and It’s Out There

But, how do you find it? That’s the question that confronts astronomers who study the large-scale structure of the universe. There are two types of matter that they can study-ordinary, baryonic matter (protons, neutrons, and the subatomic particles that make up hydrogen, helium, and other elements (which themselves combine to form stars, planets, and galaxies) and then there’s dark matter, which isn’t baryonic. To get at the problem of how much dark matter there is, you have to corral all the baryonic matter. For a long time, astronomers have known that there’s a substantial mass of baryonic matter out there. The problem was to find it. You have to do a cosmic accounting of it. And, it’s not easy. You can’t just look out at the sky and easily spot the protons, neutrons, and other particles. You have to look at what it does to light.

The way to do that is to look at light from distant quasars through a spectrograph, a device that breaks up the light into its component wavelengths. As that light travels through space between the quasars and us, it gets absorbed by baryonic matter. And that leaves little dark lines (called absorption lines) in the spectrum of the quasar’s light. Those lines correspond to various elements that exist in the intergalactic medium.

Mike Shull and Charles Danforth (of the University of Colorado) used the Space Telescope Imaging Spectrograph (STIS) and the Far Ultraviolet Spectroscopic Explorer (FUSE) to look at quasars and map the distribution of baryonic matter in the local universe (the four billion or so light-years around us). They’ve now mapped enough of that “local” intergalactic medium to be able to say they’ve found about half of the missing baryonic matter there. (Read the details here.)

Shull says they’re finding structure in this matter, which is a big deal. “We think we are seeing the strands of a web-like structure that forms the backbone of the universe,” he said. “What we are confirming in detail is that intergalactic space, which intuitively might seem to be empty, is in fact the reservoir for most of the normal, baryonic matter in the universe.”

Of course, there are many more quasars to observe, and mapping the entire universe (in all directions) will take more time and a new instrument that is scheduled to go up on HST later this year. Stay tuned!