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Grand unification epoch
1974· Cosmic event
SpeculativeA serious idea with no experimental proof yet.
A proposed era, from about 10⁻⁴³ to 10⁻³⁶ second, when the strong, weak and electromagnetic forces may have acted as one force.
See it on the Science map →How it works
Grand unified theories, starting with Howard Georgi and Sheldon Glashow's in 1974, say three of the forces merge above about 10¹⁶ GeV, about a trillion times the Large Hadron Collider's energy. If so, the universe lived under one combined force while temperatures fell from about 10³² to 10²⁸ degrees. Gravity had already split off. As things cooled, the strong force separated, like ice freezing out of water, possibly releasing energy and leaving defects such as magnetic monopoles: isolated north or south magnetic poles.
Why it matters
It is where the dream that all forces are one meets cosmic history, and where Alan Guth first looked for inflation's trigger.
How we know
Untested. Measured force strengths nearly converge near 10¹⁶ GeV. But the theories predict protons slowly decay, and experiments, lately Japan's Super-Kamiokande, show they last more than about 10³⁴ years, ruling out the simplest 1974 version. No monopoles have been found.
Still a mystery
Did unification happen at all? The strengths meet neatly only with 'supersymmetry', which the Large Hadron Collider has not found. If reheating after inflation was cooler than this scale, this era is not part of our hot history.
Connections (3)
▸Planck epoch: the first 10⁻⁴³ second gives way to Grand unification epochSpeculative
The relationship
In the standard extrapolated timeline, once the universe expanded and cooled past the Planck time, gravity became an ordinary, classical force, separate from the rest. What remained, if grand unified theories are right, was a single combined force for the strong, weak and electromagnetic interactions. The handoff is the moment gravity 'froze out' of quantum chaos into the smooth spacetime that general relativity describes.
How the link was found
Pieced together in the late 1970s and 1980s as particle physicists applied grand unified theories to the hot early universe; summarized in Edward Kolb and Michael Turner's 1990 textbook The Early Universe.
Is it proven?
Not proven. Neither era has been observed, and inflation may have erased any trace. The sequence is an extrapolation of known physics plus unconfirmed unification.
Open questions and mysteries
Was gravity ever unified with the other forces? String theory says it joins them near the Planck scale; loop quantum gravity quantizes gravity alone, without unifying it. Neither has been tested.
▸Grand unification epoch may have triggered Cosmic inflationSpeculative
The relationship
In Guth's original idea, the field that broke the grand unified force got stuck as the universe cooled, like water supercooled below freezing without turning to ice. Trapped in that false vacuum, its energy drove runaway expansion. That would also dilute the magnetic monopoles a unification transition should make in huge numbers, explaining why none are found.
How the link was found
Alan Guth, working with Henry Tye on the overproduction of monopoles in grand unified theories, realized in December 1979 that supercooling would inflate space. His paper appeared in 1981.
Is it proven?
Untested. Grand unification is unconfirmed, and Guth's unification-based model failed because its bubbles could not merge smoothly. Most current models use a separate inflaton field. Monopole dilution is an argument, not evidence.
Open questions and mysteries
Searches for inflation's gravitational waves cap its energy below about 10¹⁶ GeV, close to the unification scale. Coincidence, or a real connection? Unknown.
▸The four forces of nature may have had three unified in Grand unification epochSpeculative
The relationship
Force strengths change with energy: the strong force weakens at high energy while electromagnetism strengthens. Extending measured trends upward, the strong, weak and electromagnetic strengths approach each other near 10¹⁶ GeV. Grand unified theories say they merge there into one force, carried by extra particles that could turn quarks into positrons (anti-electrons), letting protons decay. Gravity is thought to have separated earlier, at the Planck scale.
How the link was found
Howard Georgi and Sheldon Glashow proposed the first grand unified theory in 1974; the same year Georgi, Helen Quinn and Steven Weinberg showed the strengths could converge at high energy.
Is it proven?
Not proven. The three strengths nearly meet but miss slightly unless supersymmetry exists, shown with precise CERN data in 1991. Supersymmetry has not appeared at the LHC, and proton decay, the key prediction, is unseen.
Open questions and mysteries
Are the forces really one at high energy, and does gravity join them at the Planck scale? A proton decay seen at Hyper-Kamiokande or DUNE would be the strongest clue.