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The inflaton field
1982· Theory
SpeculativeA serious idea with no experimental proof yet.
The hypothetical field whose energy drove inflation. It has never been detected, and nobody knows what it is.
See it on the Science map →How it works
It is almost certainly a 'scalar' field, one with only a strength at each point and no direction, like the Higgs; a field with a direction would have stretched space unevenly. Everything about inflation follows from the shape of its energy hill. Candidates include the Higgs itself (Bezrukov and Shaposhnikov, 2008), an extra twist of gravity (Starobinsky, 1980), axion-like fields (Freese and colleagues, 1990), and fields from string theory. Planck's data favor hills that flatten into long plateaus over simple bowl shapes.
Why it matters
Its identity would tie the universe's first instant to particle physics. Its hill shape decides how long inflation lasted, whether it is eternal, and how it handed energy to ordinary matter.
How we know
Only indirect: the ripple pattern in the microwave background fits a single slowly rolling field well. Its energy scale was at most about 10¹⁶ GeV, a trillion times what the Large Hadron Collider reaches, so no collider can recreate those conditions.
Still a mystery
One field or several? Why is its hill so flat? Is it related to the Higgs, dark matter or dark energy? Or did something other than a field drive the growth?
Connections (4)
▸Fields: what everything is made of may include a new one called The inflaton fieldSpeculative
The relationship
Every field in the Standard Model but one carries spin, a built-in sense of direction. Only the Higgs is a pure 'scalar', a single strength at each point. Inflation needs that kind: a field with a direction would have stretched space more one way than another. So either the Higgs drove inflation, or nature has at least one more scalar field.
How the link was found
Guth (1980) used scalar fields from grand unified theories. Linde's 1983 chaotic inflation freed the inflaton from any particular theory. In 2008 Fedor Bezrukov and Mikhail Shaposhnikov showed the Standard Model's Higgs could serve, if it couples strongly to gravity.
Is it proven?
Not proven. String theory and other unified theories predict many scalar fields, but none beyond the Higgs has been found. In popular models the inflaton particle weighs around 10¹³ GeV, a billion times beyond collider reach.
Open questions and mysteries
If the Higgs was the inflaton, why the unusual link to gravity? If not, how does a separate inflaton connect to ordinary particles to heat the universe afterward?
▸The inflaton field drives Cosmic inflationWell supported
The relationship
How high the inflaton sits on its hill sets how fast space inflates; the slope sets how long. A higher perch means faster expansion and stronger gravitational waves; a gentler slope means a longer run and more doublings. When the field drops off, inflation stops.
How the link was found
Guth's 1980 model used a field already present in particle theories. Andrei Linde's 1983 'chaotic inflation' showed any field starting high enough on a simple hill would do.
Is it proven?
Indirect. The ripple pattern fits a single field: Planck (2013–2018) found the ripples' statistics almost perfectly random and bell-shaped, with no sign of extra fields or exotic effects.
Open questions and mysteries
Why did the field start high on its hill, in a patch smooth enough to begin inflating? How the very first inflating patch arose is unknown.
▸The inflaton field rolls down its energy hill in Slow roll: energy rolling down a hillWell supported
The relationship
Expansion itself acts as friction on the inflaton, called Hubble friction: the faster space grows, the more it slows the field. On a gentle slope the field settles at a crawling terminal speed, as a falling raindrop does in air, and its energy stays nearly constant. Slow roll needs two things: a shallow slope, and a slope that changes only gradually.
How the link was found
Linde and, separately, Albrecht and Steinhardt introduced it in 1982. The general slow-roll approximation was formalized through the 1980s and early 1990s by Steinhardt, Michael Turner, Andrew Liddle, David Lyth and others.
Is it proven?
The equations of a field in expanding space are standard physics. They show exactly how the slope and its curvature set the ripples' strength and tilt, which Planck has measured.
Open questions and mysteries
Quantum corrections tend to steepen such hills, in supergravity theories usually enough to spoil slow roll (the 'eta problem'). What keeps the hill flat, perhaps a hidden symmetry, is unknown.
▸The inflaton field decays into particles during Reheating: inflation's energy becomes hot matterWell supported
The relationship
At the end of inflation the inflaton field sits near the bottom of its energy curve and oscillates around it. Through its links to other fields, each swing passes energy into particles: either by steady decay, or explosively when the swings pump particle production in rhythm, like pushing a swing at the right moment ('preheating'). The new particles then collide and settle at one temperature.
How the link was found
Andreas Albrecht, Paul Steinhardt, Michael Turner and Frank Wilczek; Alexander Dolgov and Andrei Linde; and Larry Abbott, Edward Farhi and Mark Wise, all in 1982. Lev Kofman, Linde and Alexei Starobinsky described explosive preheating in 1994.
Is it proven?
Not observed. A strong inference: inflation leaves space empty and cold, yet the universe was hot and full of particles by one second. Some transfer must have happened; its details are untested.
Open questions and mysteries
Did the inflaton decay completely? In 'quintessential inflation' (Peebles and Vilenkin, 1999) it never fully decays and later becomes dark energy; other proposals make leftovers the dark matter. Both are speculative. Whether preheating happened depends on unmeasured couplings.