In the very early universe, shortly after the Big Bang, all fundamental forces and particles are believed to have been highly symmetric and unified. Under these conditions, particles had no mass. As the universe cooled and expanded, a process called spontaneous symmetry breaking occurred, fundamentally altering the behavior of particles and forces.
This process is best explained through the Higgs mechanism, a key feature of the Standard Model of particle physics. According to this theory, an invisible energy field known as the Higgs field permeates all of space. Initially, the field was symmetric, and particles moved freely without mass. But as the universe cooled, the symmetry of this field spontaneously broke, causing the Higgs field to settle into a lower energy state.
When particles began to interact with the Higgs field, they acquired mass—the stronger the interaction, the heavier the particle. This is how W and Z bosons (which mediate the weak force) became massive, while photons (which mediate electromagnetism) remained massless.
Key implications of symmetry breaking:
- Explains the diversity of particle masses,
- Allows for distinct fundamental forces to emerge from a unified one,
- Was confirmed by the discovery of the Higgs boson in 2012 at CERN’s LHC,
- Plays a crucial role in shaping the structure and evolution of the universe.