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By Alan Boyle Science editor

If gravity works the way it’s supposed to, then most of the universe’s mass is invisible, existing as what’s come to be known as “dark matter.” What’s the nature of that missing mass, and what does it all mean for the fate of universe? The questions lead to some of the greatest mysteries of modern physics.

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The dark matter question arises because when it comes to galaxies and the bigger structures of the universe, there’s more than meets the eye — or any other detector of radiation, for that matter.

For decades, astronomers have been able to estimate the mass of faraway stars by comparing the characteristics of their light with those of our own sun. This method can even be extended to figure out the mass of galaxies and whole clusters of galaxies.

But astronomers can also calculate mass by closely observing the gravitational interactions of celestial bodies, then figuring out what amounts of mass would create those motions. They don’t even have to see a star’s motion directly; instead, they can use the change in the spectrum of the star, or “redshift,” to calculate the star’s velocity relative to Earth.

That was how astronomer Fritz Zwicky calculated the mass of the Coma cluster of galaxies in 1933. The only problem was, something didn’t add up.”The mass that he could account for in the stars and galaxies was only a tiny fraction of the mass needed to account for the motion observed in these clusters,” said Charles Lawrence, an astrophysicist at NASA’s Jet Propulsion Laboratory.

Other astronomers noticed the same discrepancy for other galaxy clusters. And there was more: The rotational rates of individual galaxies, measured in the 1950s and 1960s, were off-kilter as well. Based on what we see, galaxies should be torn apart as they spin. Instead, something appears to be holding them together like solid flywheels with stable rates of spin.

Even accounting for interstellar gas and dust, there’s a huge gap to fill. All the signs point to extra matter that we can’t detect directly, surrounding galaxies like a huge halo, perhaps filling in the space between galaxies and exerting an extra gravitational effect on the motions of whole galaxy clusters.

The prime suspects
What is the nature of this matter? The prime suspects break down into a number of categories:

  • MACHOs: Scientists say the universe may contain swarms of “massive compact halo objects” — perhaps dim neutron stars or dwarf stars, or brown dwarfs or rogue planets, or even collapsed stars and black holes left over from the primordial Big Bang. All these objects represent more or less ordinary states of matter (although one would be hard-put to call black holes “ordinary”).
  • Neutrinos: These ghostlike subatomic particles are produced in abundance by our sun and other stars (as well as nuclear reactors) but rarely interact with other forms of matter. Physicists say billions of neutrinos stream through your body harmlessly every second. If it turns out that such particles have even a tiny bit of mass, that could account for at least some of the missing mass.
  • Theoretical particles: Some physicists have also proposed the existence of particles that would be almost undetectable through interactions with ordinary matter, even though they have mass. Various schemes have dubbed them supersymmetric particles, or photinos, or neutralinos, or axions. Many of these particles are put under a category called WIMPs, or “weakly interacting massive particles.”

Researchers have found evidence to support all three possibilities.

For nearly a decade, astronomers have been conducting a survey known as the MACHO Project to watch for the “gravitational lensing” effect created when a massive compact halo object passes in front of a distant light source. At an August symposium in Australia, the astronomers said their results indicated that MACHOs could account for half of the universe’s “missing mass.”

At a June conference in Tokyo, physicists said they had determined that neutrinos indeed possessed mass. And a year ago, researchers said our own galaxy was surrounded by a gamma-ray halo that could hint at the existence of previously undetected WIMPs.

But all those dark matter claims are subject to debate. Even the observations relating to neutrinos aren’t conclusive. “Most people believe these days that while the measurement of mass in neutrinos is interesting, neutrinos can’t account for the missing mass,” Lawrence said.

How big is the universe?
Scientists haven’t even figured out yet how much total mass the universe contains — a no-less-weighty question that is linked to the dark matter debate. Indeed, the nature and amount of dark matter determines whether the universe itself is fated to collapse back upon itself, expand into virtual nothingness or reach a state of equilibrium.

Right now, the best bet is that there isn’t enough matter for gravity to overcome the Big Bang, meaning that the universe’s current expansion will continue forever until there’s practically nothing left. In fact, some scientists are puzzling over data indicating that the expansion is accelerating.

For a long time cosmologists worked under the assumption that there is enough matter to bring the universe into an eventual balance. Cosmologists call this balance point the critical density, and they use a variable called “omega” to describe the proportion of the universe’s actual density to the critical density.

If omega equals one, the universe is in balance and all is well for most theoretical physicists. But if omega is much less than one — as appears to be the case — then the theoreticians have a lot of explaining to do. In fact, it may indicate that we don’t fully understand how gravity works after all.

That’s why some physicists hope there’s enough undetected dark matter to fill the gap.

Figuring out the total mass of the universe may sound like an imponderable question — but surprisingly, Lawrence and other researchers hope to come up with some conclusive answers in the next decade or so. Their strategy is to measure the uneven afterglow of the Big Bang’s aftermath, known as the cosmic background radiation.

A satellite called the Cosmic Background Explorer has made a good start toward charting that afterglow. Future spacecraft such as NASA’s Microwave Anisotropy Probe and the European Space Agency’s Planck mission will map the early universe’s signature in even greater detail. By closely comparing the density differences in the background radiation, astronomers can come up with an answer for the mass question and gain some new hints as to the nature of dark matter.

“I think in 10 or 15 years we will know pretty much for sure whether the universe will expand forever, collapse back on itself or just drift,” said Lawrence, who is a principal investigator for one of the Planck research teams. “That’s pretty exciting. That’s a question that didn’t exist 100 years ago.”

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Interactive: Why does dark matter matter?


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