Breaking Open the Unknown Universe

Light Reading: Astronomers can make dark matter by tracking how it distorts the light from distant galaxies:  NASA

Darkness Doubles Down

Here's what we know about what the universe is made of: We have the ordinary, common matter, like protons and electrons. In addition, there's all the stuff that transmits a force, like photons of light, or gravitons, which pull heavy objects together. That's the universe—matter and force—and physicists have spent the past 60 years or so uncovering the details of how all the matter particles and the force particles interact. The totality of that work is called the Standard Model of particle physics, and any particle physicist will tell you that it is the most successful theory in the history of human existence, powerful enough to predict the results of experiments down to one part in a trillion.

And yet the Standard Model is almost certainly not the whole picture. While particle physicists have been busy constructing the Standard Model, astronomers and cosmologists have been working on another task, a giant cosmic accounting project. What they see—or, more precisely, don't—is a clear sign that there are far more things in heaven and earth than are dreamt of by the Ph.D.s.

If you go out and count up all the stars and galaxies and supernovae and the like, you should get an estimate of how much total mass there is in the universe. But if you estimate the mass another way—say, by looking at how quickly galaxies rotate (the more mass in a galaxy, the faster it spins) or by noting how galaxies clump together in large groups—you will conclude that the universe has much more mass than we can see. About five times as much, by the latest reckoning. Since it can't be seen, we call it dark matter.

Here's the problem: These unknown dark-matter particles—there's no column in the Standard Model for them. Another problem is that not even the people who came up with it think the Standard Model is the whole story. "The theory raises so many new questions," says David Gross, who won a Nobel Prize in 2004 for his work on the Standard Model, "that we are convinced it must be incomplete in some way." Sure, the model correctly predicts the outcome of experiments. But it is not, in the deep way that physicists want it to be, pretty.

To make the Standard Model work, there needs to be much fine-tuning, a dirty word to physicists because it implies arbitrarily tweaking lots of little variables in order to make everything come out right. Much better, physicists would argue, to have everything balance out naturally. As Dan Hooper, a physicist at Fermi National Accelerator Laboratory in Illinois, concludes in his new book Nature's Blueprint, "The Standard Model as we understand it is ultimately unstable and is in desperate need of a new mechanism to prevent it from falling apart."

Enter supersymmetry, one helluva "mechanism." Supersymmetry posits that every particle is only half the story—that every particle has a hidden twin. Remember how the universe is split into matter and force? The core idea of supersymmetry is that every matter particle has a twin force-carrying particle. Same goes the other way: Every force particle has a twin made of matter. Matter and force, in one sense, are just two manifestations of the same thing.

How does this work in practice? Electrons give rise to selectrons (as in, supersymmetric electrons), and photons beget photinos (don't ask). The extra particles, each heavier than its twin, automatically balance out the Standard Model, no fine-tuning needed. But perhaps more important, these particles, were they to exist, could very well be the hitherto invisible dark matter. The universe swarms with squarks, winos and neutralinos, and these supersymmetric particles are just heavy enough and just common enough to outweigh the "normal" stuff by a factor of five to one. Cosmology, meet particle physics.

Of course, for this to make any sense, the LHC first needs to find a supersymmetric particle. And here's the catch: Even if the LHC makes a supersymmetric particle—two protons come together with enough energy to make, say, a neutralino—that particle will still be invisible. It will pass through the walls of the detector and down into Earth's crust and back out into space. Invisible means it doesn't interact with ordinary matter, and ordinary matter is the only thing we can build detectors out of.

So what happens? How can we tell? Well, we look very closely. When two protons come together, they will generate a shower of particles. Most of them will be ordinary particles, and the detectors will catch these. Then the scientists will look for what's missing. "It's a bit like the Sherlock Holmes story where the most important clue is the dog that doesn't bark," says John Ellis, a theorist at CERN. If lots of stuff comes out going one way, there has to be an equal amount of stuff going the other way—it's just the law of conservation of momentum. Count up what you have, subtract that from what you started with, and voilà, you could find yourself with a fleeting glimpse of dark matter. Or at least, its absence.