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From Wikipedia, the free encyclopedia

An animation of the supposed behaviour of a Big Crunch
An animation of the supposed behaviour of a Big Crunch

The Big Crunch is one of the theoretical scenarios for the ultimate fate of the universe, in which the metric expansion of space eventually reverses and the universe recollapses, ultimately causing the cosmic scale factor to reach zero or causing a reformation of the universe starting with another Big Bang.

Recent experimental evidence suggests that the expansion of the universe is not being slowed down by gravity but rather accelerating.

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  • ✪ Three Ways to Destroy the Universe
  • ✪ Michio Kaku: What's the Fate of the Universe? It's in the Dark Matter
  • ✪ The Big Crunch theory
  • ✪ (In Hindi) हमारे ब्रह्मांड का अंत केसे होगा ? | Big Rip, Big Crunch, Big Freeze, Big Slurp.
  • ✪ Big Freeze. Big Rip. Big Crunch. How Will The Universe End?

Transcription

One day the universe will die. But why? And how? And will the universe be dead forever? And how do we know that? First of all, the universe is expanding. And not only that, the rate of its expansion is accelerating. The reason: dark energy. Dark energy is a strange phenomenon that, scientists believe, permeates the universe. Until 1998 we thought that the universe must work a bit like a ball that you throw into the sky. The ball goes up, but at some point it has to come down again. But the expansion of the universe is actually speeding up. That's length throwing a ball up and watching it fly away faster and faster and faster. Where is this acceleration coming from? Well, we don't know, but we call it 'dark energy'. Einstein thought of it first and then decided it was stupid. Now, astrophysicists have decided it is plausible, trouble is, this is all very theoretical and we don't actually know what the properties of dark energy are. But there are various theories and they lead us to three scenarios for the end of the universe. One: the big rip. Since its birth, the universe has been expanding. For unknown reasons new spaces created everywhere equally. The space between galaxies expands so they move apart The space inside galaxies also expands but here, gravity is strong enough to keep them together. In the big rip scenario, the expansion accelerates up to a point where space expands so fast that gravity can't compensate for this effect anymore. The result is a big rip. At first, only large structures like galaxies are torn apart since space between the single objects expands very fast. Next, big bodies like black holes, stars and planets die. Their gravity isn't strong enough to keep them together so they dissolve into their components. In the end, space would expand faster than the speed of light. Atoms would now be affected and they would just disband. One space is expanding faster than light, no particle than the universe can interact with any other particle anymore. The universe would dissolve into countless lonely particles that won't be able to touch anything else in a strange timeless universe. Hmm, and you thought you felt lonely (ironic voice) Two: Heat death or a big freeze. In a nutshell the difference between the big rip and heat death is that in a heat death scenario matter stays intact and is converted over an incredibly long but finite period of time into radiation, while the universe expands forever. But how does this work? Let's talk about entropy. Every system tends towards the state of highest entropy, like when we have a latte macchiato. Initially, it has different regions but over time they will cool down and disintegrate until its uniform and this also applies to the universe. So, while the universe gets bigger and bigger, matters slowly decays and spreads out. At some point after lots of generations of stars, all the gas clouds necessary to form stars will be exhausted, so the universe will turn dark. The remaining suns will die, black holes will slowly degenerate and evaporate over trillions of years due to what's known as Hawking Radiation. When this process is complete, only a dilute gas of photons and light particles remains until even this decays. All activity in the universe seizes at this point, entropy is at its maximum and the universe is dead forever. Unless, theoretically it might be possible, but after an incredibly long amount of time, there might be a spontaneous entropy decrease as a result of something called 'quantum tunneling' leading to a new big bang. Three: Big crunch and big bounce. This is the most uplifting scenario. If there is less dark energy than we think or it decreases over time, gravity will be the dominating force in the universe one day. In a few trillion years the rate of expansion of the universe will slow down and stop. After that, it reverses. Galaxies will race at each other, merging as the universe becomes smaller and smaller. Since a smaller universe also means a hotter universe, temperatures rise everywhere all at once. One hundred thousand years before the big crunch, background radiation would be hotter than the surfaces of the most stars, which means that they would be cooked from the outside. Minutes before the big crunch happens, atom cores are ripped apart before super massive black holes devour everything. Finally, all black holes would emerge into a super massive mega black hole that contains the entire mass of the universe and in the last moment before the big crunch it would devour the universe, including itself. The big bounce theory states that this has happened a lot of times and that the universe goes through an infinite cycle of expansion and contraction. Well, wouldn't that be nice? So what will actually happen to the universe in the end? At the moment, heat death seems the most likely, but we, at ‘kurzgesagt’, hope, that this 'dead forever' stuff is wrong and the universe will start over and over again. We don't know for sure either way, so let's just assume that the most uplifting theory is true. By the way, we have a twitter account. Subtitles by the Amara.org community

Overview

If the universe's expansion speed does not exceed the escape velocity, then the mutual gravitational attraction of all its matter will eventually cause it to contract. If entropy continues to increase in the contracting phase (see Ergodic hypothesis), the contraction would appear very different from the time reversal of the expansion. While the early universe was highly uniform, a contracting universe would become increasingly clumped.[1] Eventually all matter would collapse into black holes, which would then coalesce, producing a unified black hole or Big Crunch singularity.

The idea behind the theory is that the expansion of the universe is linked to the energy released in the Big Bang, therefore the outward speed of the matter would decrease over time due to gravity (mutual attraction). This would act as ballast and would eventually lead to a halt of the expansion. As matter attracts and there is no matter beyond the maximum expansion point, eventually all matter would begin to travel inwards again, accelerating as time passes.

The exact details of the events that would take place before such final collapse depend on the length of both the expansion phase as well as the previous contraction phase; the longer both lasted, the more events expected to take place in an ever-expanding universe would happen; nonetheless it's expected that the contraction phase would not immediately be noticed by hypothetical observers because of the delay caused by the speed of light, that the temperature of the cosmic microwave background would rise during contraction symmetrically compared to the previous expansion phase, and that the events that took place during the Big Bang would occur in opposite order.[2] For a contracting Universe similar to ours in composition it's expected that superclusters would merge among themselves followed by galaxy clusters and later galaxies. By the time stars were so close together that collisions among them were frequent, the temperature of the cosmic microwave background would have increased so much that stars would be unable to expel their internal heat, slowly cooking until they exploded, leaving behind a hot and highly heterogeneous gas, whose atoms would break down into their constituent subatomic particles because of the increasing temperature, that would be absorbed by the already coalescing black holes before the Big Crunch itself.[2]

The Hubble Constant measures the current state of expansion in the universe, and the strength of the gravitational force depends on the density and pressure of matter in the universe, or in other words, the critical density of the universe. If the density of the universe is greater than the critical density, then the strength of the gravitational force will stop the universe from expanding and the universe will collapse back on itself[1]—assuming that there is no repulsive force such as a cosmological constant. Conversely, if the density of the universe is less than the critical density, the universe will continue to expand and the gravitational pull will not be enough to stop the universe from expanding. This scenario would result in the heat death of the universe, where the universe reaches the maximum state of entropy that is thermodynamic equilibrium. In the state of thermodynamic equilibrium energy in the universe is evenly distributed so heat transfer or any other energy transfer is impossible so no reactions can happen in such universe making it "dead".[3][not in citation given] One theory proposes that the universe could collapse to the state where it began and then initiate another Big Bang,[1] so in this way the universe would last forever, but would pass through phases of expansion (Big Bang) and contraction (Big Crunch).[4] Another scenario results in a flat universe which occurs when the critical density is just right. In this state the universe would always be slowing down, and eventually come to a stop in an interminable amount of time. Although, it is now understood that the critical density has been measured and determined to be a flat universe.[5]

Experimental evidence in the late 1990s and early 2000s (namely the observation of distant supernovae as standard candles, and the well-resolved mapping of the cosmic microwave background)[6][7] led to the conclusion that the expansion of the universe is not being slowed down by gravity but rather accelerating. However, more recent research, based on larger datasets, has cast doubt on this conclusion[8].

See also

References

  1. ^ a b c How the Universe Works 3. End of the Universe. Discovery Channel. 2014.
  2. ^ a b Davies, Paul (January 9, 1997). The Last Three Minutes: Conjectures About The Ultimate Fate Of The Universe. Basic Books. ISBN 978-0-465-03851-0.
  3. ^ Dr. Gary F. Hinshaw, WMAP Introduction to Cosmology. NASA (2008)
  4. ^ Jennifer Bergman, The Big Crunch, Windows to the Universe (2003)
  5. ^ Fraser Cain (2013-10-17), How Will The Universe End?, retrieved 2016-06-13
  6. ^ Y Wang, J M Kratochvil, A Linde, and M Shmakova, Current Observational Constraints on Cosmic Doomsday. JCAP 0412 (2004) 006, astro-ph/0409264
  7. ^ McSween, Stephen A. "Dark Energy and the Red Shift in a Contracting Universe." [1]
  8. ^ J. T. Nielsen, A. Guffanti & S. Sarkar, Marginal evidence for cosmic acceleration from Type Ia supernovae. Nature (2016)
This page was last edited on 27 February 2019, at 20:04
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