11 March 2005
The discovery in the 1990s that there could be some kind of mysterious dark energy that is accelerating the expansion of the universe came from studies of supernovae billions of light years away. Now, it turns out that the evidence for dark energy was there in our cosmic backyard all along, and that astronomers could have discovered it nearly 30 years ago.
It has been known since the late 1920s that the universe is expanding and that this so-called Hubble expansion is the dominant factor controlling the speed of galaxies in the distant universe.
In 1972, Allan Sandage of the Carnegie Observatories in Pasadena, California, US, reasoned that in the nearby universe - where the expansion is at its slowest - the gravitational attraction between groups and clusters of galaxies should produce significant deviations in their velocities from the otherwise largely smooth speed of expansion.
These deviations are called "peculiar velocities", but Sandage pointed out that galaxies in our vicinity - those lying just beyond our "Local Group" of the Milky Way and its immediate neighbours - showed abnormally low peculiar velocities.
Now, a team of astronomers is claiming that this anomaly could have alerted cosmologists to the idea of dark energy way back then. In 1997, before dark energy was thought of, Fabio Governato of the University of Washington, Seattle, US, made a computer model of galaxy formation from shortly after the big bang to the present day. But this model failed to predict the observed low peculiar velocities for nearby galaxies.
"Islands in a sea"
Governato and his colleagues have now incorporated dark energy into their computer model, and find that it does indeed match extremely well the observed peculiar velocities for galaxies in regions resembling the Local Group and just beyond. "This proves that the galaxies are islands in a sea of dark energy," says Governato.
Not everyone is convinced by the study. Astronomer John Peacock of the Royal Observatory in Edinburgh, UK, says that to posit the presence of dark energy in the local universe: "You have to find a signature that cannot be confused with anything else. This is not it." According to Peacock, galaxies naturally form in regions where the gas has cooled and slowed down enough to form stars, and these regions have low peculiar velocities, which can explain the observations.
Governato's colleague Andrea Macciò of the University of Zurich in Switzerland agrees that low-density regions of the universe can give galaxies low peculiar velocities, but rejects the idea that this alone explains the observations. "Our analysis clearly shows that the measured velocities of galaxies beyond the Local Group are too low to be explained without dark energy," he says.
So could astronomers have used the observations to propose dark energy in the 1970s? Governato is not sure how far they could have got without today's computer power. Also, "even if someone had written a paper claiming dark energy back in the 1970s, I don't think anyone would have believed them," he says.