Space is festooned with vast "hyperclusters" of galaxies, a new cosmic map suggests. It could mean that gravity or dark energy – or perhaps something completely unknown – is behaving very strangely indeed.
Galaxies, clusters, and superclusters - mere local details?
We know that the universe was smooth just after its birth. Measurements of the cosmic microwave background radiation (CMB), the light emitted 370,000 light years after the big bang, reveal only very slight variations in density from place to place. Gravity then took hold and amplified these variations into today's galaxies and galaxy clusters, which in turn are arranged into big strings and knots called superclusters, with relatively empty voids in between.
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.
But the universe, it seems, did not get the memo. Shaun Thomas of University College London (UCL), and colleagues have found aggregations of galaxies stretching for more than 3 billion light years. 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 theory predicts.
"This is a challenging result for the standard cosmological models," says Francesco Sylos Labini of the University of Rome, Italy, who was not involved in the work.
Colour guide
The clumpiness emerges from an enormous catalogue of galaxies called 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. "Before this survey people were looking at smaller areas," says Thomas. "As you look at more of the sky, you start to see larger structures."
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 colour 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.
Coagulating dark energy
The result hints at some profound new physical phenomenon, perhaps involving dark energy – the mysterious entity that is accelerating the expansion of space. 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.
Fractal structure?
"It will be essential to confirm this with another technique," says Spergel. The best solution would be to get detailed spectra of a large number of galaxies. Researchers would be able to work out their distances from Earth much more precisely, since they would know how much their light has been stretched, or red-shifted, by the expansion of space.
Sylos Labini has made such a map using a subset of Sloan data. It reveals clumpiness on unexpectedly large scales – though not as vast as these. He believes that the universe may have a fractal structure, looking similar at all scales.
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 colours 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.
Source New Scientist
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