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Reheating: inflation's energy becomes hot matter
1982· Cosmic event
Well supportedStrong evidence or math, but not fully tested.
The handoff, roughly 10⁻³² second in (timing uncertain), when the field driving inflation dumped its energy into particles, making the universe hot again.
See it on the Science map →How it works
Inflation stretched space so fast that any earlier matter was diluted to nothing and the temperature fell almost to absolute zero. All the energy sat in the inflaton field. When inflation stopped, that field swung back and forth around its lowest point and shed energy into particles, possibly explosively, in a burst called preheating. The particles collided until they shared one temperature, somewhere between about 10¹⁰ and 10²⁸ degrees. In inflation theory, this is the start of the hot Big Bang, not t = 0.
Why it matters
It is the bridge from a cold, empty, inflating space to the hot early universe. If inflation is right, every particle that exists traces back to this energy release.
Think of it like
Like a struck bell: the field rings back and forth around its resting point, and each swing sheds energy, here as particles instead of sound.
How we know
Indirect. Big Bang element amounts and the neutrino background require it to have reached at least tens of billions of degrees. Microwave-background ripple data place weak limits on how long it lasted. Nothing from reheating itself has been observed.
Still a mystery
Its temperature is uncertain by a factor of about a billion billion. Which particles the inflaton fed, and whether reheating made dark matter or left detectable gravitational waves, are open.
Connections (5)
▸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.
▸Slow roll: energy rolling down a hill ends in Reheating: inflation's energy becomes hot matterWell supported
The relationship
Slow roll lasts while the field's energy hill is gentle, so its stored energy barely changes as it moves. Near the bottom the slope steepens, the field speeds up, and inflation stops once its energy of motion rivals its stored energy. The field then overshoots and swings around the low point, and those swings power reheating. The shape of the hill's bottom controls how reheating goes.
How the link was found
Built into the 1982 slow-roll models of Andrei Linde and of Andreas Albrecht and Paul Steinhardt, whose fields ended in oscillations; the decay was worked out that year with Michael Turner and Frank Wilczek.
Is it proven?
The slow-roll phase fits microwave-background ripple data well. The exit and oscillations follow from the same math, but no observation confirms them yet.
Open questions and mysteries
Does the hill end in a smooth bowl, a sharp drop, or a handoff to another field? Ripple data weakly limit how long reheating took, but not how it worked.
▸Cosmic inflation ends with Reheating: inflation's energy becomes hot matterWell supported
The relationship
Inflation by itself only makes space: huge, smooth, flat and empty, with all the energy held in the field driving it. To give a universe like ours, inflation must end and that energy must turn into matter almost evenly everywhere. Quantum jitter made it end a hair earlier in some places than others; those tiny timing differences became the 1-in-100,000 density ripples later seen in the microwave background.
How the link was found
Even reheating became possible with 1982's slow-roll models. How its uneven timing seeds the ripples was worked out that year, around a Cambridge workshop, by Hawking, Starobinsky, Guth and So-Young Pi, and Bardeen, Steinhardt and Turner.
Is it proven?
The ripple pattern these timing differences predict matches Planck's maps. Reheating itself is inferred rather than seen: if inflation occurred and the universe was hot afterward, it must have happened.
Open questions and mysteries
Was reheating quick or gradual? Did other regions keep inflating while ours reheated, as eternal inflation suggests?
▸Reheating: inflation's energy becomes hot matter starts the hot phase of The Big BangWell supported
The relationship
'Big Bang' has two meanings: the extrapolated moment t = 0, and the hot, dense, expanding state that made the light elements and the microwave background. In inflation theory, the second begins at reheating. All the classic Big Bang evidence describes what happened after reheating; the infinitely dense start may never have been part of our past.
How the link was found
Became the standard reading after the 1982 slow-roll models; cosmology textbooks since the 1990s describe inflation as coming before the hot Big Bang.
Is it proven?
The hot-phase evidence (microwave background, light elements, neutrinos) is strong. That this phase began at reheating rests on inflation, which is well supported but not proven.
Open questions and mysteries
Did anything precede inflation, and so was there a true t = 0? Many cosmologists now use 'Big Bang' for the hot phase only, which changes what 'before the Big Bang' means.
▸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?