Science › Creation theories
The Big Bounce
2006· Theory
SpeculativeA serious idea with no experimental proof yet.
The idea that our expanding universe was preceded by a contracting one, which reached a maximum density and rebounded instead of hitting infinity.
See it on the Science map →How it works
Run expansion backward and, in general relativity, density climbs without limit. In loop quantum cosmology, a simplified version of loop quantum gravity, quantum effects make gravity repulsive near the Planck density, about 10⁹⁶ kilograms per cubic meter. A shrinking universe slows, stops and turns around. Martin Bojowald found hints in 2001; Abhay Ashtekar, Tomasz Pawłowski and Parampreet Singh showed a clean bounce in 2006. Other bounce models use exotic matter or string ideas. Inflation can still follow the bounce.
Why it matters
It removes the singularity at the start and turns the Big Bang from the beginning of everything into a transition, making 'before' a real time.
Think of it like
Like a ball thrown at the floor: it squashes, stops and springs back up, rather than crushing to a point.
How we know
None confirmed. Some models predict weaker ripples at the largest angles in the microwave background. The Planck satellite does see slightly low power there, but chance explains it equally well.
Still a mystery
Does information or structure survive the bounce? Why was the contracting universe so orderly? The full loop theory, not just simplified models, has yet to confirm it.
Connections (3)
▸The Big Bounce answers "a shrinking universe" to What came before the Big Bang?Speculative
The relationship
Its answer is an earlier universe much like ours run in reverse: contracting, growing denser and hotter, until quantum effects halted the collapse and turned it into expansion. On this view the Big Bang was the moment of maximum density, not the start of time, and our past continues back through it.
How the link was found
Alexander Friedmann noted a possible 'periodic' universe in 1922, and Richard Tolman explored oscillating universes in the 1930s. Bojowald argued in 2007 that a bounce would erase some details of the earlier universe.
Is it proven?
No observational evidence yet. Any message from before would have to survive extreme density and then inflation, which stretches and dilutes it.
Open questions and mysteries
Did the earlier universe have its own big bang, making the past infinite? Why was it so orderly before the bounce?
▸Loop quantum gravity predicts, in simplified models, The Big BounceSpeculative
The relationship
Loop quantum gravity says space comes in smallest chunks, so it cannot be squeezed indefinitely. Applied to a uniform universe, this adds a correction to the equations of expansion that matters only near the Planck density. There gravity effectively turns repulsive: contraction slows, stops at a maximum density and reverses into expansion, with no infinity.
How the link was found
Martin Bojowald showed singularity avoidance in 2001. Abhay Ashtekar, Tomasz Pawłowski and Parampreet Singh computed a clean bounce in 2006 by following a quantum universe's evolution numerically.
Is it proven?
Mathematically solid in the simplified models, which assume a perfectly uniform universe. Whether the full theory gives the same result is not established, and no observation tests it.
Open questions and mysteries
Do the simplifications hide effects that would change the result? Could the bounce leave a measurable imprint on the microwave background?
▸The Big Bounce replaces Singularities: where physics breaksSpeculative
The relationship
The singularity theorems of general relativity assume gravity always attracts. Near the Planck density, bounce models' quantum corrections break that assumption, so the theorems no longer apply. Instead of infinite density and curvature at t = 0, the universe reaches a large but finite maximum. If so, the infinity was the theory failing, as many suspected, not a physical object.
How the link was found
Hawking and Penrose noted from the start that their theorems rely on classical gravity. Loop quantum cosmologists (2001–2006) and other bounce theorists showed specific ways quantum effects remove the singularity.
Is it proven?
Accepted as possible in principle, but shown only in model calculations. Without a confirmed theory of quantum gravity, which replacement is right is unknown.
Open questions and mysteries
Could the same mechanism stop collapse inside black holes too, and what would that mean for what falls in?