A MYSTERIOUS pattern seen in the cosmic microwave background 鈥� the faint afterglow of the big bang 鈥� has left some physicists wondering whether this central plank in the evidence for the big bang is somehow flawed. But now there may be a simpler explanation for the pattern: 鈥淚t is being caused by the gravity of a tremendous concentration of galaxies in our cosmic backyard,鈥� says Chris Vale of Fermilab in Chicago and the University of California, Berkeley.
Dubbed the 鈥渁xis of evil鈥� by cosmologist Jo茫o Magueijo of Imperial College London, the pattern appears in the map of the microwave backtround (CMB) built up by NASA鈥檚 Wilkinson Microwave Anisotropy Probe (WMAP). As part of their analysis, astronomers break up the subtle temperature variations in the CMB into components called the dipole, the quadrupole and the octupole (see Graphic), like breaking up an orchestral score into tunes played by different instruments. If the CMB really is the afterglow of the big bang, then the orientations of the hot and cold regions of the quadrupole and the octupole should be random. 鈥淏ut they are not,鈥� says Vale. 鈥淭he big surprise is they are aligned 鈥� along the axis of evil.鈥�
Magueijo and his colleagues have suggested there may be something wrong with the big bang model (New 杏吧原创, 2 July, p 30), but Vale鈥檚 idea is less radical. The axes of the quadrupole and the octupole lie in the same plane, he says, which is perpendicular to the direction of the dipole. 鈥淚t hints at a connection.鈥�
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Vale suspects the alignment is being caused by an enormous group of galaxies known as the Shapley supercluster, which lies about 450 million light years away and spans an area of sky at least 1000 times the apparent size of the full moon. The gravity of this supercluster could warp the CMB in such a way that some of the temperature variation in the dipole could 鈥渟pill over鈥� into the quadrupole and the octupole. 鈥淭he dipole variation is hundreds of times bigger than the quadrupole, so only a little need spill over,鈥� says Vale ().
聯Gravity from the enormous Shapley supercluster of galaxies could be warping the cosmic microwave background聰
To test his hunch, Vale developed a computer simulation that modelled the supercluster as a gigantic spherical mass, and found that he could replicate the apparent alignment of the quadrupole and octupole. He says that better observations of the supercluster鈥檚 mass distribution could help confirm the theory.
WMAP scientist Gary Hinshaw of NASA鈥檚 Goddard Space Flight Center in Greenbelt, Maryland, is impressed with Vale鈥檚 work. 鈥淗is toy model generates a good match to our quadrupole and octupole signal, which I think is remarkable.鈥� Hinshaw鈥檚 colleague and the principal investigator of WMAP, Charles Bennett, adds, 鈥淚t is not a crazy idea.鈥�
But if Vale鈥檚 idea is correct, it raises a new question. The measured quadrupole signal is already much smaller than expected by theory, and Vale鈥檚 mechanism would mean that some of that signal is actually spillover from the dipole. So the true quadrupole must be even smaller, and no one knows how that could have happened. 鈥淚 might have solved one problem but created another,鈥� admits Vale.
It is possible that some feature of the big bang may have suppressed the quadrupole signal. One such scenario is that the universe is a peculiar shape like a flat slab or a doughnut. 鈥淭hat way some of the sloshing motions of matter that caused the temperature variations in the cosmic background would not have been able to occur,鈥� says Vale.
