Dark matter’s lens


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.

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galaxies cluster


Updating Einstein’s Universe

Galaxies cluster in Einstein’s way

To prove Albert Einstein wrong and so share a little of his glory has been a goal for generations of physicists and astronomers. But the findings of X-ray astronomy help to show that his theory of gravity, called General Relativity or GR for short, remains stubbornly resistant to detectable error 95 years after Einstein promulgated it. In a report soon to be published in Monthly Notices of the Royal Astronomical Society, David Rapetti and colleagues at Stanford and Honolulu say, “Our results represent the most robust consistency test of General Relativity on cosmological scales to date.”

X-rays from galaxy cluster Abell 3376, 600 million light-years away in the Columba constellation. Chandra (NASA/CXC/SAO/A.Vikhlinin) and ROSAT

For seeing whether Einstein’s writ runs reliably throughout known space and time, clusters of galaxies can serve like the standard weights and measures used to check a shopkeeper’s scales. Bound together by gravity as the largest objects in the Universe, galaxy clusters fill the chasm between local distances (Earth, Solar System, Milky Way Galaxy and its neighbours) and the microwave background radiation from the edge of the observable cosmos. By visible light a galaxy cluster resembles a swarm of flies, but to X-ray telescopes in space it looks like a big balloon. That’s because a very hot gas cloud, more massive than all the galaxies put together, fills the space between them.

Theorists can reckon how big the clusters ought to be, and how they should grow over time, according to various theories of gravity, and check the expectations against the X-ray observations. Rapetti and Co. used results from NASA’s Chandra X-ray Observatory to enhance the data on 238 clusters of galaxies seen by Germany’s Rosat X-ray satellite, which ceased operating in 1999. The aim was to gauge how quickly the galaxy clusters grew over cosmic time. If a rival theory called DGP were right, gravity should leak away into some other cosmic dimension and the growth of the clusters would be slowed. It wasn’t.

Another recent “X-ray test” for General Relativity also uses observations of galaxy clusters by Chandra and Rosat – in this case 49 relatively close ones. Fabian Schmidt of the California Institute of Technology and his colleagues found that the cluster masses were too low to fit another theory, called f(R), but appropriate to Einstein’s theory.

Prompting these tests is one of the biggest issues in cosmology. Since 1998, when astronomers unexpectedly found that the expansion of the Universe is accelerating, theorists have been divided about whether or not to accept the Einsteinian view of the matter. The possibility of acceleration always lurked in a “cosmological constant” that Einstein introduced into his 1917 equation describing the Universe as a whole. But its implementation requires a huge invisible driver called Dark Energy. The other theories mentioned here, DGP and f(R), are among the attempts to do without Dark Energy by revising the theory of gravity – by inventing, in effect, a new kind of force in Nature.

Having seen off one of the possibilities, Schmidt and his colleagues write that “The abundance of galaxy clusters promises to be a good probe of other modified gravity scenarios as well.” Meanwhile, Uncle Albert scores twice. The way the clustering galaxies behave fits both his theory of gravity and his cosmological constant very nicely, thank you.


D. Rapetti et al., Mon. Not. R. Astron. Soc. in press, released online 13 April 2010. Text available at: http://arxiv.org/PS_cache/arxiv/pdf/0911/0911.1787v2.pdf

F. Schmidt et al., Phys Rev D, 80, 083505, 2009