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Inphysical cosmology, theBig Rip is ahypotheticalcosmological model concerning theultimate fate of the universe, in which thematter of theuniverse, from stars and galaxies to atoms and subatomic particles, and evenspacetime itself, is progressively torn apart by theexpansion of the universe at a certain time in the future, until distances between particles will infinitely increase.
According to the standard model of cosmology, thescale factor of the universe isaccelerating, and, in the future era of cosmological constant dominance, will increase exponentially. But this expansion is similar for every moment of time (hence the exponential law—the expansion of a local volume is the same number of times over the same time interval), and is characterized by an unchanging, smallHubble constant, effectively ignored by any bound material structures. By contrast, in the Big Rip scenario the Hubble constant increases to infinity in a finite time. According to recent studies, the universe is set for a constant expansion andheat death,[1] because theequation of state parameterw = −1.
The possibility of sudden rip singularity occurs only for hypothetical matter (phantom energy) with implausible physical properties.[2]
The truth of the hypothesis relies on the type ofdark energy present in ouruniverse. The type that could prove this hypothesis is a constantly increasing form of dark energy known asphantom energy. If the dark energy in the universe increases without limit, it could overcome all forces that hold the universe together. The key value is theequation of state parameterw, theratio between the dark energy pressure and itsenergy density. If −1 < w < 0, the expansion of the universe tends to accelerate, but the dark energy tends to dissipate over time, and the Big Rip does not happen. Phantom energy hasw < −1, which means that its density increases as the universe expands.
A universe dominated by phantom energy is anaccelerating universe, expanding at an ever-increasing rate. But this implies that the size of theobservable universe and thecosmological event horizon is continually shrinking—the distance at which objects can influence an observer becomes ever closer, and the distance over which interactions can propagate becomes ever shorter. When the size of the horizon becomes smaller than any particular structure, no interaction by any of thefundamental forces can occur between the most remote parts of the structure, and the structure is "ripped apart". The progression of time itself will stop. The model implies that after a finite time there will be a final singularity, called the "Big Rip", in which the observable universe eventually reaches zero size and all distances diverge to infinite values.
The authors of this hypothesis, led byRobert R. Caldwell ofDartmouth College, calculate the time from the present to the Big Rip to bewherew is defined above,H0 isHubble's constant andΩm is the present value of the density of all the matter in the universe.
Observations ofgalaxy cluster speeds by theChandra X-ray Observatory seem to suggest the value ofw is between approximately −0.907 and −1.075, meaning the Big Rip cannot be ruled out. Based on the above equation, if the observation determines thatw is less than −1 but greater than or equal to −1.075, the Big Rip would occur in approximately 152 billion years at the earliest.[3] More recent data from Planck mission indicates the value ofw to be −1.028 (±0.031), pushing the earliest possible time of Big Rip to approximately 200 billion years into the future.[4]
In their paper, the authors consider a hypothetical example withw = −1.5,H0 = 70 km/s/Mpc, andΩm = 0.3, in which case the Big Rip would happen approximately 22 billion years from the present. In this scenario,galaxies would first be separated from each other about 200 million years before the Big Rip. About 60 million years before the Big Rip, galaxies would begin to disintegrate as gravity becomes too weak to hold them together.Planetary systems like theSolar System would become gravitationally unbound about three months before the Big Rip, and planets would fly off into the rapidly expanding universe. In the last minutes, stars and planets would be torn apart, and the now-dispersedatoms would be destroyed about 10−19 seconds before the end (the atoms will first beionized aselectrons fly off, followed by thedissociation of theatomic nuclei). At the time the Big Rip occurs, even spacetime itself would be ripped apart and the scale factor would be infinity.[5]
Evidence indicatesw to be very close to −1 in our universe, which makesw the dominating term in the equation. The closerw is to −1, the closer the denominator is to zero and the further the Big Rip is in the future. Ifw were exactly −1, the Big Rip could not happen, regardless of the values ofH0 orΩm.
According to the latest cosmological data available, the uncertainties are still too large to discriminate among the three casesw < −1,w = −1, andw > −1.[6][7]
Moreover, it is nearly impossible to measurew to be exactly at −1 due to statistical fluctuations. This means that the measured value ofw can be arbitrarily close to −1 but not exactly at −1, hence the earliest possible date of the Big Rip can be pushed back further with more accurate measurements but the Big Rip is very difficult to completely rule out.[8]
