Astronomers are used to the notion that the universe is expanding. But the recently announced discovery that our corner of the cosmos is going off on an excursion of its own came as a surprise. Superficially, at least, it runs counter to one of the current basic concepts of how the universe is organized -- the so-called cosmological principle. This holds that, by and large, the universe is homogeneous and looks the same when viewed in any direction. In other words, relatively small-scale clumpiness -- such as the concentration of matter in galaxies and clusters of galaxies -- smooths out when you consider the universe as a whole.
Thus astronomers expect to find the universe generally expanding uniformly, even though individual galaxies or galaxy clusters may head off in directions of their own. However, a cosmic volume 350 million light-years across is a substantial portion of the 10 billion to 20 billion-light-year universe. Astronomers would not expect the galaxies distributed over that large a volume to share an average motion different from that of the general expansion. Yet this is exactly what a team of seven American and British astronomers has found.
According to an announcement released last week by the US National Optical Astronomy Observatories (NOAO), those galaxies -- including our own Milky Way -- are off on a group excursion at a rate of 600 to 700 kilometers a second. They're heading toward the Southern Cross at around a million miles an hour beyond the velocity attributed to cosmic expansion.
The team that made this discovery includes David Burstein of Arizona State University, Roger Davies of NOAO, Alan Dressler of the Carnegie Institution of Washington, Dandra Faber of Lick Observatory, Gary Wegner of Dartmouth College, and British astronomers Donald Lynden-Bell of Cambridge University and Roberto Terlevich of the Royal Greenwich Observatory. They don't know why this vast collection of galaxies is headingoff toward the Southern Cross. They note that a huge and so far undetected concentration of mass may pull in that direction. But this is only speculation.
Their finding does put our galaxy's motion into new perspective. Astronomers had thought that two relatively nearby galaxy concentrations, called the Virgo and Hydra-Centaurus Superclusters, were drawing us toward them. Now it seems we're all moving together toward the Southern Cross.
Such a large-scale group motion, if confirmed by other observers, would seem to strain the cosmological principle. The strain would be even greater if a vast, previously unsuspected mass were found to be causing that motion.
There's nothing sacred about principles like this. In the absence of contrary evidence, cosmologists, like other scientists, go for the simplest concepts available. This includes assuming that the physical laws we have developed on Earth hold true throughout the universe and throughout time, although theorists sometimes speculate about different laws holding in the past.
Cosmologists won't abandon these comfortable assumptions easily. Recent studies that suggest the cosmos has a bubble-bath structure, with galaxies lying on the surface of voids, also appears to challenge cosmic uniformity. Yet this could be an illusion. David H. Politzer and John P. Preskill of the California Institute of Technology have shown statistically that a random distribution of galaxy groupings could appear bubble-like just by chance.
It will take much more study to put all of these findings, including the anomalous motion, into sound cosmic perspective. But they do warn experts not to take the cosmological principle for granted.
Meanwhile, there's new support for assuming the laws of physics have operated unchanged for billions of years. A team from the (US) Lawrence Livermore National Laboratory, the National Bureau of Standards, and the British Geological Survey has made the most accurate study to date of the radioactive decay rate of the element rhenium 187. Geochemists and cosmologists use this element as a cosmic chronometer. It decays at a known rate into the element osmium 187. They can tell the age of material that originally contained osmium-free rhenium by the amount of osmium that has accumulated.
If the physical law governing such radioactive decay varied with time, this could upset the dating. The relevant factor in this law is a physical quantity called the fine-structure constant. The team reports in Nature that the mathematical value of that constant must have changed less than one part in 100,000 over the past 4.5 billion years.
That's important to cosmologists who like to keep their assumptions tidy.
Robert C. Cowen is the Monitor's natural science editor.