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TheBig Crunch is a hypothetical scenario for theultimate fate of the universe, in which theexpansion of the universe eventually reverses and the universe recollapses, ultimately causing the cosmicscale factor to reach absolute zero, an event potentially followed by a reformation of the universe starting with anotherBig Bang. The vast majority of current evidence, however, indicates that this hypothesis is not correct. Instead, astronomical observations show that the expansion of the universe isaccelerating rather than being slowed bygravity, suggesting that aBig Chill orBig Rip is much more likely to occur.[1][2][3] Nonetheless, some physicists have proposed that a "Big Crunch-style" event could result from adark energy fluctuation.[4]
The hypothesis dates back to 1922, with Russian physicistAlexander Friedmann creating a set of equations showing that the end of the universe depends on itsdensity. 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 ablack hole.[5]
As the universe collapses in on itself, it would get filled withradiation from stars andhigh-energy particles; when this is condensed andblueshifted to higher energy, it would be intense enough to ignite the surface of stars before they collide.[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.[7]
The Big Crunch[8] scenario hypothesized that the density of matter throughout the universe is sufficiently high that gravitational attraction will overcome the expansion that began with the Big Bang. TheFLRW cosmology can predict whether the expansion will eventually stop based on the averageenergy density,Hubble parameter, andcosmological constant. If the expansion stopped, then contraction will inevitably follow, accelerating as time passes and finishing the universe in a kind ofgravitational collapse,turning the universe into a black hole.
Experimental evidence in the late 1990s and early 2000s (namely the observation of distantsupernovas asstandard candles; and the well-resolved mapping of thecosmic microwave background) led to the conclusion that the expansion of the universe is not being slowed by gravity but is insteadaccelerating.[9] The 2011Nobel Prize in Physics was awarded to researchers who contributed to this discovery.[1]
The Big Crunch hypothesis also leads into another hypothesis known as theBig Bounce, in which after the big crunch destroys the universe, it begins a new expansionary epoch, causing another big bang.[10] This could potentially repeat forever in a phenomenon known as a cyclic universe.
Richard Bentley, a churchman and scholar, sent a letter toIsaac Newton in preparation for a lecture on Newton's theories and the rejection ofatheism:
If we're in a finite universe and all stars attract each other together, would they not all collapse to a singular point, and if we're in an infinite universe with infinite stars, would infinite forces in every direction not affect all of those stars?
This question is known asBentley's paradox, an early predecessor of the Big Crunch. It is now known, however, that stars move around and are not static.[11]
Albert Einstein favored an unchanging model of the universe. He collaborated in 1917 with Dutch astronomerWillem de Sitter to help demonstrate that the theory ofgeneral relativity would work with a static model; Willem demonstrated that his equations could describe a very simple universe. Finding no problems initially, scientists adapted the model to describe the universe. They ran into a different form of Bentley's paradox.[13]
The theory of general relativity also described the universe as restless. Einstein realized that for a static universe to exist—which was observed at the time—ananti-gravity would be needed to counter the gravity contracting the universe together, adding an extra force that would ruin the equations in the theory of relativity. In the end, thecosmological constant, the name for the anti-gravity force, was added to the theory of relativity.[14]
Edwin Hubble working in theMount Wilson Observatory took measurements of the distances of galaxies and paired them withVesto Silpher andMilton Humason's measurements of red shifts associated with those galaxies. He discovered a rough proportionality between the red shift of an object and its distance. Hubble plotted a trend line from 46 galaxies, studying and obtaining theHubble Constant, which he deduced to be 500 km/s/Mpc, nearly seven times more than what it is considered today, but still giving the proof that the universe was expanding and was not a static object.[15]
After Hubble's discovery was published, Einstein abandoned the cosmological constant. In their simplest form, the equations generated a model of the universe that expanded or contracted. Contradicting what was observed, hence the creation of the cosmological constant.[16] After the confirmation that the universe was expanding, Einstein called his assumption that the universe was static his "biggest mistake". In 1931, Einstein visited Hubble to thank him for "providing the basis of modern cosmology".[17] After this discovery, Einstein's and Newton's models of a contracting, yet static universe were dropped for the expanding universe model.
A hypothesis called "Big Bounce" proposes that the universe could collapse to the state where it began and then initiate another Big Bang, so in this way, the universe would last forever but would pass through phases of expansion (Big Bang) and contraction (Big Crunch).[10] This means that there may be a universe in a state of constant Big Bangs and Big Crunches.
Cyclic universes were briefly considered byAlbert Einstein in 1931. He hypothesized that there was a universe before the Big Bang, which ended in a Big Crunch, which could create a Big Bang as a reaction. Our universe could be in a cycle of expansion and contraction, a cycle possibly going on infinitely.
There are more modern models of cyclic universes as well. TheEkpyrotic model, formed byPaul Steinhardt, states that the Big Bang could have been caused by two parallelorbifold planes, referred to asbranes colliding in a higher-dimensional space.[18] The four-dimension universe lies on one of the branes. The collision corresponds to the Big Crunch, then a Big Bang. The matter and radiation around us today arequantum fluctuations from before the branes. After several billion years, the universe has reached its modern state, and it will start contracting in another several billion years.Dark energy corresponds to the force between the branes, allowing for problems, like theflatness andmonopole in the previous models to be fixed. The cycles can also go infinitely into the past and the future, and anattractor allows for a complete history of the universe.[19]
This fixes the problem of the earlier model of the universe going intoheat death fromentropy buildup. The new model avoids this with a net expansion after every cycle, stopping entropy buildup. There are still some flaws in this model, however. The basis of the model, branes, are still not understood completely by string theorists, and the possibility that thescale invariant spectrum could be destroyed from the big crunch. Whilecosmic inflation and the general character of the forces—or the collision of the branes in the Ekpyrotic model—required to makevacuum fluctuations is known. A candidate fromparticle physics is missing.[20]
PhysicistRoger Penrose advanced a general relativity-based theory called theconformal cyclic cosmology in which the universe expands until all the matter decays and is turned to light. Since nothing in the universe would have any time or distance scale associated with it, it becomes identical with the Big Bang (resulting in a type of Big Crunch that becomes the next Big Bang, thus starting the next cycle).[21] Penrose and Gurzadyan suggested that signatures of conformal cyclic cosmology could potentially be found in thecosmic microwave background; as of 2020, these have not been detected.[22]
There are also some flaws with this model as well: skeptics pointed out that in order to match up an infinitely large universe to an infinitely small universe, that all particles must lose their mass when the universe gets old. 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.[23]
Loop quantum cosmology is a model of the universe that proposes a "quantum-bridge" between expanding and contracting universes. In this model quantum geometry creates a brand-new force that is negligible at low spacetime curvature, but that rises very rapidly in thePlanck regime, overwhelming classical gravity and resolving singularities ofgeneral relativity. Once the singularities are resolved the conceptual paradigm of cosmology changes, forcing one to revisit the standard issues—such as the horizon problem—from a new perspective.[24]
Under this model, due to quantum geometry, the Big Bang is replaced by the Big Bounce with no assumptions or any fine tuning. 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. Numerical simulations have confirmed the validity of effective dynamics, which provides a good approximation of the full loop quantum dynamics. It has been shown when states have very large quantum fluctuations at late times, meaning they do not lead to macroscopic universes as described by general relativity, but the effective dynamics departs from quantum dynamics near bounce and the later universe. In this case, the effective dynamics will overestimate the density at bounce, but it will still capture qualitative aspects extremely well.[25]
If a form ofquintessence 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, theaccelerating 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 withcyclic cosmologies and recent conjectures aboutquantum gravity". The study suggests that the slow contraction phase would "endure for a period of order 1 billion y before the universe transitions to a new phase of expansion".[26][27][28]
Paul Davies considered a scenario in which the Big Crunch happens about 100 billion years from the present. In his model, the contracting universe would evolve roughly like the expanding phase in reverse. First,galaxy clusters, and then galaxies, would merge, and the temperature of thecosmic microwave background (CMB) would begin to rise as CMB photons getblueshifted. Stars would eventually become so close together that they begin to collide with each other. Once the CMB becomes hotter thanM-type stars (about 500,000 years before the Big Crunch in Davies' model), they would no longer be able to radiate away their heat and would cook themselves until they evaporate; this continues for successively hotter stars untilO-type stars boil away about 100,000 years before the Big Crunch. In the last minutes, the temperature of the universe would be so great thatatoms andatomic nuclei would break up and get sucked up into already coalescingblack holes. At the time of the Big Crunch, all the matter in the universe would be crushed into an infinitely hot, infinitely densesingularity similar to theBig Bang.[29] The Big Crunch may be followed by another Big Bang, creating a new universe.[4]
InThe Restaurant at the End of the Universe, a novel byDouglas Adams, the concept is that a restaurant, Milliways, is set up to allow patrons to observe the end of the Universe, or "Gnab Gib", as it is referred to, as they dine.[30] The term is sometimes used in the mainstream, for example (as "gnaB giB") inPhysics I For Dummies and in a posting discussing the Big Crunch.[31]In theDoctor Who television series between the years 1980 and 1989 the various opening credits follow a "Big Bang" and expansion of the universe, while the closing credits show the reverse: a contraction and eventual "Big Crunch".
