Dark Energy and the Fate of the Universe
For millennia, human beings have been pondering two ultimate questions: "Where do we come from?" and "Where are we going?" Over that time, these questions have spurred theological and philosophical debate. Thanks to the rapid development of modern cosmology in the past three decades, scientists nowadays have obtained some important clues to answer these questions. The standard "inflation + hot big bang" framework has been developed to explain the origin of the Universe. However, to forecast the destiny of the Universe, researchers have realized that the nature of dark energy is key.
In the absence of a consensus on what dark energy is, a phenomenological description of the equation-of-state parameter w—the ratio of pressure and density of dark energy—provides an important means for investigating dark energy dynamics. Properties of dark energy will decide the ultimate fate of the Universe. In particular, if w<-1 at some time in the future, dark energy density will grow to infinity in finite time, and its gravitational repulsion will tear apart all the objects in the Universe. This "big rip" (or "cosmic doomsday") scenario is the major focus of the paper. "We want to infer from the current data what the worst fate would be for the Universe", said the authors.
To foresee that fate, it is important to choose an appropriate parameterization that covers the overall expansion history of the Universe. The most popular Chevallier-Polarski-Linder (CPL) parameterization, in fact, is not suitable in predicting the future evolution of the Universe because in this form w will diverge when the redshift parameter approaches -1. Thus, the authors invoke a divergence-free parameterization, called the Ma-Zhang (MZ) parameterization, to predict the evolution of the Universe.
Thus the constrained parameter space indicates that it is very likely that in the future w<-1. If so, one may ask another interesting question: "How about the destinies of the gravitationally bound objects in the Universe, such as galaxies and stars?" In fact, if w indeed ever becomes less than -1, dark energy's gravitational repulsion will continuously increase until it overcomes all forces holding objects together and all objects will be torn apart. No object would escape this fate, but obviously systems more tightly bound would exist for longer. Using the MZ parameterization, the authors speculated on a series of possible consequences before the cosmic doomsday. For example, for the worst situation, namely the 95.4% CL lower limit, the Milky Way will be torn apart 32.9 Myr before the big rip; two months before doomsday, the Earth will be ripped from the Sun; five days before the doomsday, the moon will be ripped from the Earth; the Sun will be destroyed 28 min before the end of time; and 16 min before the end, the Earth will explode.
However, from what we already know of the dynamical properties of dark energy, one thing is all very clear, we still have a very long future ahead.