Updating Magic Universe
Dark matter’s lens on the cosmic scenery
Since 1996 the efforts of the French astrophysicist Jean-Paul Kneib to exploit natural lenses in the sky, created by the dark matter that surrounds clusters of galaxies, have fascinated me. While other stargazers used the “gravitational lenses”, bending light in the Einsteinian manner, to see galaxies far beyond the range of unaided telescopes, Kneib’s aim was to chart the mysterious dark matter itself. He wanted to see how visible matter and the far weightier dark matter have interacted through cosmic time – to see “the whole history of the Universe from start to finish”, as Kneib remarked to me in 2002.
It’s been taxing work, but now Kneib is one of the team reporting in today’s Science magazine about the dark matter around one the richest known clusters of galaxies. Abell 1689 lies 2.2 billion light-years away in the Virgo constellation, and a couple of years ago its extraordinary lensing power revealed a very distant and early object in the sky, Galaxy A1689-zD1, 12.8 billion light-years away. But that’s by the way
The new report not only gauges the cluster’s dark matter but uses the galaxies beyond it to infer the overall nature of space-time itself, dominated by the even more massive dark energy that drives the accelerating expansion of the Universe.
In Magic Universe, much of the story called “Dark matter: a wind of wimps or the machinations of machos?” was concerned with unsuccessful efforts to settle the question of whether exotic particles (wimps) or bodies made of ordinary matter too faint to see (machos) predominated in dark matter. But it concluded with Kneib’s work, as in this extract.
Part of the scenery
With the negative results concerning cluster evolution, and all the uncertainties about wimps versus machos, cosmologists entered the 21st Century like zookeepers unable to say whether the creatures in their care were beetles or whales. Recall that the dark matter outweighs the ordinary matter of galaxies and clusters by about ten to one. A vote might have shown a majority of experts favouring wimps, but issues in science are not settled that way.
In consolation, astronomers had a new way of picturing the dark matter. Its prettiest effects come in gravitational lenses. The unseen mass concentrated in a cluster of galaxies bends light like a crude lens, producing distorted, streaked and often multiple images of more distant galaxies. This makes a natural telescope that helps to extend the range of manmade telescopes. At first glance the distortions of the images seem regrettable — until you realise that they can be interpreted to reveal the arrangement in space of the dark matter.
The young French pioneer of this technique was Jean-Paul Kneib of the Observatoire Midi-Pyrénées. In the mid-1990s, with colleagues at Cambridge, he obtained extraordinary pictures of galaxy clusters with the Hubble Space Telescope, showing dozens of gravitationally imaged features. Kneib used them to work out the shape and power of the natural lens. He deduced the magnification of the background galaxies, and charted the dark matter in the foreground cluster.
Thus dark matter became part of the scenery, along with radio galaxies, or gas masses detected by X-rays. The analysis was indifferent to the nature of the dark matter, because it measured only its gravitational and tidal action. The Hubble Space Telescope’s successor, the NASA-Europe-Canada James Webb Telescope (2010) using infrared light, would amplify the power of the technique amazingly, in Kneib’s opinion.
‘By studying clusters of galaxies at different distances,’ he said, ‘and probing the most distant ones with infrared detectors, we’ll explore the relationship between visible and dark matter, and how it has evolved. We’ll see the whole history of the Universe from start to finish.’
Update for Magic Universe
A minor point is that the James Webb Telescope is now scheduled for launch in 2014. There’ll also be a need to cross-relate this update to another story in Magic Universe, called “Dark energy: revealing the power of an accelerating Universe”. But here’s the main addition.
Kneib’s dream began to come true by 2010, when he was based at the Laboratoire d’Astrophysique de Marseille and working with colleagues in France, the USA and the UK. They used intricate methods to analyse the dark matter in the rich galactic cluster Abell 1689 – methods so subtle that they could use the distant galaxies seen in the lens to deduce the evolving shape of the Universe. Team member Priyamvada Natarajan of Yale explained how the trick was done.
“The precise effects of lensing depend on the mass of the lens, the structure of space-time, and the relative distance between us, the lens and the distant object behind it. It’s like a magnifying glass, where the image you get depends on the shape of the lens and how far you hold it from the object you’re looking at. If you know the shape of the lens and the image you get, you can work out the path that light followed between the object and your eye.”
The results were in good agreement with what cosmologists had come to expect, but the new technique was gratifying. Kneib commented: “Using our unique method in conjunction with others, we were able to come up with results that were far more precise than any achieved before.”
Eric Jullo, Priyamvada Natarajan, Jean-Paul Kneib, Anson D’Aloisio, Marceau Limousin, Johan Richard and Carlo Schimd, “Cosmological Constraints from Strong Gravitational Lensing in Clusters of Galaxies”, Science, Vol. 291, pp. 924-7, 2010 Preprint available at http://www.spacetelescope.org/static/archives/releases/science_papers/heic1014.pdf
Hubble Europe press release at http://www.spacetelescope.org/news/heic1014/
N. Calder, Magic Universe, p. 192, Oxford UP, 2003