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Straight from my 7 year old to you, exactly what it says on the cover:

What is the hottest thing in the universe?

To make it Stack Exchange-friendly, I'll add the following caveats:

  • it should be bounded, as in an actual compact object, or class of objects, or part of an object
  • it should be observable
  • it should be an astronomical object, ie a Quark Gluon Plasma created by collisions at the Large Hadron Collider doesn't count.

Thanks, Bruce

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    $\begingroup$ Possible duplicate of astronomy.stackexchange.com/q/8324/5506 $\endgroup$ – Bruce Becker Aug 13 at 17:00
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    $\begingroup$ I would recommend your 7-year old this video: youtube.com/watch?v=4fuHzC9aTik $\endgroup$ – SpaceBread Aug 13 at 17:22
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    $\begingroup$ @BruceBecker I think the two questions are distinctly different. One asks for an astronomical object, the other seems ... entirely unrelated to astronomy, actually. $\endgroup$ – BMF Aug 13 at 18:22
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    $\begingroup$ I assume you don't want to hear "the Big Bang", right? :) The question is a bit tricky because what we observe today isn't the hottest thing anymore (given the interstellar distances and speed of light); and if you do include things we only observe today as "the hottest thing right now", the Big Bang would probably still be the answer, since we're still bathing in the "afterglow" 15 billion years later. $\endgroup$ – Luaan Aug 14 at 7:10
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    $\begingroup$ @Peteris The same is true of the accepted answer - the supernova we're now observing is no longer the hottest thing in the universe - it cooled down over the 200k years it took the neutrinos to get to us. If you count the original temperature and run the clock backwards on the microwave background radiation, you get relatively hot. But the MBR is still just from the point where everything cooled down enough for space to become largely transparent - the temperature of the actual Big Bang was much higher, though the estimates involve lots of uncertainty. $\endgroup$ – Luaan 2 days ago
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Energetic neutrinos have been observed from the core of a supernova (SN 1987A). The inferred temperature at the "neutrinosphere" is about 4 MeV (equivalent to 50 billion K - ($5\times 10^{10}$ K, Valentim et al. 2017). Hence it is observable and has been observed.

The very centre of the proto-neutron star that is responsible for the neutrino emission is likely to be a factor of two or so hotter, but cannot be observed, even with neutrinos, because the "neutrinosphere" is opaque to neutrinos. By the time this "clears", the proto-neutron star is much cooler - its surface would be orders of magnitude cooler.

Arguably we could study the very core of a supernova through gravitational waves if one were to explode in our own Galaxy. Whether this counts as "observing" a hot object, I'm not sure.

In a similar vein, we have observed "kilonova" that appear to be due to the merger of two neutron stars. The temperatures generated in these events are also likely to be of order 100 billion K ($10^{11}$ K), but again these temperatures are not observed directly - the gravitational waves and gamma rays produced in these events are caused by "non-thermal" mechanisms.

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    $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – called2voyage yesterday
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Note that while we haven't observed anything even close, there is a theorized Absolute Hot along the lines of absolute zero. Its theorized value is ~ $1.416 \cdot 10^{32}$ Kelvin. Above this temperature, it would be impossible to pump more energy into a system, even gravitationally.

That gives an upper bound on the maximum temperature we could measure.

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    $\begingroup$ That's the Planck temperature. Without a theory of quantum gravity, we cannot make any predictions about what (if anything) happens as you approach or reach it. Nor is there any foreseeable way to observe it. $\endgroup$ – OrangeDog 2 days ago
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If you're ruling out the big bang, then the most extreme releases of energy in our universe should be cases of runaway gravitational collapse. There is a rigorous theorem in general relativity (Penrose singularity theorem) showing that these will generically lead to the creation of singularities. For realistic gravitational collapse, it's expected that in the end state of this process you will have a black hole, which has an event horizon surrounding a certain specific type of singularity described as spacelike and not a strong curvature singularity (not s.c.s.).

However, during the initial process of formation of the black hole, it's not really fully established what kind of singularity you would have. It could be timelike rather than spacelike, could be an s.c.s., and might even not be surrounded by an event horizon (which would violate the cosmic censorship hypothesis -- but we don't know whether the CCH is true or even the best way to state it). If it's an s.c.s., then general relativity predicts that the infalling matter will be infinitely compressed, and therefore probably heated to infinite temperature. GR is a classical theory, so this should probably be interpreted as a statement that an s.c.s. would heat the matter to the Planck temperature.

So if an observer were to jump into a black hole during its initial formation, and if the observer was able to withstand the temperatures, then they might get a millisecond during which they could observe the matter around them being heated up to very high temperatures. Whether these temperatures would rise to the Planck temperature is not really known (probably not), and whether any of this might ever be observable from far away, without suicide, is not really known (but probably not).

Straight from my 7 year old to you, exactly what it says on the cover: What is the hottest thing in the universe?

So at this level, scientists don't really know for sure, but they think if you jump into a black hole while it's in the process of being born, you might be able to see matter heated to extremely high temperatures, probably hotter than anything else in the universe since the big bang.

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    $\begingroup$ This is a good addition, but I wonder really how "observable" this is. You need lot more than a ms and if you are relying on instruments, they have to survive. What would be the requirements to actually measure some sort of thermal radiation in situ? $\endgroup$ – Rob Jeffries yesterday

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