IT IS the kind of mystery that could keep a cosmologist up at night: the discovery of a non-random pattern in the structure of the universe. Now fresh evidence for an 鈥渁xis of asymmetry鈥�, along which many more spiral galaxies seem to spin one way than the other, threatens to undermine our understanding of the cosmos.
The first hint of such an axis emerged last year in a survey of about 15,000 spiral galaxies, which have a spin that is either left or right-handed (快猫短视频, 17聽October 2011, p聽44).
The new evidence comes from a survey with more than 10 times as many galaxies, stretching even deeper into space. It is the work of a different scientist using a different technique.
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Though the feeling among cosmologists not involved in the work tends to be that the asymmetry will reveal itself to be a statistical fluke, or an artefact of measuring techniques, the new analysis has raised the stakes.
鈥淚 am not going to abandon my impression that the universe does not have a handedness asymmetry, but I am going to be following this,鈥� says of Princeton University, a pioneer of the field of physical cosmology. 鈥淚t鈥檚 good to look outside the bounds of accepted ideas.鈥�
If the universe does contain such an axis, it would contradict our current view of the cosmos, which assumes that matter and energy are uniformly distributed throughout, and that the universe looks pretty much the same in all directions.
This view dates back to the 1920s, and is supported by both the near-uniform distribution of far-flung, radio wave-emitting galaxies 鈥� the only ones with radiation able to reach us from the distant universe 鈥� and the smoothness of the cosmic microwave background (CMB), the radiation left over from the big bang.
Tantalising hints that something might be amiss emerged last year. at the University of Michigan in Ann Arbor studied 15,158 spiral galaxies, extracted from a database of galaxies observed by the (SDSS), which scans the northern skies from a telescope at the Apache Point Observatory in Sunspot, New Mexico.
In most directions, Longo found an even spread of right and left-handed galaxies. But when he looked along a line about 25 degrees off from the direction of the Milky Way鈥檚 north pole 鈥� a cosmic reference point that sits directly above the centre of our galaxy 鈥� he found more left-handed spirals than right-handed ones ().
He worked manually, with the help of graduate students who inspected images of galaxies on a computer screen and classified their spin direction. The furthest of these galaxies were about 1.2 billion light years away.
Now, of the Lawrence Technological University at Southfield, Michigan, has automated the process and looked deeper into space. His software classified the handedness of almost 250,000 spiral galaxies up to 3.4 billion light years away, which were surveyed by SDSS and another project called the . The technique plots how the intensity of a galaxy鈥檚 light varies as you move away from its centre, resulting in a characteristic waxing and waning that reveals the galaxy鈥檚 handedness.
Like Longo, Shamir found more left-handed spirals than right-handed ones along a particular line (). This time, though, the axis of asymmetry pointed about 60 degrees to the other side of the Milky Way鈥檚 north pole (see diagram). Despite being separated by 85 degrees, both axes have such large uncertainties that they could be aspects of the same axis.
鈥淭he observation is so strange that it鈥檚 difficult to interpret its meaning,鈥� says Shamir. 鈥淎 pattern in the structure of the universe at such a large scale is not something that we expect to see.鈥�
鈥淎 pattern in the structure of the universe at such a large scale is not something we expected鈥�
Nature is no stranger to surprise asymmetries. Amino acids are more often left-handed than right, while all subatomic neutrinos that have been observed have a left-handed spin.
A similar bias among structures of cosmic proportions would have deep implications. For example, if more galaxies are spinning one way than the other, this implies that the universe has a net spin, or angular momentum, in a particular direction. Since angular momentum can neither be created or destroyed, the universe must have been come into existence in a spin. What set it spinning, though, and what is it spinning relative to? Such questions will need to be addressed if Longo and Shamir鈥檚 conclusions hold up.
鈥淭his implies a net spin for the universe 鈥� but what set it spinning, and what is it spinning relative to?鈥�
Given these profound implications, of Harvard University demands stronger evidence. 鈥淥n the face of it you think, 鈥榳ow, this is pretty weird. If this is true, it鈥檇 be hard to understand鈥�,鈥� he says. 鈥淛ust because something seems strange doesn鈥檛 mean it鈥檚 wrong, but it does mean that the threshold for convincing people is going to be pretty high.鈥�
Kirshner reckons that the observed asymmetry could be due to subtle side effects of how the SDSS telescope scans the sky, or collects the light, or how the information is read out from the camera 鈥� none of which has been analysed in Shamir鈥檚 work.
In 2005, an apparent pattern in an otherwise even spread of warm and cold spots in the CMB emerged and was dubbed the 鈥�axis of evil鈥�. But the team behind NASA鈥檚 WMAP telescope, which has created detailed images of the CMB, now say it is most likely a statistical fluke.
Shamir agrees that the way SDSS collects and processes the data, or his analysing software, could make the survey more sensitive to galaxies with a certain handedness, leading to an apparent excess of one type. But he points out that this bias should exist no matter which way he looks, whereas the asymmetry he detects is in only one direction.
Future galaxy surveys, such as ones that will be carried out by the in Chile, which will image 10 billion or so galaxies, should resolve the issue. 鈥淚t will provide an extremely large sample of clear, high-quality galaxy images,鈥� says Shamir. 鈥淭hat sample will provide a definite answer to the question.鈥�
