In the immediate aftermath of the Big Bang, there were no atoms. Instead, the universe was filled with what physicists call a hot soup of elementary particles. It may not have lasted long, but this primeval broth helped shape everything that came afterward – us included. In a quest to understand its nature, physicists have come to a remarkable conclusion; the “soup” flowed like water, despite being unimaginably dense.
Technically known as quark-gluon plasma, the first matter is thought to have filled the early universe microseconds after there was a universe to fill. Professor Kostya Trachenko of Queen Mary University is attempting to reconstruct the nature of that plasma in the crucial fraction of a second before it cooled to allow atoms to form. He has explored its viscosity in SciPost Physics.
Quark-gluon plasma is not merely a thing of the past. It can exist today when temperatures are hot enough that the quarks that normally make up protons and neutrons get released, taking with them the gluons that carry the strong force to mediate their interactions. However, since it takes temperatures of trillions of degrees for this to happen (a million times hotter than the Sun's center), it's not easy to study. We can, however, recreate it – albeit exceptionally briefly – in particle colliders.
Since the entire universe was still packed unbelievably tightly, the original plasma-gluon soup was about 16 orders of magnitude (ie ten million billion) times denser than water, Trachenko has concluded, at the time he is interested in. However, by a remarkable coincidence, it was also 16 orders of magnitude more viscous (resistant to flowing).
Since a fluid's flow (somewhat confusingly known as its kinematic viscosity) is dictated by the ratio of its dynamic viscosity to its density, this means the plasma flowed at rates very similar to that of water. If there had been taps at the time, and anyone around to turn them, hot and (relatively) cold running quark-gluon plasma might have been on offer. Jokes aside, the finding suggests particles within the plasma moved like those in water, improving our capacity to understand their behavior.
Water's exceptional properties, such as the fact it gets less dense when it freezes, are a constant source of wonder to physicists since so few other liquids share them. Therefore it is strange to find something so different has a point of commonality with our most familiar substance.
Liquids' viscosity varies with temperature – sometimes quite dramatically, as anyone who has gently warmed honey knows. However, it has been observed that there is a near-universal lower limit for liquids' kinematic viscosity of νm, approximately equal to 10−7 meters squared per second. It seems the same rules apply to quark-gluon plasma.
Remarking on the similarity in flow of water and quark-gluon plasma, Trachenko said in a statement that "We do not fully understand the origin of this striking similarity yet but we think it could be related to the fundamental physical constants which set both the universal lower limit of viscosity for both ordinary liquids and quark-gluon plasma."