Some say the world will end in fire,
Some say in ice.
I say it will end in an infinite-density singularity driven by a scalar field with a negative kinetic term.
Who knew that Robert Frost had such a deep grasp of cosmology?
A couple of weeks ago, New Scientist asked me to comment on a recent paper about the Big Rip. What's that all about?
As the universe expands, all of the ordinary matter becomes less dense. That makes intuitive sense -- if you have a fixed amount of matter in an expanding box, then the amount of mass per unit volume has to go down. But in 1999, Rob Caldwell at Dartmouth made a radical suggestion: suppose that as the universe expanded, the density of the dominant form of energy increased instead of decreasing. This leads to very weird behavior -- the Universe enters a superaccelerated phase, with the expansion factor going to infinity at a finite time, at which point the equations break down at a singularity. It was later pointed out by Caldwell, Marc Kamionkowski, and Nevin Weinberg that as the universe approaches this singularity, it expands so rapidly that all of bound structures in the universe would be torn apart. First galaxies would dissolve, then the solar system would disintegrate, followed by the destruction of the earth, our bodies, and then the atoms in our bodies and the nuclei inside the atoms. And we would be dead by then. Caldwell, Kamionkowski, and Weinberg coined the term "Big Rip" to describe this fate for the universe.
So what was my contribution to this? Nothing, actually. I later worked with Paul Frampton and Kevin Ludwick to come up with the "Little Rip." In what sense is the Little Rip "littler" than the Big Rip?
To get a Little Rip, you still need a weird form of energy with a density that increases as the universe expands. But if the density increased slowly enough, the universe wouldn't tear itself apart at a finite time. Instead, the universe would continue to expand forever, and everything would eventually get ripped apart (including us), but we would never hit a singularity and the equations would never break down. That's right, as the expansion tore the universe apart, you could console yourself with the thought that, "Thank goodness, at least the equations aren't breaking down." To be honest, my chief contribution to the Little Rip was coming up with the name itself. Good marketing is everything.
Most scientists (including me) don't think that we inhabit a Big Rip/Little Rip type of universe. To make that happen, the exotic energy would have to violate something called the "weak energy condition", and when that happens, pretty much all hell breaks loose: you could make time machines, wormholes, and even reliable printers. (Just kidding, you can never make a reliable printer). So it's unlikely that the universe will eventually be destroyed in a Big Rip or a Little Rip.
You might think that science fiction would have exploited this idea to death, but it really hasn't. The one memorable story I can think of that's based on the Big Rip is Stephen Baxter's "Last Contact," which I discussed in this earlier post. I guess the paucity of stories on this topic is understandable -- if you're going to write about the Big Rip, there's only one possible ending, and it's not a happy one.