It is often said that the universe was in a hot, dense state in its earliest moments. While that’s a pretty accurate description, it’s also pretty vague. What exactly was hot and dense and what condition was it in? Answering this question requires both complex theoretical models and high-energy experiments in particle physics. But as a recent study shows, we learn quite a bit.
According to particle physics and the Standard Cosmological Model, matter appeared within the first microsecond of the universe. This initial affair is believed to be a thick soup of quarks interacting in a sea of gluons. This state of matter is known as quark-gluon plasma (QGP). The behavior of QGP is determined by the strong force that follows the laws of quantum chromodynamics (QCD). While we understand QCD relatively well, the math of the theory is so complex that it is difficult to compute. Even with supercomputers it is difficult to calculate the state of dense quark-gluon interactions.
A look in ALICE at the Large Hadron Collider. ALICE is one of the LHC’s four particle detectors. Image: CERN / LHC
The alternative is to use the Large Hadron Collider at CERN. Smash particles at almost the speed of light and you can make a quark-gluon soup for a brief moment. The ALICE Collaboration studied these types of collisions to not only study the state of QGP, but also how the plasma transitions to hadrons. The two most common types of hadrons are protons and neutrons, which make up the atomic nucleus.
One of their surprising discoveries is that, like other plasmas, quark-gluon plasma does not behave like a dense gas. Instead, like water, QGP acts more dense than a dense liquid. As a result, its overall density is smoother. This difference is subtle, but it could hold keys to understanding the critical shift that likely occurred in the early universe.
In the Standard Cosmological Model, the early universe underwent a dramatic phase change to transform into the universe we see today. Before the QGP period, the universe had a period of exponential expansion. Almost immediately, the observable universe expanded by a factor of 1026 and cooled down by a factor of 100,000. This expansion and hypothermia ushered in the QGP period. Understanding fluid behavior therefore helps us study this transition period.
There is still a lot to learn about the early universe. Studies like this one from the ALICE collaboration are crucial to our understanding. They are reaching the limits of high energy physics and continue to exceed our expectations.
Reference: Acharya, S. et al. “Measurements of mixed harmonic cumulants in Pb-Pb collisions at sNN = 5.02 TeV.” Physics Letters B (2021): 136354.