Largest cosmic structures 'too big' for theories

by Stephen Battersby
New Scientist 21 June 2011http://www.newscientist.com/article/dn20597-largest-cosmic-structures-too-big-for-theories.html
 
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.
 
We know that the universe was smooth just after its birth.
 
Measurements of the cosmic microwave background radiation (CMB),
the light emitted 370,000 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.
 
Journal reference: Physical Review Letters, DOI: 10.1103/PhysRevLett.106.241301