Strong Nuclear Force Is Not So Weak at Electroweak Temperatures
Soon after the big bang, the Universe was filled with a hot primordial particle soup, with freely streaming quarks and gluons among the ingredients. As the Universe cooled, the strong force steadily strengthened until, at a temperature in energy units of about 0.15 giga-electron-volts (GeV), or 2 × 1013 K, it could bind the quarks and gluons into protons, neutrons, and other hadrons. Now a new computation by researchers at the University of Milano-Bicocca and the National Institute for Nuclear Physics (INFN) in Italy has traced this thermal history of quarks and gluons back even further to evaluate how important the strong force was before the hadrons emerged [1].
The researchers used a computational technique called lattice quantum chromodynamics. The method discretizes continuous space-time into the largest and finest grid of points that can fit within a supercomputer’s memory. At the end of the simulation, the spacing between the points is extrapolated to zero. The need for extrapolation limited previous simulations to temperatures below 1 GeV. The researchers found that by keeping a certain quark–gluon coupling constant fixed as they made the space-time grid finer, the spurious effects of the grid are considerably reduced, enabling controlled extrapolations at high temperatures.
Putting their new method to work, the researchers computed the pressure of quark–gluon plasma made of up, down, and strange quarks for temperatures from 3 to 165 GeV. Surprisingly, the pressures even at these high early temperatures could not be described using a model of weakly interacting quark–gluon plasma, implying that the strong force was influential sooner after the big bang than previously assumed.
–Nikhil Karthik
Nikhil Karthik is an Associate Editor for Physical Review Letters.
References
- M. Bresciani et al., “QCD equation of state with Nf = 3 flavors up to the electroweak scale,” Phys. Rev. Lett. 134, 201904 (2025).