Science › Cosmic timeline
The Higgs switches on (electroweak epoch)
1972· Cosmic event
ProvenTested many times; part of working physics.
The era ending about a trillionth of a second in, near 10¹⁵ degrees, when the Higgs field settled and particles gained their masses.
See it on the Science map →How it works
Before this, the Higgs field averaged zero, so electrons, quarks and the W and Z bosons were massless and moved at light speed, and the weak and electromagnetic forces acted as one. As the universe cooled to about 10¹⁵ degrees, the field settled to a steady value everywhere. Particles that interact with it gained mass, and the forces took their present form. For the measured Higgs mass, computer calculations show a smooth change, not a sudden one.
Why it matters
It gave matter its masses and set the forces as they are today. It is also the earliest era whose physics colliders have tested directly.
How we know
The Large Hadron Collider exceeds these energies, producing W, Z and Higgs particles. Lattice calculations since 1996 show a smooth change for a heavy Higgs; a 2014 one, using the 125 GeV mass found in 2012, put it near 160 GeV, about 2×10¹⁵ degrees.
Still a mystery
Was the change smooth, as the Standard Model says, or sharp because of undiscovered particles? A sharp one could help explain matter's excess and leave gravitational waves for the LISA space mission, planned for about 2035.
Connections (4)
▸Reheating: inflation's energy becomes hot matter cools into The Higgs switches on (electroweak epoch)Well supported
The relationship
After reheating the universe was a hot soup of every particle type, expanding and cooling: double its size and the temperature halves. It kept cooling until, at about 10¹⁵ degrees, roughly a trillionth of a second in, the Higgs field could settle. Physicists call the long span in between the 'desert': unless undiscovered particles exist, nothing notable happens in it.
How the link was found
The cooling history follows from the Friedmann equations for hot radiation, applied by Gamow, Alpher and Herman from 1948 and extended to the full set of known particles in the 1970s.
Is it proven?
The cooling law is confirmed by the microwave background's temperature measured in distant gas clouds. Whether reheating was hot enough to restore the unified electroweak force depends on unknown inflaton details; most models say yes.
Open questions and mysteries
Is the 'desert' really empty, or does it hide dark-matter particles, supersymmetry, or the origin of matter's excess?
▸The Higgs field and boson (2012) switches on in The Higgs switches on (electroweak epoch)Proven
The relationship
Heat reshapes the Higgs field's energy curve. In the hot plasma, particles that feel the field add energy that is lowest when the field is zero, so it sat at zero. Below about 2×10¹⁵ degrees (160 GeV) that extra energy faded, zero became a hump, and the field slid into the surrounding valley: gradually, like a dimmer, not a switch.
How the link was found
David Kirzhnits and Andrei Linde (1972) showed heat would switch the Higgs field off. Kajantie, Laine, Rummukainen and Shaposhnikov showed by computer in 1996 that for a heavy Higgs the change is smooth.
Why it's proven
The Higgs field and its 125 GeV mass are measured (2012). The high-temperature behavior is calculated, not observed, but rests on well-tested theory.
Open questions and mysteries
The Higgs field's interaction with itself, which shapes its energy curve, is barely measured. If it differs from prediction, the switch-on may have been sharp. The High-Luminosity LHC, from about 2030, aims to measure it.
▸The weak force and radioactivity splits from electromagnetism in The Higgs switches on (electroweak epoch)Proven
The relationship
Above about 10¹⁵ degrees, the weak and electromagnetic forces were one electroweak force with four massless carriers. When the Higgs field settled, three carriers became the heavy W⁺, W⁻ and Z, and the weak force shrank to subatomic range; the fourth stayed massless as the photon. From then on the two forces behaved as separate. At very high collision energies they still act alike.
How the link was found
The unified theory came from Glashow (1961), Weinberg (1967) and Salam (1968). Its claim that the forces act alike at high energy was tested directly at Germany's HERA collider, 1992–2007.
Why it's proven
At HERA's highest energies, weak and electromagnetic interaction rates became equal, as predicted. The W and Z masses also match the theory.
Open questions and mysteries
Did the split leave relics? Some propose it generated tiny magnetic fields that later seeded the magnetism seen in galaxies; this is unconfirmed.
▸The Higgs switches on (electroweak epoch) is followed by Quarks bind into protons and neutronsProven
The relationship
After the Higgs switched on, the universe was a plasma of quarks and electrons, now with mass, plus gluons, neutrinos and light. Over the next few millionths of a second it cooled a thousandfold, to about 2 trillion degrees. Along the way the heaviest particles, top quarks, Higgs, W and Z bosons, decayed away, because the cooling soup could no longer make new ones. Then the strong force took hold.
How the link was found
The sequence follows from the Standard Model plus the cooling law; it was laid out for general readers in Steven Weinberg's 1977 book The First Three Minutes.
Why it's proven
Both transitions are calculated with lattice simulations and partly recreated at the LHC, which makes W, Z, top quarks and quark–gluon plasma. Their order in time follows from their measured energies.
Open questions and mysteries
If dark matter is a heavy particle, it probably stopped annihilating ('froze out') in this window. Detectors such as LZ have not found it, so the window may be empty of it.