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.