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Main article: Ultimate fate of the universe As with interpretations of what happened in the very early universe, advances in fundamental physics are required before it will be possible to know the ultimate fate of the universe with any certainty. Below are some of the main possibilities. Fate of the Solar system: 1 to 5 billion years Main articles: Formation and evolution of the Solar System#Future, Stability of the Solar System, Future of the Earth#Solar evolution, and Red giant#The Sun as a red giant Relative size of our Sun as it is now (inset) compared to its estimated future size as a red giant Over a timescale of a billion years or more, the Earth and Solar System are unstable. Earth's existing biosphere is expected to vanish in about a billion years, as the Sun's heat production gradually increases to the point that liquid water and life are unlikely;[20] the Earth's magnetic fields, axial tilt and atmosphere are subject to long term change; and the Solar System itself is chaotic over million- and billion-year timescales;[21] Eventually in around 5.4 billion years from now, the core of the Sun will become hot enough to trigger hydrogen fusion in its surrounding shell.[20] This will cause the outer layers of the star to expand greatly, and the star will enter a phase of its life in which it is called a red giant.[22][23] Within 7.5 billion years, the Sun will have expanded to a radius of 1.2 AU—256 times its current size, and studies announced in 2008 show that due to tidal interaction between Sun and Earth, Earth would actually fall back into a lower orbit, and get engulfed and incorporated inside the Sun before the Sun reaches its largest size, despite the Sun losing about 38% of its mass.[24] The Sun itself will continue to exist for many billions of years, passing through a number of phases, and eventually ending up as a long-lived white dwarf. Eventually, after billions more years, the Sun will finally cease to shine altogether, becoming a black dwarf.[25] Big freeze: 100 trillion years and beyond Main articles: Future of an expanding universe and Heat death of the universe This scenario is generally considered to be the most likely,[citation needed] as it occurs if the universe continues expanding as it has been. Over a time scale on the order of 1014 years or less, existing stars burn out, stars cease to be created, and the universe goes dark.[26], §IID. Over a much longer time scale in the eras following this, the galaxy evaporates as the stellar remnants comprising it escape into space, and black holes evaporate via Hawking radiation.[26], §III, §IVG. In some grand unified theories, proton decay after at least 1034 years will convert the remaining interstellar gas and stellar remnants into leptons (such as positrons and electrons) and photons. Some positrons and electrons will then recombine into photons.[26], §IV, §VF. In this case, the universe has reached a high-entropy state consisting of a bath of particles and low-energy radiation. It is not known however whether it eventually achieves thermodynamic equilibrium.[26], §VIB, VID. Big Crunch: 100+ billion years from now See also: Big Crunch If the energy density of dark energy were negative or the universe were closed, then it would be possible that the expansion of the universe would reverse and the universe would contract towards a hot, dense state. This is a required element of oscillatory universe scenarios, such as the cyclic model, although a Big Crunch does not necessarily imply an oscillatory Universe. Current observations suggest that this model of the universe is unlikely to be correct, and the expansion will continue or even accelerate. Big Rip: 20+ billion years from now See also: Big Rip This scenario is possible only if the energy density of dark energy actually increases without limit over time.[citation needed] Such dark energy is called phantom energy and is unlike any known kind of energy. In this case, the expansion rate of the universe will increase without limit. Gravitationally bound systems, such as clusters of galaxies, galaxies, and ultimately the Solar System will be torn apart. Eventually the expansion will be so rapid as to overcome the electromagnetic forces holding molecules and atoms together. Finally even atomic nuclei will be torn apart and the universe as we know it will end in an unusual kind of gravitational singularity. At the time of this singularity, the expansion rate of the universe will reach infinity, so that any and all forces (no matter how strong) that hold composite objects together (no matter how closely) will be overcome by this expansion, literally tearing everything apart. Vacuum metastability event See also: False vacuum If our universe is in a very long-lived false vacuum, it is possible that a small region of the universe will tunnel into a lower energy state (see Bubble nucleation). If this happens, all structures within will be destroyed instantaneously and the region will expand at near light speed, bringing destruction without any forewarning. Heat death: 10150+ years from now See also: Heat death of the universe The heat death is a possible final state of the universe, estimated at after 10150 years, in which it has "run down" to a state of no thermodynamic free energy to sustain motion or life. In physical terms, it has reached maximum entropy (because of this, the term "entropy" has often been confused with Heat Death, to the point of entropy being labelled as the "force killing the universe"). The hypothesis of a universal heat death stems from the 1850s ideas of William Thomson (Lord Kelvin)[27] who extrapolated the theory of heat views of mechanical energy loss in nature, as embodied in the first two laws of thermodynamics, to universal operation |
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