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Thursday, Jan. 8, 2004
Shedding light on dark matter
When science is more imaginative than science fiction
By ROWAN HOOPER
These days, you never hear people complaining that science destroys the wonder of the world. They wouldn't dare. For a beautiful example, look at what was discovered last year. A satellite -- the Wilkinson Microwave Anisotropy Probe (WMAP) -- confirmed one of the strangest, most wondrous proposals about the universe. It is this: that all the matter we can see, the matter that makes you and me, everything on Earth, all the planets and all the stars, all that matter accounts for only 4 percent of the matter in the universe.
Where is the rest? Why can't we see it? WMAP confirmed that 23 percent of the universe is made of "dark matter," which astrophysicists believe is made of an unknown particle. The remaining 73 percent is made of a mysterious "dark energy." Moreover, the dark matter of the universe is being stretched apart by the force of the dark energy.
In short, our universe is dark, and it is expanding. The discoveries were awarded top spot in Science magazine's Breakthroughs of the Year, 2003.
We know that the universe is expanding through observations of light from distant galaxies. This is the main evidence for the idea of the Big Bang. Information from WMAP and from telescopes being used in the Sloan Digital Sky Survey (a ground-based survey that will ultimately map a quarter of the sky) have confirmed the rate of the expansion, and with this data, scientists calculated back to when the universe wasn't expanding -- to the moment it came into existence. WMAP's data showed that this occurred 13.7 billion years ago.
The problem facing cosmologists has been that their calculations for the expansion of the universe demand more matter than they can detect. We can see billions of stars in countless galaxies, but it's not enough. It's not enough by a long way. Cosmologists concluded that there must be missing matter, and lots of it. Science writers Mary and John Gribbin put it like this in their book, "The Science of Philip Pullman's 'His Dark Materials' ":
"There must be at least five times as much extra material, not made of atoms at all, out there. It cannot be made of atoms, so it must be made of some sort of particles never yet detected on Earth. And it cannot shine, or we would see it. It is dark material."
The idea of dark matter has been around for many years, but it wasn't until 1998 that the theory of dark energy was proposed. This was when physicists determined that the expansion of the universe was accelerating. The discovery challenged the prevailing view that the universe would end when expansion slowed down and eventually reversed. The idea was that the deflated and compacted universe would end in a "Big Crunch," the opposite of how it started. But when it was found that the expansion was speeding up, dark energy was proposed to drive the expansion, to counter the effects of gravity. It is this strange idea that has now been confirmed.
"The implications of these discoveries about the universe are truly stunning," said Don Kennedy, editor of Science. "Cosmologists have been trying for years to confirm the hypothesis of a dark universe."
It was one of the reasons NASA launched the WMAP satellite in 2001. The probe measures the "cosmic microwave background," the faint afterglow of radiation still lingering from the colossal explosion that was the Big Bang. The microwaves come in ripples, and the size of the ripples reveal information about the beginning of the universe, and about its current size.
WMAP observations suggest that the universe is not infinite. It is probably "only" 70 billion light years across, cosmologists said last year. This might be big, but it's not infinite, and if confirmed this would resolve many age-old philosophical puzzles about the universe. For example, some philosophers worry that in an infinite universe, everyone on Earth has an infinite number of alien doubles leading parallel lives. "If we could prove that the universe was finite and small, that would be Earth-shattering," said David Spergel of Princeton University, N.J. And it would smooth those philosophers' furrowed brows (leaving them free to fret about something else).
Until recently the age of the universe was given at anything between 10 and 20 billion years. WMAP's unprecedented survey of the temperature of the ripples and bumps of microwaves in the early universe has pinned the age down to 13.7 billion years.
Only 150 years ago, Darwin's theory of natural selection was ridiculed because he had used geology to estimate the age of the Earth at several hundred million years. This was enough time, he believed, for natural selection to produce the millions of different species on Earth. The great physicist Lord Kelvin disputed this, claiming that Earth was much younger. When nuclear energy was discovered in the early 20th century, Darwin and Kelvin were proved wrong: the Earth was far older than either had suspected (4.5 billion years). Now, finally, the age of the universe, too, has been accurately determined.
With discoveries like these, about an ancient, dark universe filled with unknown matter, pulled by an unknown energy, it's not enough to say that science fiction has become science fact. Science is more imaginative than science fiction. And it will require the most inspired scientific imagination to determine the nature of the dark materials of the universe.
Mary and John Gribbin's book, "The Science of Philip Pullman's 'His Dark Materials' " is published by Hodder. The WMAP Web site is at map.gsfc.nasa.gov/ Rowan Hooper is a researcher at Trinity College Dublin. He welcomes comments at firstname.lastname@example.org