AN ASTROPHYSICS experiment designed to tell us about the cosmic microwave
background may scupper one of the central tenets of modern cosmology鈥攖hat
the Universe is crammed full of dark matter.
The Boomerang experiment surveyed the cosmos from a balloon above Antarctica,
and its results reported earlier this year provided the most detailed map yet of
the microwave background鈥攑hotons that echo the big bang. The conclusion
was that the Universe is 鈥渇lat鈥�: finely balanced between expanding forever or
collapsing back into a 鈥渂ig crunch鈥�. But the Boomerang data may be telling us
something else too.
Maps of the background show 鈥渂lobs鈥� where the temperature of the radiation
varies slightly from the average. The size of the blobs reflects how long it
took photons to cross the early Universe, and how much matter was around
then.
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Analysing the Boomerang data produced a chart of blob sizes that peaked at
just the right size for the Universe to be flat. But astronomers were surprised
by the absence of a second peak, predicted by the theory of dark matter. Many
astronomers believe that, for galaxies to behave as they do, more than 90 per
cent of the mass of the Universe must be dark matter. It should cause clumping
in normal matter, producing more peaks in the Boomerang data.
But Stacy McGaugh of the University of Maryland has an explanation. His
studies of dim galaxies thought to contain a lot of dark matter convinced him
that they were better explained by a modification to the law of gravity called
Modified Newtonian Dynamics (MOND). Last year he predicted what would happen to
the microwave background if there wasn鈥檛 any dark matter. 鈥淚t turns out my model
does fit the [Boomerang] data,鈥� he says.
Other astronomers admit the standard model of the Universe needs changing.
But if there is no dark matter, 鈥渨e鈥檒l need more normal matter in the early
Universe to provide drag on the photons to smooth out the spectrum,鈥� says Max
Tegmark at the University of Pennsylvania, 鈥渁nd that might contradict our theory
of . . . how matter forms after the big bang.鈥�