Big Rip

Overview

The hypothesis relies crucially on the type of dark energy in the universe. The key value 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 form of quintessence.

Expansion

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.

The authors of this hypothesis, led by Robert Caldwell of Dartmouth College, calculate the time from the present to the end of the universe as we know it for this form of energy to be

t r i p − t 0 ≈ 2 3 | 1 + w | H 0 1 − Ω m

where w is defined above, H₀ is Hubble's constant and Ω_m is the present value of the density of all the matter in the universe.

Author's example

In their paper, the authors consider a hypothetical example with w  = −1.5, H₀ = 70 km/s/Mpc, and Ω_m = 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. 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. 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, the scale factor will be infinity.

Observed universe

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 Ω_m.

According to the latest cosmological data available, the uncertainties are still too large to discriminate among the three cases w  < −1, w  = −1, and w  > −1.