Big Crunch

The Big Crunch is a hypothetical scenario for the ultimate fate of the universe, in which the expansion of the universe eventually reverses and the universe recollapses, ultimately causing the cosmic scale factor to reach absolute zero, an event potentially followed by a reformation of the universe starting with another Big Bang. The vast majority of valid evidence, however, indicates that this hypothesis is not correct. Instead, astronomical observations show that the expansion of the universe is accelerating rather than being slowed by gravity, suggesting that a Big Chill or Big Rip is much more likely to occur. Nonetheless, some physicists have proposed that a "Big Crunch-style" event could result from a dark energy fluctuation.

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. It could either expand or contract rather than stay stable. With enough matter, gravity could stop the universe's expansion and eventually reverse it. This reversal would result in the universe collapsing on itself, not too dissimilar to a black hole.

As the universe collapses in on itself, it would get filled with radiation from stars and high-energy particles; when this is condensed and blueshifted to higher energy, it would be intense enough to ignite the surface of stars before they collide. 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.

Overview

The Big Crunch is an old idea about what might happen to the universe in the far future. It suggested that if there was enough matter, gravity might eventually pull the universe back together after the Big Bang. This would cause everything to collapse into a single point.

But studies in the late 1990s and early 2000s showed that the universe is actually expanding faster and faster, not slowing down. This discovery won a Nobel Prize in Physics in 2011. Because of this, most scientists now think the Big Crunch is unlikely, and other ideas like the Big Bounce might happen instead.

History

Richard Bentley, a churchman and scholar, once asked Isaac Newton if stars in a finite universe would eventually collapse together. This idea is called Bentley's paradox and is an early thought related to the Big Crunch. We now know that stars move and are not fixed in place.

Albert Einstein originally supported a model where the universe stayed the same size. He introduced something called the cosmological constant to balance out the forces pulling the universe together. Later, observations by Edwin Hubble showed that the universe was actually expanding, leading Einstein to change his ideas.

Cyclic universes

A hypothesis called "Big Bounce" suggests that the universe might collapse and then begin again with another Big Bang. This could mean the universe goes through endless phases of expanding and contracting.

Cyclic universes were an early idea by Albert Einstein in 1931. He thought there might have been a universe before the Big Bang that ended in a Big Crunch, leading to our Big Bang. Modern ideas, like the Ekpyrotic model by Paul Steinhardt, suggest the Big Bang happened when two special planes called branes collided. Another theory, conformal cyclic cosmology by Roger Penrose, proposes that the universe expands until everything turns to light, then starts over again. There is also loop quantum cosmology, which suggests a quantum effect prevents total collapse, leading to a Big Bounce instead of a Big Crunch.

Empirical scenarios from physical theories

Some theories suggest that if a special kind of energy called quintessence behaves in a certain way, the universe's expansion might eventually stop and start shrinking. This could happen within the next 100 million years. According to a study by Andrei-Ijjas-Steinhardt, this shrinking phase might last about a billion years before the universe begins expanding again. These ideas connect to theories about the universe going through cycles and ideas about quantum gravity.

Current observations, however, show that the universe is continuing to expand and this shrinking scenario might not actually happen.

Effects

Imagine the universe shrinking back in on itself billions of years from now. As it contracts, galaxies would move closer together and eventually merge. The space between stars would grow smaller until stars bump into each other. The heat in space would rise, causing stars to break apart. In the very end, everything would be crushed into an extremely hot and dense point, much like the moment the universe began. Some thinkers wonder if this could lead to a new universe beginning again.

Main article: Big Bang

In culture

In the novel The Restaurant at the End of the Universe by Douglas Adams, there is a restaurant where people can watch the end of the Universe, called the "Gnab Gib". The idea of the Big Crunch has also appeared in other places, like a book called Physics I For Dummies and in discussions about space. In the Doctor Who television series from 1980 to 1989, the opening credits show the universe expanding after a Big Bang, and the closing credits show it shrinking towards a Big Crunch.