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In physical cosmology, the Big Rip is a hypothetical cosmological model concerning the ultimate fate of the universe, in which the matter of the universe, from stars and galaxies to atoms and subatomic particles, and even spacetime itself, is progressively torn apart by the expansion of the universe at a certain time in the future. According to the hypothesis, first published in 2003, the scale factor of the universe and with it all distances in the universe, which is known to be increasing, will be found to increase exponentially. If this were to happen, the model speculates that eventually in many billions of years' time the scale factor would become infinitely large, pulling even subatomic particles and the fabric of spacetime apart, and ending our universe.
The hypothesis relies crucially on the type of dark energy in the universe. The type that could cause this hypothesis is a constantly increasing form of dark energy. If the dark energy in the universe increases without limit, it could overcome all forces that hold the universe together. The key value[why?] is the equation of state parameter w, the ratio between the dark energy pressure and its energy density. If w < −1, this dynamical vacuum energy is known as phantom energy, an extreme[how?] form of quintessence.
A universe dominated by phantom energy is an accelerating universe, expanding at an ever-increasing rate. However, this implies that the size of the observable universe is continually shrinking; the distance to the edge of the observable universe which is moving away at the speed of light from any point moves ever closer. When the size of the observable universe becomes smaller than any particular structure, no interaction by any of the fundamental forces can occur between the most remote parts of the structure. When these interactions become impossible, the structure is "ripped apart". The model implies that after a finite time there will be a final singularity, called the "Big Rip", in which all distances diverge to infinite values.
where w is defined above, H0 is Hubble's constant and Ωm is the present value of the density of all the matter in the universe.
In their paper, the authors consider a hypothetical example with w = −1.5, H0 = 70 km/s/Mpc, and Ωm = 0.3, in which case the Big Rip will happen approximately 22 billion years from the present.
For w = −1.5, the galaxies would first be separated from each other. About 60 million years before the Big Rip, gravity would be too weak to hold the Milky Way and other individual galaxies together. Galaxies would be destroyed as stars separate from the main black hole. Approximately three months before the Big Rip, the Solar System (or systems similar to our own at this time, as the fate of the Solar System 22 billion years in the future is questionable) would be gravitationally unbound. Planets would be detached from the star’s orbit. In the last minutes, stars and planets would be torn apart, and an extremely short amount of time before the Big Rip, atoms would be destroyed. At the time the Big Rip occurs, even spacetime itself will be ripped apart and the scale factor will be infinity.
Evidence indicates w to be very close to −1 in our universe, which makes w the dominating term in the equation. The closer that w is to −1, the closer the denominator is to zero and the further the Big Rip is in the future. If w were exactly equal to −1, the Big Rip could not happen, regardless of the values of H0 or Ωm.
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