Science › Creation theories
Eternal inflation and bubble universes
1983· Theory
SpeculativeA serious idea with no experimental proof yet.
The idea that inflation, once started, never stops everywhere: it ends in pockets, each a universe, while the space between keeps inflating.
See it on the Science map →How it works
Two routes lead here. In slow roll, random quantum kicks push the inflaton back uphill in some regions, and those regions grow fastest. In a landscape of false vacuums, bubbles of lower energy keep forming but never eat all the inflating space. Either way, inflating space outgrows the pockets forever. Each pocket where inflation stops heats up and has its own Big Bang. Our whole observable universe would sit inside one. Endless into the future, it probably still had a beginning.
Why it matters
It turns our universe from the whole of reality into one pocket among countless others, and it is the main physical route to a multiverse.
How we know
None direct; other pockets lie beyond any possible view. It follows mathematically from many, not all, inflation models once quantum kicks are included. Paul Steinhardt noticed it in 1982; Vilenkin (1983) and Linde (1986) developed it.
Still a mystery
Steinhardt, its first discoverer, now argues it lets inflation predict anything, and so nothing. Whether it is real, and whether it can ever be tested, is debated.
Connections (5)
▸Alexander Vilenkin co-proposed Eternal inflation and bubble universesSpeculative
The relationship
Vilenkin took the inflaton's quantum jitters, the same ones that seed galaxies, and asked what they do to inflation itself. He found that in some regions they push the field back uphill faster than it rolls down. Those regions keep inflating and grow fastest, so inflating space expands forever, while ordinary universes form in pockets within it.
How the link was found
Alexander Vilenkin at Tufts University, in his 1983 paper 'Birth of inflationary universes', which applied the 1982 calculations of quantum jitters to the inflaton's whole history.
Is it proven?
It follows from the standard quantum treatment of the inflaton, applied to regions far beyond view. Most inflation theorists accept the math; no observation can directly confirm it.
Open questions and mysteries
Vilenkin and Jaume Garriga argued (2001) it implies every possible history repeats endlessly in distant pockets. How to assign probabilities among infinitely many pockets is unsolved.
▸Vacuum decay and bubbles of new space makes the bubbles of Eternal inflation and bubble universesSpeculative
The relationship
In this version, space sits in a high-energy false vacuum that inflates. Now and then a bubble of lower-energy vacuum forms. With gravity included, a bubble's interior looks, from inside, like an infinite expanding universe. But the false vacuum between bubbles grows faster than bubbles consume it, so new bubbles keep forming forever. Our universe would be the inside of one.
How the link was found
Sidney Coleman and Frank De Luccia (1980) found the universe-like bubble interior; J. Richard Gott (1982) proposed our universe is one. Guth and Erick Weinberg (1983) showed bubbles never fill all space.
Is it proven?
Not proven. The math is consistent, and it predicts our space should be very slightly curved, like a saddle. Measurements find space flat to a fraction of a percent, which neither confirms nor rules this out.
Open questions and mysteries
Whether string theory really allows long-lived, high-energy vacuums is disputed; a 2018 'swampland' conjecture by Cumrun Vafa and colleagues suggests it may not.
▸Andrei Linde developed Eternal inflation and bubble universesSpeculative
The relationship
In 1983 Linde proposed chaotic inflation: the field starts with random values in different patches, and wherever it starts high enough, that patch inflates. In 1986 he showed quantum jumps keep driving some patches higher, so inflation reproduces itself forever, branching into domains that might even have different laws. He called it a self-reproducing universe.
How the link was found
Andrei Linde, 'Eternally existing self-reproducing chaotic inflationary universe', Physics Letters B, 1986. In 1994, with his son Dmitri Linde and Arthur Mezhlumian, he pictured it in computer simulations as ever-growing mountain ranges of field values.
Is it proven?
It is a mathematical consequence of chaotic inflation plus quantum jumps. Nothing observable confirms it; regions beyond our horizon cannot be seen.
Open questions and mysteries
Linde saw the whole as a stationary, possibly beginningless universe; later theorems suggest a beginning is still needed. How to compare probabilities across endless domains is unsolved.
▸Cosmic inflation may never fully stop, giving Eternal inflation and bubble universesSpeculative
The relationship
While the inflaton rolls, it keeps getting random quantum kicks. In each e-fold of time, every region grows into about 20 regions. If more than about one in 20 is kicked uphill rather than rolling down, the volume still inflating grows rather than shrinks. In many models that happens high on the hill, so inflation ends only in patches, never everywhere.
How the link was found
Alexei Starobinsky built the standard math for these random kicks, 'stochastic inflation', in 1986. Guth later gave the simple false-vacuum version: if the half-life exceeds the doubling time, inflation is eternal.
Is it proven?
It uses the same quantum-jitter physics that correctly predicted the microwave background's ripples, but applies it to field values far beyond anything observable. Whether it occurs depends on the hill's shape.
Open questions and mysteries
In their final joint paper (2018), Stephen Hawking and Thomas Hertog argued that eternal inflation yields a smoother, finite universe rather than an endless fractal of pockets. Unresolved.
▸Eternal inflation and bubble universes creates bubble universes, a kind of The multiverseSpeculative
The relationship
Eternal inflation supplies the many pockets, but different laws in different pockets need more: an energy landscape with many valleys. With only one valley, every pocket has the same constants, just copies of one kind of universe. String theory's landscape lets each pocket land in a different valley, with different particles, forces and dark energy. Eternal inflation then makes every option real somewhere.
How the link was found
Linde (1986) described pockets that could differ in laws and constants. KKLT (Shamit Kachru, Renata Kallosh, Linde and Sandip Trivedi, 2003) offered string-theory vacuums that eternal inflation could fill.
Is it proven?
Not proven, and other pockets are probably unobservable forever. Each ingredient, eternal inflation and the landscape, is a consequence of untested theory, not an observation.
Open questions and mysteries
Counting pockets goes wrong: some counting rules predict that brains assembled by random fluctuations would vastly outnumber evolved observers like us. No agreed counting rule exists.