THE 21st century is bringing astronomers more questions than answers, as evidence grows that the Universe consists mainly of utterly baffling stuff called dark energy. Though some astronomers remain unconvinced, new studies of the afterglow of the big bang make the strongest case yet for this idea.
“We only understand 3 per cent of the Universe,” laments Keith Grainge of Cambridge University, one of those involved. That small fraction is the ordinary matter found in stars and galaxies. Some of the rest is dark matter, which we can’t see but know must be there because of its gravitational effect. But that’s not all.
A few years ago, studies of distant supernova explosions showed that not only is our Universe expanding, but that the expansion is accelerating (91av, 11 April 1998, p 26). Scientists concluded that something exerting a field like gravity—only in reverse—must dominate the Universe. They called it dark energy.
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Now completely independent measurements have arrived to support this idea. Anthony Readhead of Caltech in Pasadena and his colleagues used an array of microwave receivers high in the Chilean desert to look in detail at the radiation left over from the big bang. A second team from the universities of Cambridge and Manchester have made similar observations using telescopes on Mount Teide in Tenerife.
Variations, or ripples, in this radiation correspond to “lumpiness” in the early Universe and carry lots of information about the make-up of matter and whether space is curved or flat. Both teams deduced that space is flat: in other words, two light beams that start off parallel would stay that way. Taken with other observations, this is the most convincing evidence yet that dark energy does exist.
Despite some astronomers’ doubts (see “Resisting the dark side”), most physicists now accept that ordinary matter is just a drop in the ocean. Around 30 to 35 per cent of the Universe is dark matter, which astronomers can’t identify despite decades of trying. And a whopping 60 or 70 per cent is dark energy. “If anything, that’s even less well understood than the dark matter,” says Grainge. “It’s really very mysterious indeed.”