ but even ignoring the very early (and very hot) times that we can really only speculate about, it is easy to get temperatures above 100 trillion trillion degrees.

Kevin A. Pimbblet

Senior Lecturer, Astrophysics, University of Hull, UK

If we include in our definition of the Universe everything that has happened in the past, then the hottest object in the Universe was the Big Bang itself. Current thinking places the temperature of the Universe close to the moment of the Big Bang as 10 raised to the power of 32 Kelvin (a number with 32 zeroes at the end of it, or 100 million million million million million!). Clearly this is unimaginably hot to our everyday lives and not something we can readily measure.

So what about things we can measure? Surprisingly, we have measured some very hot temperatures right here on planet Earth. CERN’s Large Hadron Collider produced a temperature of over 5 trillion Kelvin in 2012 when they smashed together heavy ions travelling a fractionally below the speed of light. This is the hottest measured temperature that we know of.

But what about more recently? The answer is likely to be when a massive star collapses in which its core can reach a temperature of some 10s of billion Kelvin.

Do you have a burning question for Giz Asks? Email us at [email protected]

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