Big Crunch

Instead, astronomical observations show that the expansion of the universe is accelerating rather than being slowed by gravity, suggesting that a Big Freeze is much more likely to occur.

[1][2][3] Nonetheless, some physicists have proposed that a "Big Crunch-style" event could result from a dark energy fluctuation.

[4] The hypothesis dates back to 1922, with Russian physicist Alexander Friedmann creating a set of equations showing that the end of the universe depends on its density.

[6] In the final moments, the universe would be one large fireball with a near-infinite temperature, and at the absolute end, neither time, nor space would remain.

Experimental evidence in the late 1990s and early 2000s (namely the observation of distant supernovas as standard candles; and the well-resolved mapping of the cosmic microwave background) led to the conclusion that the expansion of the universe is not getting slowed by gravity but is instead accelerating.

The Ekpyrotic model, formed by Paul Steinhardt, states that the Big Bang could have been caused by two parallel orbifold planes, referred to as branes colliding in a higher-dimensional space.

Dark energy corresponds to the force between the branes, allowing for problems, like the flatness and monopole in the previous models to be fixed.

The basis of the model, branes, are still not understood completely by string theorists, and the possibility that the scale invariant spectrum could be destroyed from the big crunch.

[20] Physicist Roger Penrose advanced a general relativity-based theory called the conformal cyclic cosmology in which the universe expands until all the matter decays and is turned to light.

[21] Penrose and Gurzadyan suggested that signatures of conformal cyclic cosmology could potentially be found in the cosmic microwave background; as of 2020, these have not been detected.

Penrose presented evidence of CCC in the form of rings that had uniform temperature in the CMB, the idea being that these rings would be the signature in our aeon—An aeon being the current cycle of the universe that we're in—was caused by spherical gravitational waves caused by colliding black holes from our previous aeon.

In this model quantum geometry creates a brand-new force that is negligible at low spacetime curvature, but that rises very rapidly in the Planck regime, overwhelming classical gravity and resolving singularities of general relativity.

The approach of effective dynamics has been used extensively in loop quantum cosmology to describe physics at the Planck scale, and also the beginning of the universe.

[25] If a form of quintessence driven by a scalar field evolving down a monotonically decreasing potential that passes sufficiently below zero is the (main) explanation of dark energy and current data (in particular observational constraints on dark energy) is true as well, the accelerating expansion of the Universe would inverse to contraction within the cosmic near-future of the next 100 million years.

According to an Andrei-Ijjas-Steinhardt study, the scenario fits "naturally with cyclic cosmologies and recent conjectures about quantum gravity".

[30] The term is sometimes used in the mainstream, for example (as "gnaB giB") in Physics I For Dummies and in a posting discussing the Big Crunch.