Motus Mentis

Here’s an item that I am trying to get my head around: According to Raphael Bousso at the University of California, there are physical constraints that make an eternally expanding universe impermissible.

Cosmologists have been bickering over the fate of the universe since its expansion was confirmed by Edwin Hubble back in 1929. On the assumption that the forces at work were purely gravitational, boffins worked for decades totting up all the mass they could find, which total they then used as a term in an equation also containing ever-more-accurate values of the Hubble constant (the current rate of expansion). There were three possible outcomes: a) the amount of mass was not enough to slow the universe’s expansion, so it would expand forever; b) the amount of mass exactly balanced the outward momentum, so the rate of expansion would asymptotically approach zero; or c) there was so much mass that the expansion would eventually stop, reverse itself, and end with a Big Crunch (possibly followed by a new Big Bang).

(These scenarios, by the way, described different spacetime geometries. In model a), the curvature of space is considered to be negative, like a saddle; in c) it is positive, like a sphere; and in b) it is flat.)

For a long time cosmologists had a sneaking suspicion that the answer would turn out to be b): that the total mass in the universe exactly balanced the momentum of its expansion. But in the late 1990’s everyone was completely gobsmacked by the discovery that the expansion of the universe seemed not to be slowing as expected, but was actually accelerating, for reasons that are still far from clear.

And that’s how matters have stood since then: scenario a) was declared the winner, and the universe would just go on and on, its constituent parts spreading farther and farther apart, growing colder and colder and darker and darker. Eventually the stars would die, and there would only be blackness everywhere as their lifeless cinders drifted through the void, forever.

Nice, right? Sounds like a nice quiet way to wrap things up, after all we’ve been through. Well, don’t get your hopes up, because according to Dr. Bousso and his chums, there are good reasons why you can’t have an eternal universe, no matter how dull. Instead, he says, something has to come along and bring down the curtain, and his team has estimated that it is actually going to happen pretty soon: in about 3.7 billion years, that is, or roughly the same amount of time the Earth has been around.

This doesn’t mean you can keep your library books, and I don’t think even Outlook 2010 lets you make calendar entries that far in advance, but it is still rather a shock.

Bad news for Cubs fans, too.