What is the hottest thing in the universe?

Pinpointing the absolute "hottest thing" in the universe is tricky, as it depends on what you mean by "thing" and how you measure temperature. There isn't a single, universally agreed-upon answer. However, some extremely hot phenomena are strong contenders:

• The moments after the Big Bang: The very earliest moments of the universe are believed to have been unimaginably hot. Temperatures in the Planck epoch (the first ~10^-43 seconds) are theoretically so high that our current physics models break down, making it impossible to assign a definite temperature. We're talking about temperatures far beyond anything we can even conceive of.

• Quark-gluon plasma: Created in high-energy particle collisions, this is a state of matter where quarks and gluons—normally confined within protons and neutrons—exist freely. Experiments at the Large Hadron Collider (LHC) have generated quark-gluon plasma with temperatures of trillions of degrees Celsius – incredibly hot, but still likely far cooler than the immediate aftermath of the Big Bang.

• The cores of neutron stars: These incredibly dense remnants of massive stars are believed to have temperatures of around 10¹² Kelvin (a trillion Kelvin). While significantly hot, this is still dwarfed by the temperatures immediately after the Big Bang or those hypothetically achieved in quark-gluon plasma.

It's important to note that:

• Temperature definitions change at extreme scales: The concept of temperature as we understand it (based on the average kinetic energy of particles) may not accurately describe conditions at energies approaching the Planck scale.
• Measurements are indirect: We infer the temperatures of these extremely hot phenomena through observations and complex theoretical models, not direct measurement.

In summary, while the cores of neutron stars and quark-gluon plasma are incredibly hot by any standard, the moments immediately following the Big Bang remain the strongest candidate for the "hottest thing" in the universe, even if we cannot accurately quantify its temperature with current physics.