by Amanda Gefter
25 June 2008
Is the matter in the universe arranged
in a fractal pattern? A new study of nearly a million galaxies
suggests it is - though there are no well-accepted theories to
explain why that would be so.
Cosmologists trying to reconstruct the entire history of the
universe have precious few clues from which to work. One key clue is
the distribution of matter throughout space, which has been sculpted
for nearly 14 billion years by the competing forces of gravity and
If there is a pattern in the sky, it
encodes the secrets of the universe.
A lot is at stake, and the matter
distribution has become a source of impassioned debate between those
who say the distribution is smooth and homogeneous and those who say
it is hierarchically structured and clumpy, like a fractal.
Nearly all physicists agree that on relatively small scales the
distribution is fractal-like: hundreds of billions of stars group
together to form galaxies, galaxies clump together to form clusters,
and clusters amass into superclusters.
The point of contention, however, is what happens at even larger
scales. According to most physicists, this Russian doll-style
clustering comes to an end and the universe, on large scales,
But a small team of physicists, including
Francesco Sylos Labini of the
Enrico Fermi Centre in Rome and Luciano Pietronero of
the University of Rome argue that the data shows the
opposite: the universe continues to look fractal as far out as our
telescopes can see.
The best data for looking at the galaxy distribution comes from the
Sloan Digital Sky Survey (SDSS),
which is constructing the largest 3D map of the universe. When
completed, it will map the positions of about a million galaxies and
When SDSS data was released in 2004, physicists David Hogg of
New York University and Daniel Eisenstein of the
University of Arizona, both in the US, published an
analysis of 55,000 luminous red galaxies
suggesting that the fractal pattern smoothed out at scales over 200
million light years.
But Sylos Labini and Pietronero were not convinced. They believed
that the apparent smoothing was an illusion caused by weak
statistics - the smoothing seemed to occur at the largest scales the
survey was capable of studying, where there were too few large
regions to be able to reliably compare their densities, they said.
Only a bigger map could resolve the
Now, SDSS has released its sixth round of data, which plots the
locations of roughly 800,000 galaxies and 100,000 quasars, bright
objects powered by violent supermassive
The Sloan Digital Sky Survey
According to their latest paper, which has been submitted to
Nature Physics, Sylos Labini
and Pietronero, along with physicists Nikolay Vasilyev and
Yurij Baryshev of St Petersburg State University in
Russia, argue that the new data shows that the galaxies exhibit an
explicitly fractal pattern up to a scale of about 100 million
And they say if the universe does become homogeneous at some point,
it has to be on a scale larger than a staggering 300 million light
years across. That's because even at that scale, they still observe
large fluctuations - a cluster here, a void there - in the matter
Most cosmologists interpret such fluctuations as being no more
significant than small waves on the surface of the sea, but Sylos
Labini and colleagues say that these are more like tsunamis.
Many cosmologists find fault with their analysis, largely because a
fractal matter distribution out to such huge scales undermines the
standard model of cosmology. According to the accepted story of
cosmic evolution, there simply hasn't been enough time since the big
bang nearly 14 billion years ago for gravity to build up such large
What's more, the assumption that the distribution is homogeneous has
allowed cosmologists to model the universe fairly simply using
Einstein's theory of general relativity
- which relates the shape of space to the distribution of matter.
Modeling a fractal universe with general relativity is
possible in theory, but in reality it would be devilishly
complicated. That would leave cosmologists without a working model,
like acrobats without a net.
To support the homogeneity assumption, cosmologists point to the
smoothness of the cosmic microwave background (CMB),
relic radiation from the nascent universe.
The CMB is perfectly uniform up to one
part in 100,000, suggesting the early universe was nearly
"The standard picture of a
homogeneous universe on large scales is holding up very well
when tested with very large-scale observations like those
mapping the cosmic background radiation, X-rays and radio
galaxies," says physicist
Neil Turok of Cambridge
University in the UK.
"If the observations of galaxies in optical surveys don't agree,
there may be a number of possible explanations, without
resorting to an extremely inhomogeneous, fractal universe," he
told New Scientist.
But inferring the matter distribution from the CMB is not always
CMB maps show a 3D distribution
projected onto a 2D surface, and it is possible for a clumpy 3D
distribution to appear smooth when projected in 2D. The same is true
of the X-ray background, which appears homogeneous in two
dimensions. Finally, using galaxies that are bright at radio
wavelengths is also problematic, as it is
difficult to measure their distances
accurately enough to pinpoint their positions in 3D.
So what could produce such a fractal pattern in galaxy surveys like
Sloan? Some of the clumpiness may be a sort of optical illusion
known as the Bull's-eye effect, says Adrian Melott of
the University of Kansas in the US.
That's because nearby galaxies fall towards each other due to their
mutual gravitational attraction - even as space itself expands.
That movement can enhance the apparent
clumpiness of matter in surveys like Sloan, since those surveys rely
on measurements of the galaxies' velocities to determine their
distance from Earth.
But according to their paper, Sylos Labini's team says the
Bull's-eye effect is only relevant on very small scales, about
16 million light years and below, and has no influence on the
clumpiness at the large scales in question.
Melott disagrees, saying it should magnify clumpiness at any scale.
But he adds that the effect only "enhances structures that [already]
What's at stake if the universe is indeed a fractal on the largest
scales? Besides a radical rethink of the laws and history of the
cosmos, researchers have placed something more down-to-Earth on the
More than a decade ago, Sylos Labini and Pietronero wagered a bet
with physicist Marc Davis of the University of California,
Berkeley, US. The bet, refereed by Turok, held that if the galaxy
distribution turned out to be fractal beyond scales of approximately
50 million light years, Davis would owe Sylos Labini and
Pietronero a case of California wine.
Should the fractal pattern begin to disintegrate at scales less than
50 million light years, Davis would receive a case of Italian wine -
which some would say is a better deal.
Turok has yet to declare a winner.