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Wednesday 1 July 2009
The Extinction Oscillator

Using the revised timescales and Fourier analysis, Rohde and Muller looked for a periodic signal in the history of biodiversity. They began by subtracting out biodiversity’s long-term growth—a vital step if you want to find any short-term signal (the wiggles) superimposed upon the rising curve. They were looking for evidence of a 26-millionyear cycle that had been hinted at in the 1980s; the strong peak in their power spectrum indicating a 62-million-year cycle was a surprise. Using the same data, Bruce Lieberman and I checked their results. We estimated the 62-million-year peak had a 1 in 100 probability of arising through random chance. Then, collaborating with paleobiologist Richard Bambach, we found evidence of the same cycle in three more data sets.

Back to my sleepless nights. When a long, nearly regular cycle is found, an astronomical event or interaction may be the source, because orbits under gravity usually maintain regular rhythms for very long times. Some cycles of the Earth’s motion around the Sun are already known. But these have periods of hundreds of thousands of years, possibly a million or two, and no more. Nothing known in the motion of the Earth itself can make a 62-million- year cycle. Further, the laws of celestial mechanics rule out any object orbiting the Sun with such a long period; it would be so distant that the gravity of other stars would pull it away. But other astronomical cycles are still in play.

It takes about 200 million years for the Sun to complete one orbit around the center of our Milky Way galaxy. Moreover, the galaxy is a thin disk, and there is also a motion along a vertical direction. As our solar system slowly orbits the Milky Way’s center, it oscillates through the galactic plane with a period of around 65 million years. When we move up in the disk, we are pulled back down by gravity, coasting past the midpoint, then rising back up again, akin to a weight bobbing up and down on a spring.

Was this the missing mechanism? In fact, Rohde and Muller had considered this and dismissed it, for the same reason almost anyone would: One would think that any effect would occur when we passed through the disk of the galaxy, or perhaps when we got very far away from it. But that would happen twice per cycle, every 30 million years or so, which doesn’t explain the 62-million-year signal.

Trying to understand all this, I did something that in retrospect is fairly obvious: I looked at the phase. That is, how did the cycles of biodiversity and the Sun’s bobbing motion correspond? People had already computed the history of the Sun’s galactic orbit. It turns out that the biodiversity minima of the 62-million- year cycle happens when the Sun is “bobbed up” on only one side of the galaxy, when the solar system is on the disk’s upper, “north” side. So I visited my colleague Barbara Anthony-Twarog in the office next door. She has a beach ball painted with constellations, the Milky Way, and astronomical coordinate systems. It confirmed what I recalled: The galaxy’s north side lies toward the constellation Virgo, as well as the largest concentration of mass in our neighborhood, the Local Supercluster some 60 million light-years away. This supercluster is so massive that its gravity pulls our galaxy toward it at a velocity of about 200 kilometers per second.

This realization was the key for what follows, which I developed with my collaborator Mikhail Medvedev. The space between galaxies is not empty. It’s actually full of rarefied hot gas. As our galaxy falls into the Local Supercluster, it should disturb this gas and create a shock wave, like the bow shock of a jet plane. Shocks in hot gas at such high speeds generate cascades of high-energy subatomic particles and radiation called “cosmic rays.” These should be showering the north side of the galaxy’s disk. We are protected by the galactic magnetic field, much as the Earth’s magnetic field protects our planet. When we rise to the north side, we are less protected—and the ensuing flux of cosmic rays contains particles of such energy that they can reach the Earth’s surface.

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