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.”
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