Compare · The Fate of the Universe

Classical Closed Recollapse vs Dynamical Dark Energy Turnaround

Recollapse and the Big Crunch· within family

	Classical Closed Recollapse Rejected	Dynamical Dark Energy Turnaround Speculative
Proposed	1922	2002
Key figures	Alexander Friedmann	Renata Kallosh, Andrei Linde
In one sentence	Classical Closed Recollapse is the original Big Crunch, the closed Friedmann universe that dominated cosmology before dark energy was discovered. If the total density exceeds a critical value, space has positive curvature and is finite, and gravity eventually halts the expansion and pulls everything back into a hot, dense crunch. Current measurements of a flat universe with dark energy have made this the disfavoured route, but it remains the foundational picture of a recollapsing cosmos.	The Dynamical Dark Energy Turnaround shows that a flat, currently accelerating universe can still end in a Big Crunch. If dark energy is a scalar field whose potential is slowly decreasing and will eventually go negative, its repulsion turns into an attraction, the expansion halts, and the universe recollapses. Kallosh and Linde 2002, working in supergravity, showed this can happen within a time comparable to the present age of the universe, so a crunch is not ruled out by today's acceleration.
Predictions	The total density exceeds the critical value, giving positive spatial curvature and a finite, closed universe Expansion reaches a maximum and reverses purely under gravity, with no dark energy required The contraction reverses cosmic history: blueshifted, reheating background radiation ending in a hot dense crunch A measurement of significantly positive spatial curvature would revive this route to a crunch	Dark energy is a dynamical field whose potential decreases and eventually becomes negative The cosmic acceleration reverses, expansion halts, and the universe recollapses into a Big Crunch in finite time The time to the crunch depends on the slope of the potential, and in viable supergravity models can be comparable to the current age of the universe A measurement that the dark-energy density is decreasing toward zero, with w staying above -1 as the field rolls down toward a negative potential, would support an approaching turnaround
Where it breaks	Precision cosmological measurements are consistent with a spatially flat universe, removing the positive curvature this scenario requires A positive dark energy, which is observed, drives eternal expansion even in a closed universe, overriding the gravitational recollapse On current data this is not a viable future, which is why it carries historical status	There is no evidence that the dark-energy potential is heading negative; the data are consistent with a positive cosmological constant and eternal expansion The time and even the occurrence of a turnaround depend sensitively on the assumed potential, which is not measured, so predictions are model-dependent Whether the resulting crunch is a true singularity or transitions to a bounce depends on unknown Planck-scale physics
Key unresolved problem	The scenario needs a closed, super-critical universe, but the measured near-perfect flatness and the observed positive dark energy together leave it with no room in the current data.	The whole scenario turns on the future sign of the dark-energy potential, a feature no current observation constrains, so it can place the crunch anywhere from soon to never.
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Classical Closed Recollapse

1922 · Rejected

Dynamical Dark Energy Turnaround

2002 · Speculative

Proposed

1922

2002

Key figures

Alexander Friedmann

Renata Kallosh, Andrei Linde

In one sentence

Classical Closed Recollapse is the original Big Crunch, the closed Friedmann universe that dominated cosmology before dark energy was discovered. If the total density exceeds a critical value, space has positive curvature and is finite, and gravity eventually halts the expansion and pulls everything back into a hot, dense crunch. Current measurements of a flat universe with dark energy have made this the disfavoured route, but it remains the foundational picture of a recollapsing cosmos.

The Dynamical Dark Energy Turnaround shows that a flat, currently accelerating universe can still end in a Big Crunch. If dark energy is a scalar field whose potential is slowly decreasing and will eventually go negative, its repulsion turns into an attraction, the expansion halts, and the universe recollapses. Kallosh and Linde 2002, working in supergravity, showed this can happen within a time comparable to the present age of the universe, so a crunch is not ruled out by today's acceleration.

Predictions

The total density exceeds the critical value, giving positive spatial curvature and a finite, closed universe
Expansion reaches a maximum and reverses purely under gravity, with no dark energy required
The contraction reverses cosmic history: blueshifted, reheating background radiation ending in a hot dense crunch
A measurement of significantly positive spatial curvature would revive this route to a crunch

Dark energy is a dynamical field whose potential decreases and eventually becomes negative
The cosmic acceleration reverses, expansion halts, and the universe recollapses into a Big Crunch in finite time
The time to the crunch depends on the slope of the potential, and in viable supergravity models can be comparable to the current age of the universe
A measurement that the dark-energy density is decreasing toward zero, with w staying above -1 as the field rolls down toward a negative potential, would support an approaching turnaround

Where it breaks

Precision cosmological measurements are consistent with a spatially flat universe, removing the positive curvature this scenario requires
A positive dark energy, which is observed, drives eternal expansion even in a closed universe, overriding the gravitational recollapse
On current data this is not a viable future, which is why it carries historical status

There is no evidence that the dark-energy potential is heading negative; the data are consistent with a positive cosmological constant and eternal expansion
The time and even the occurrence of a turnaround depend sensitively on the assumed potential, which is not measured, so predictions are model-dependent
Whether the resulting crunch is a true singularity or transitions to a bounce depends on unknown Planck-scale physics

Key unresolved problem

The scenario needs a closed, super-critical universe, but the measured near-perfect flatness and the observed positive dark energy together leave it with no room in the current data.

The whole scenario turns on the future sign of the dark-energy potential, a feature no current observation constrains, so it can place the crunch anywhere from soon to never.

Reader vote

No votes yet