Originally posted on sciy.org by Ron Anastasia on Fri 09 Nov 2007 11:11 PM PST
Cosmic rays
They came from outer space
Nov 8th 2007
From The Economist print edition
A 40-year-old mystery is solved
SMALLER than an atom, they arrive with the energy of a tennis ball served by a champion. When they hit the atmosphere they create showers of daughter particles that zap mountaineers and people in aeroplanes. And no one knows where they come from—nor how, in apparent defiance of the laws of physics, they get to this planet in the first place.
Actually, that last sentence is no longer true. The super-particles in question are a particular type of high-energy cosmic ray and fittingly, given their extreme properties, their origin has now been worked out by a team of 444 researchers from 17 countries, using the biggest piece of scientific apparatus on Earth—the Pierre Auger observatory, which occupies 3,000 square kilometres of western Argentina.
Ordinary cosmic rays are puny things. Indeed, they are not really “cosmic†at all. They originate from various events (supernovae and so on) within the Milky Way galaxy that is home to the Earth. A few, however, are real whoppers—the products of events far more powerful than occur in the Milky Way. These are the tennis-ball equivalents and their existence is a puzzle.
Two puzzles, in fact. The first is: what created them? The second is: how did they get to Earth at such speed?
One hypothesis about their creation is that they are the result of stars being sucked into giant black holes. A second is that they come from colliding galaxies. A third is that they are caused by the collapse of massive but invisible relics from the beginning of the universe. All those events would be powerful enough, but all tend to happen a long way away. And that is where the second puzzle comes in.
In 1966 Kenneth Greisen, Vadim Kuzmin and Georgiy Zatsepin showed that high-energy charged particles (cosmic rays are mostly atomic nuclei, and thus positively charged) should be slowed by collisions with the photons of the cosmic microwave background (radiation left over from the Big Bang that permeates all space). This would bring them below a well-defined speed limit. Yet that limit is regularly exceeded. So, either the laws of physics are wrongly understood, or the super-rays are coming from close by, even if not from the Milky Way itself.
To find answers to these questions, the team trawled through the data that have accumulated since the Pierre Auger observatory began operating three years ago. The observatory, which is named after the physicist who discovered the showers caused by cosmic rays, has 1,600 detectors on the ground to record the arrival of such cascades and 24 telescopes pointing at the sky to locate the flashes of light produced by the collisions that create them.
So far, it has recorded a million or so showers. Around 80 of these were caused by cosmic rays more energetic than theory allows. The collaborators whittled these down to the 27 biggest events, so that there would be no ambiguity. They then decided to test the first, and easiest, hypothesis: that energetic cosmic rays are caused by hungry black holes. They did this by taking a peek at an astronomers' catalogue of 318 active galaxies located within 300m light years of Earth.
An active galaxy is a star system with a humongous black hole at its centre. Such black holes regularly chomp up stars, and that produces a lot of radiation. Only the 318 in question, however, are close enough to the Milky Way for the predicted galactic speed limit not to have been imposed (300m light years may sound a long way, but it is a short hop on the cosmic scale).
As the team report in this week's Science, all 27 of the cosmic rays they looked at did, indeed, come from the direction of galaxies in the catalogue. Both mysteries thus seem to have been solved. High-energy cosmic rays are caused by black holes consuming stars. And the laws of physics do not have to be torn up after all.
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