The universe in perfect balance
In the first moments of its existence, less than a trillionth of a second after the Big Bang, the universe was an hot and dense place. At temperatures above 10¹⁵ Kelvin, four fundamental forces of nature we know today were not distinct. The electromagnetic force and the weak nuclear force were unified into a single, elegant electroweak interaction. In this primordial state, all fundamental particles were massless, traveling at the speed of light. The universe exhibited a underlying symmetry; the laws of physics governing this electroweak force did not change, no matter how one's viewpoint was rotated in an abstract mathematical space. This state of high-energy perfection, however, was fleeting. The universe was expanding and cooling rapidly, led to for a dramatic transformation.
A cosmic phase transition
As the universe aged to about 10⁻¹² seconds, the temperature dropped below a critical point of approximately 10¹⁵ K. At this threshold, the vacuum of spacetime underwent a phase transition, conceptually similar to water freezing into ice. This event is known as electroweak symmetry breaking. The change was driven by a quantum field that permeates all of space: the Higgs field. Above the critical temperature, the lowest energy state of the Higgs field was zero, preserving the perfect electroweak symmetry. Below this temperature, the zero-energy state became unstable. The field settled into a new, lower-energy ground state with a non-zero value. This process is called spontaneous symmetry breaking because the underlying physical laws remained symmetric, but the vacuum state of the universe did not.
The shape of the Higgs field's potential energy is often compared to the bottom of a wine bottle or a "Mexican hat". At high energy, a ball rests on the central peak, a symmetric but unstable position. As the energy drops, the ball must roll down into the circular trough at the bottom, randomly picking one specific location and thus breaking the rotational symmetry. This new, non-zero value of the Higgs field is what gives fundamental particles their mass. Particles that interact strongly with the field, like the W and Z bosons, acquire a large mass. The photon, which does not interact with the Higgs field, remains massless.
This theoretical framework, called the Brout-Englert-Higgs mechanism, was proposed in 1964 by three groups of physicists: Robert Brout and François Englert; Peter Higgs; and Gerald Guralnik, C. R. Hagen, and Tom Kibble. Nearly 50 years later, on July 4, 2012, scientists at CERN's Large Hadron Collider (LHC) announced the discovery of the Higgs boson, the particle associated with the Higgs field, confirming the theory. François Englert and Peter Higgs were awarded the Nobel Prize in Physics for their work in 2013; Robert Brout had passed away in 2011. The coordinates for this point of interest lead to CERN, the laboratory where this fundamental component of our universe was finally observed.