Quark-gluon plasma, a phase of matter made up of nearly free quarks and gluons, is currently considered the rarest substance in the universe. It was the normal state of matter in the first millionth of a second after the Big Bang and can now be created in particle accelerators. However, quark stars, if they exist, may be even rarer and denser. Other exotic particles also vie for the title of rarest substance.
The rarest substance in the universe is probably quark-gluon plasma or something similar. This is a phase of matter generated only under the most intense temperatures and pressures. For most of the first millionth of a second after the Big Bang, the explosive event that created our universe, all matter was in the form of quark-gluon plasma. Quarks and gluons are particles that make up nucleons such as neutrons and protons, which in turn make up the atoms that make up all matter. Quarks are the particles with mass, while gluons are the force-carrying particles that “glue” the quarks together.
Although quark-gluon plasma is currently a contender for the rarest substance in the universe, in the beginning it was the normal state of matter. A quark-gluon plasma is a bath of nearly free quarks and gluons, which are typically tightly locked into nucleons. Conventional nucleons are so tightly held together that even a nuclear explosion or the temperature and pressure at the center of the Sun aren’t enough to shake them. Free quarks have never been observed, and some physicists think that the very phenomenon of free quarks is physically impossible.
Quark-gluon plasma is created under some unusual circumstances outside of the Big Bang. We’ve been able to produce it at will in particle accelerators, using huge amounts of energy focused on heavy ions, since the year 2000. It took us about two decades to try and create it, the rarest substance we know of. The feat was accomplished at the CERN particle accelerator in Switzerland. More recently, CERN’s Large Hadron Collider is conducting quark-gluon plasma experiments.
Quark-gluon plasma may not actually be the rarest substance if it happens to exist in the centers of extremely massive stars. Some neutron stars (the remnant left over from some of the largest supernovae) are denser than predicted by theory, leading some scientists to suspect that these aren’t actually neutron stars, but actually quark stars. Neutron stars have a radius of between 10 and 20 km (6 – 12 mi), but a mass slightly larger than that of the Sun. In contrast, quark stars, if they exist, would have a radius of between 3 and 9 km (2-6 mi) and a mass comparable to neutron stars, which would make them the densest objects in the universe. Supernova remnant RX J1856.5-3754, the closest neutron star to Earth, is a potential candidate for being a quark star.
There are other substances that vie for the title of the rarest substance in the universe. These include exotic particles created under very high-energy cosmic ray collisions and other exotic particles that existed at the dawn of the universe but have never been seen since. Antimatter doesn’t qualify as the rarest substance in the universe because it can still be found floating in space just about anywhere, albeit in very low proportions.
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