Cosmic rays, the high energy protons and helium nuclei that constantly bombard the Earth, have puzzled astronomers for the best part of one hundred years. Where do they come from and how are they accelerated to energies in excess of 10^20 eV—that’s about the energy that Roger Federer gives a tennis ball during a serve? (By contrast, the  Large Hadron Collider will be able to accelerate protons to a mere  10^12 eV.)

To tackle these questions, astronomers have built a giant cosmic ray telescope about the size of Rhode Island in Argentina. It’s called the Pierre Auger telescope and in the short time it has been operating, it is already challenging astronomers’ views about the origin of cosmic rays. In particular, it’s beginning to look as if the highest energy comsic rays come from active galactic nuclei.

Serguei Vorobiov from University of Nova Gorica in Slovenia summarises the highlights. Worth a read if you want to get up to speed on a new generation of astronomy.

Ref: arxiv.org/abs/0811.0752: The Pierre Auger Observatory — a New Stage in the Study of the Ultra-High Energy Cosmic Rays

One of the first tests of Einstein’s theory of general relativity was to explain the precession of the perihelion of Mercury, which had long bamboozled astronomers. Newton’s law of gravity simply cannot account for it. But relativity does.

Now it’s Saturn’s turn to flummox astrophysicists. The Russian astronomer Elean Pitjeva, who heads the Laboratory of Ephemeris Astronomy at the Institute of Applied Astronomy in St Petersburg, has analysed a huge data set of planetary observations dating back to 1913, including 3D observations of the Cassini spacecraft now orbiting Saturn.

She says that the precession of Saturn’s perihileon, as predicted by general relativity, needs to be corrected to fit the data. The correction is tiny: -0.006 arcseconds per century.

That’s an astonishing claim but perhaps not surprising given the growing body of evidence that some kind of correction to gravity is needed to explain various puzzling phenomena such as the Pioneer and Flyby anomalies.

Obviously Pitjeva’s work needs to be independently verified but already the astronomy-mill is hard at work guessing what might cause the deviation from Einsteinian physics.

It’s possible that known physics will do the trick: for example, our knowledge of trans-neptunian objects may have enough uncertainty to allow for this kind of correction.

Lorenzo Iorio at the National Institute of Nuclear Physics in Pisa Italy, outlines various explanations of known physics:

Our knowledge of trans-neptunian objects may have enough uncertainty to allow for this kind of correction but this turns out to generate a prograde precession no the retrograde precession found by Pitjeva

The Lense-Thirring effect generates a force that is four orders of magnitude too small to account for the difference

Mutual cancellations among unmodelled or mismodelled effects may have conspired to cause the effect but Iorio says this looks exceedingly unlikely

Neither do various exotic modifications of gravity or the DGP braneworld model explain the figures, says Iorio

So what’s left? A magnificent conundrum for astronomers to puzzle over until they get better data and/or a new theory of gravity that explains all.

Ref: arxiv.org/abs/0811.0756: On the Recently Determined Anomalous Perihelion Precession of Saturn