If today's dark energy is a field sliding toward negative values, the expansion can reverse into a collapse.
Dynamical Dark Energy Turnaround
A dark energy carried by a scalar field whose potential dips below zero can halt and reverse the expansion, sending even a flat universe into a Big Crunch within a calculable time.
Looping ambient scene for Recollapse and the Big Crunch. A recollapsing universe is the mirror image of the expanding one: gravity, or a dark energy that turns negative, eventually halts the expansion and pulls everything back together into a Big Crunch. Three routes lead here. A dynamical dark energy whose potential becomes negative can trigger a turnaround even in a flat universe. A negative cosmological constant has the same effect through different bookkeeping, giving an anti-de-Sitter-like collapse. And a closed spatial geometry, the classical pre-dark-energy route to a crunch, recollapses under its own gravity once expansion stalls. Current data, a flat universe with w near -1, disfavour an imminent crunch, but a future measurement that w is drifting negative or that space is slightly closed would put a recollapse back on the table.
§1 · The claim, in one sentence
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.
§2 · Why it might be true
Today the universe accelerates, which looks like a one-way ticket to eternal expansion. But that conclusion assumes the dark energy stays positive. If dark energy is a dynamical field rather than a fixed cosmological constant, its energy can change sign. A field rolling down a potential that passes through zero into negative values will, at some point, make the total energy density negative.
When that happens the acceleration reverses. Kallosh and Linde 2002 worked this out in models motivated by supergravity, where negative potentials arise naturally. They found that a universe matching all current observations, flat, accelerating, with the measured dark-energy density, can nonetheless turn around and collapse, and that the time to the crunch can be as short as 10 to 20 billion years in some viable models.
The lesson is that present-day acceleration does not settle the fate. The sign of the dark energy in the future, set by the shape of its potential, is what matters, and that shape is exactly what current data cannot pin down. A crunch driven by negative dark energy is fully consistent with everything we have measured.
The family stance
The universe ends by falling back together. Expansion is not guaranteed to last forever; if the dark energy turns negative or the geometry is closed, gravity wins, the expansion reverses, and everything collapses into a hot, dense Big Crunch, possibly followed by a bounce.
§2.5 · Evidence
- Negative potentials arise naturally in supergravity and string-theory constructions, so a sign change in the dark energy is well motivated theoretically
- Kallosh and Linde showed explicit models consistent with all current data that nonetheless recollapse, so the scenario is not excluded by observation
- Hints of evolving dark energy from DESI 2024 keep dynamical models, including turnaround scenarios, observationally live
§3 · What you'd need to test it
- 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
§4 · Where it breaks
- 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
Go deeper
Kallosh, Linde, and collaborators 2002 (Phys. Rev. D 66, 123503) studied dark energy from a scalar field in supergravity-motivated potentials that pass through zero into negative values. Once the potential energy goes negative, the total energy density can reach zero and then negative, at which point the Friedmann equation forces the expansion to stop and reverse. They computed the time to the turnaround as a function of the potential's slope and found viable cases collapsing within roughly the present age of the universe.
The deeper point connects to the cosmological constant problem. A positive cosmological constant is famously hard to obtain in string theory, while negative values (anti-de-Sitter vacua) are generic. If the observed dark energy is a slowly evolving field perched near a transition to a negative-energy vacuum, a future crunch becomes natural rather than exotic. This is the link to the negative-cosmological-constant recollapse variant in this family.
Cross-references: this variant is the fate-framed counterpart of the Quintessence variant in the Chapter 5 Dark Energy Candidates family, taken into the regime where the field's potential turns negative. The negative-lambda-recollapse variant here treats the constant-negative-energy limit, and the cyclic and bouncing models bridged from Chapter 1 consider what happens if the crunch bounces rather than ending in a singularity.
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