Cooling and the formation of fundamental particles
How, as the Universe cooled from extremely high temperatures, quarks combined into protons and neutrons
One of the most important periods in the early Universe was the transition from a hot, dense "soup" of quarks and gluons to a state in which these quarks began to combine into their constituent particles, namely protons and neutrons. This transition had a decisive impact on the present Universe, as it set the stage for the subsequent formation of nuclei, atoms, and all the matter that followed. We discuss below:
- Quark-gluon plasma (QGP)
- Expansion, cooling and confinement
- The formation of protons and neutrons
- Impact on the early Universe
- Open questions and ongoing research
Understanding how quarks formed hadrons (protons, neutrons, and other short-lived particles) as the Universe cooled gives us a better understanding of the very foundations of matter.
1. Quark-Gluon Plasma (QGP)
1.1 High energy state
In the very early moments after the Big Bang—up to about a few microseconds (10−6 s) — The temperature and density of the universe were so high that protons and neutrons could not exist in bound states. Instead, quarks (the basic building blocks of nucleons) and gluons (the carriers of the strong interaction) existed quark-gluon plasmas (QGP) In this plasma:
- Quarks and gluons were deconfine, that is, it was not "locked" in composite particles.
- The temperature was probably above 1012 K (approximately 100–200 MeV in energy units), significantly higher than QCD (quantum chromodynamics) confinement limit.
1.2 Data from particle accelerators
Although we cannot exactly recreate the Big Bang, heavy ion accelerator experiments—such as Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory and Large Hadron Collider (LHC) At CERN — provided much evidence for the existence and properties of QGP. These experiments:
- Accelerates heavy ions (such as gold or lead) to almost the speed of light.
- They collide, briefly creating an extremely dense and hot "fireball" state.
- They are studying this "fireball," which reflects similar conditions that existed in the early Universe during the quark epoch.
2. Expansion, cooling and confinement
2.1 Cosmic expansion
After the Big Bang, the Universe expanded rapidly. As it expanded, it it's getting cold, in simple terms, there is a relationship between the temperature T and the scaling factor a(t) of the Universe T ∝ 1/a(t). In other words, the larger the Universe, the colder it is, and new physical processes can begin to dominate at different times.
2.2 QCD phase transition
Approximately between 10−5 and 10−6 seconds after the Big Bang, the temperature dropped below a critical value (~150–200 MeV, or about 1012 K). Then:
- Hadronization: Quarks have become "confined" in hadrons due to the strong interaction.
- Color confinement: The laws of QCD predict that "colored" quarks cannot exist alone at low energy levels. They combine into color-neutral combinations (e.g., three quarks form a baryon, a quark and antiquark pair a meson).
3. Formation of protons and neutrons
3.1 Hadrons: Baryons and Mesons
Baryons (e.g. protons, neutrons) are made up of three quarks (qqq), and mesons (e.g. pions, kaons) — from a quark and antiquark pair (q̄q). Through the hadron epoch (about 10−6–10−4 (about 1 second after the Big Bang) a large number of hadrons were formed. Most of them were short-lived and decayed into lighter, more stable particles. By about 1 second after the Big Bang, most of the unstable hadrons had decayed, and the main surviving particles were protons and neutrons (the lightest baryons).
3.2 Proton-neutron ratio
Although abundant protons (p), both neutrons (n) quantities, neutrons are slightly heavier than protons. A free neutron decays relatively quickly (~10 minutes half-life) into a proton, electron, and neutrino. In the early Universe, the ratio of neutrons to protons was determined by:
- Weak interaction speeds: Interchanges such as n + νe ↔ p + e−.
- "Freezing": As the universe cooled, these weak interactions broke away from thermal equilibrium, "freezing" the ratio of neutrons to protons to about 1:6.
- Further decomposition: Some of the neutrons decayed before nuclear fusion began, slightly changing the ratio that led to the subsequent formation of helium and other light elements.
4. Impact on the early Universe
4.1 The origins of nuclear fusion
Stable protons and neutrons were prerequisite Big Bang Nuclear Fusion (BBNF), which occurred between approximately 1 second and 20 minutes after the Big Bang. Via BBN:
- Protons (1H nuclei) combined with neutrons to form deuterium, which further combined into helium nuclei (4He) and small amounts of lithium.
- Today's observed primordial abundances of light elements are in excellent agreement with theoretical predictions—an important confirmation of the Big Bang model.
4.2 Transition to a photon-dominated era
As matter cooled and stabilized, the energy density of the Universe became increasingly dominated by photons. By about 380 000 years after the Big Bang, the Universe was filled with a hot plasma of electrons and nuclei. Only electrons recombining with nuclei and the formation of neutral atoms, the Universe became transparent, radiating cosmic microwave background (CMB)which we are observing today.
5. Open questions and ongoing research
5.1 The precise nature of the QCD phase transition
Current theories and numerical simulations of QCD suggest that the transition from quark-gluon plasma to hadrons may be smooth. crossover), rather than a sudden first-order phase transition when the baryonic density is close to zero. However, a small baryonic asymmetry may have existed in the early Universe. Theoretical work is ongoing and better digital QCD studies are trying to clarify these details.
5.2 Quark-hadron phase transition markers
If the quark-hadron phase transition left behind some unique cosmological signatures (e.g., gravitational waves, remnant particle distributions), this could help indirectly reveal the earliest moments in the history of the Universe. Researchers continue to search for these possible signatures, both through observations and experiments.
5.3 Experiments and simulations
- Heavy ion collisions: The RHIC and LHC programs recreate certain aspects of QGP, helping physicists study the properties of strongly interacting matter at high densities and temperatures.
- Astrophysical observations: Accurate KMF Measurements (Planck satellite) and estimates of the abundances of light elements verify BBN models, indirectly constraining the laws of physics in the quark-hadron transition period.
References and further reading
- Kolb, EW, & Turner, MS (1990). The Early Universe. Addison-Wesley. – A comprehensive textbook covering the physics of the early Universe, including the quark-hadron transition.
- Mukhanov, V. (2005). Physical Foundations of Cosmology. Cambridge University Press. – Provides a deeper perspective on cosmological processes, including phase transitions and nuclear fusion.
- Particle Data Group (PDG). https://pdg.lbl.gov – Provides broad overviews of particle physics and cosmology.
- Yagi, K., Hatsuda, T., & Miake, Y. (2005). Quark-Gluon Plasma: From Big Bang to Little Bang. Cambridge University Press. – Discusses experimental and theoretical aspects of QGP.
- Shuryak, E. (2004). "What RHIC Experiments and Theory Tell Us about Properties of Quark–Gluon Plasma?" Nuclear Physics A, 750, 64–83. – The focus is on QGP research in accelerators.
Final thoughts
The transition from a free quark-gluon plasma to a bound state of protons and neutrons was one of the decisive events in the early evolution of the Universe. Without it, stable matter would not have formed, and later stars, planets, and life. Today, experiments recreate it in miniature the quark era in heavy-ion collisions, and cosmologists are refining theories and simulations to understand every nuance of this complex but fundamental phase transition. Together, these efforts are increasingly revealing how the hot, dense primordial plasma cooled and formed into the basic building blocks of the present Universe.