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Darkness Made Visible
By looking at galaxies far in the background, and then tracking how gravity deflects their light as it passes through the galaxies clustered in the foreground, the Hubble Space Telescope has mapped out the cluster's dark matter (shown in blue).
Credit: J. P. Kneib, Observatoire Midi-Pyrenees, Caltech, et al.; ESA; NASA

Title: What Is the Universe Made Of, and How Does It Work?
Once again, science has given us a big part of the answer. According to the physicists’ standard model, pretty much everything we can see in the universe is made from the same basic building blocks: elementary particles with names like quark, lepton and gluon. From raindrops to galaxies, it’s all the same stuff.

According to Einstein’s general theory of relativity, likewise, a subtle curvature in the fabric of space and time can explain every known effect of gravity: the universal force that pulls apples to the ground and holds the planets in their orbits. Indeed, it’s this curvature that determines how the universe has expanded in the aftermath of the big bang and what happens when matter collapses into a black hole.

For all their successes, however, the standard model and general relativity leave some serious gaps. In particular, neither one can explain why most of the universe is utterly invisible.

Astronomers stumbled onto this fact only a few decades ago, when they began to realize that the stars, galaxies and nebulae they can see through their telescopes are just a tiny fraction of what's actually out there. Far more prevalent is "dark matter"--a kind of cosmic ectoplasm that makes itself known only though its immense mass, which produces an equally immense gravitational field. Without that field, individual galaxies like our own Milky Way would fly apart like broken pinwheels, and clusters of galaxies would disperse into the void. In fact, without it, none of those galaxies and clusters would have formed in the first place, and we would not be here to wonder what dark matter is.

Meanwhile, in 1998, two independent teams of NSF-supported astronomers announced an even more astonishing discovery--the expansion of the universe isn’t slowing down, as most observers had expected. The expansion is actually speeding up. It is as if some previously unknown force, now known as “dark energy,” is driving the galaxies apart at an ever-increasing rate. Moreover, this dark energy is incredibly abundant. According to the best current measurements, ordinary matter accounts for no more than 5 percent of the stuff in the universe, on average, while dark matter amounts to some 20 percent. Dark energy is 75 percent.

The discovery of dark matter and dark energy has electrified the scientific community. Here is observational proof that there’s something about fundamental physics we still don’t understand (Science magazine has a recent review). In an effort to fill the gap, astronomers are planning a new generation of telescopes; theoreticians are exploring exotic concepts such as superstrings and extra dimensions; and laboratory scientists are deploying a host of new experiments. Among those experiments are searches for new fundamental particles and laws at the soon-to-be-completed Large Hadron Collider in Geneva, Switzerland; the LIGO gravitational wave detectors; dark matter searches; measurements of ultra-rare particle interactions; cosmic ray observatories; and more.

NSF is supporting all of these endeavors, in part or in whole. Indeed, they were among the leading topics at a recent NSF symposium on ground-based astronomy--as was the dark matter/dark energy mystery itself. With luck, we could be on the verge of a radically new picture of the universe, and of the fundamental nature of space, time, matter and energy.

What is our Place in the Cosmos? [Next]