### What is the hottest thing in the universe?

• 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

@BruceBecker I think the two questions are distinctly different. One asks for an astronomical object, the other seems ... entirely unrelated to astronomy, actually.

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.

@Luaan the currently observable temperature of Big Bang's afterglow i.e. cosmic microwave background is 2.7K, which is not that hot compared even to me.

Thanks for that video link, @SpaceBread! It was really interesting and fun. I watched the whole thing.

@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.

3 years ago

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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