The large-scale structure of the Universe appears to be dominated by vast "hyperclusters" of galaxies, according to the new the Sloan Digital Sky Survey, compiled with a telescope at Apache Point, New Mexico. The survey plots the 2D positions of galaxies across a quarter of the sky. The science team has concluded that it could mean that gravity or dark energy – or something completely unknown – is behaving very strangely.
On even larger scales, though, cosmological models say that the expansion of the universe should trump the clumping effect of gravity. That means there should be very little structure on scales larger than a few hundred million light years across.
"Should be." But according to Shaun Thomas of University College London (UCL), and colleagues aggregations of galaxies stretching for more than 3 billion light years have been found. The hyperclusters are not very sharply defined, with only a couple of per cent variation in density from place to place, but even that density contrast is twice what the tandard cosmological models theory predict.
A 2D picture of the sky cannot reveal the true large-scale structure in the universe. To get the full picture, Thomas and his colleagues also used the color of galaxies recorded in the survey. More distant galaxies look redder than nearby ones because their light has been stretched to longer wavelengths while travelling through an expanding universe. By selecting a variety of bright, old elliptical galaxies whose natural colour is well known, the team calculated approximate distances to more than 700,000 objects. The upshot is a rough 3D map of one quadrant of the universe, showing the hazy outlines of some enormous structures.
Dark energy is usually assumed to be uniform across the cosmos. If instead it can pool in some areas, then its repulsive force could push away nearby matter, creating these giant patterns.
Alternatively, we may need to extend our understanding of gravity beyond Einstein's general theory of relativity. "It could be that we need an even more general theory to explain how gravity works on very large scales," says Thomas.
A more mundane answer might yet emerge. Using colour to find distance is very sensitive to observational error, says David Spergel of Princeton University. Dust and stars in our own galaxy could confuse the dataset, for example. Although the UCL team have run some checks for these sources of error, Thomas admits that the result might turn out to be the effect of foreground stars either masking or mimicking distant galaxies.
A comprehensive catalogue of spectra for Sloan galaxies is being assembled in a project called the Baryon Oscillation Spectroscopic Survey. Meanwhile, the Dark Energy Survey will use a telescope in Chile to measure the colors of even more galaxies than Sloan, beginning in October. Such maps might bring hyperclusters out of the haze – or consign them to the status of monstrous mirage.
The richest supercluster of galaxies in the nearby universe is the Shapley Concentration, made up of 25 separate clusters grouped together in one giant supercluster some 700 million light-years away and with a total mass of about 1016 (10,000 trillion) solar masses. Its gravitational attraction contributes perhaps 25% of the motion of the Local Group.
At the core of the Shapley Concentration is an extraordinary remarkable complex formed by several rich clusters of galaxies from the Abell catalog – A3558, A3562, and A3556 – and by the two minor groups SC 1327-312 and SC 1329-314. The central and most massive cluster is A3558, which is dominated by a cD galaxy.
Image at top of page showsthe Shapley Supercluster or Shapley Concentration (SCl 124) -- the largest concentration of galaxies in our nearby universe that forms a gravitationally interacting unit, thereby pulling itself together instead of expanding with the universe. It appears as a striking overdensity in the distribution of galaxies in the constellation of Centaurus, it is 650 million light years away. The image is a radio map of galaxies in the core of the Shapley Concentration.
The Daily Galaxy via newscientist.com and the Physical Review Letters, DOI: 10.1103/PhysRevLett.106.241301
Image credit: Credit: T. Venturi, S. Bardelli, R. Morganti, & R. W. Hunstead, Istituto di Radioastronomia.