Astronomers see era of rapid galaxy formation
New findings pose a challenge for cold dark matter theory
“The universe is always more complicated than our cosmological theories would have it,” says Nigel Sharp, program officer for extra-galactic astronomy and cosmology at the National Science Foundation (NSF). Witness a collection of new and recently announced discoveries that, taken together, suggest a considerably more active and fastmoving epoch of galaxy formation in the early universe than prevailing theories had called for.
The findings, each of which was obtained at facilities supported in whole or in part by the NSF, include the following:
- Evidence from the Sloan Digital Sky Survey (SDSS) that at least a few ultra-massive black holes had come into existence less than a billion years after the universe began in the Big Bang, some 13.7 billion years ago. Each of these black holes is several billion times more massive than stars like our own Sun, and is sitting in the middle of an otherwise normal galaxy, swallowing up the surrounding gas and dust; the thermonuclear energy released in that process is visible to the SDSS astronomers as a brilliant, but very distant, point of light known as a quasar. The puzzle is that the SDSS black holes are as large as any ever seen, including those observed in nearby quasars that are much older. So how did they manage to form and grow to such a size before the universe was a tenth its present age? “The formation should have taken time,” says Michael A. Strauss of Princeton University, who is the scientific spokesperson for the SDSS project and a co-principal investigator on this study. A formal report will be published in the Astronomical Journal in March 2004. A press release is available online at http://www.sdss.org/news/releases/20031217.lensing.html.
- Evidence from the Gemini Observatory that at least some galaxies had formed, grown, and “matured” by the time the universe was only 20 to 40 percent of its present age. This observation, which comes from a systematic scan of early galaxies known as the Gemini Deep Deep Survey (GDDS), gives us a snapshot of the cosmos at a considerably later time than the Sloan observations: several billion years after the Big Bang, versus 1 billion years. But it is surprising nonetheless. A “mature” galaxy is one that, like our own Milky Way, has already lived through its tumultuous youth-a period that typically includes the violent accretion of smaller galaxies and multiple bursts of star formation-and has finally settled down into a quieter, more stable middle age. Finding such individuals in the Gemini survey is like walking into a junior-high classroom and finding it full of adults. How did these galaxies manage to grow up so fast? The GDDS finding was presented on January 5, 2004, at the Atlanta meeting of the American Astronomical Society. A press release, along with artwork and illustrations, is available online at http://www.gemini.edu/gdds/.
- Evidence from a U.S.-Australian team, working in part at the NSFs Cerro Tololo Inter-American Observatory in Chile, that galaxies in this same era of the early universe had already begun to collect into well-defined clusters. Again, the question is how clustering could have progressed so rapidly. This discovery was reported at the astronomical society meeting on January 7, 2004. A press release, along with images and background material, is available at http://www.gsfc.nasa.gov/topstory/2004/0107filament.html. The teams formal paper will be published by the Astrophysical Journal in February 2004.
In sum, says Princetons Strauss, these results give us “a variety of different types of hints that at least some types of galaxies settled down very early in the universe.” Yet that fact, if true, is hard to understand in the prevailing theory of galaxy formation. According to the “Cold Dark Matter” model, as its known, galaxies and clusters grew in a bottom-up fashion-that is, with small structures forming first, and the bigger structures accumulating only much later. But does that mean that the Cold Dark Matter model is wrong? Or does it just mean that weve still got a lot to learn about how ordinary matter formed that first generation of stars?
Whatever the answer, says NSFs Sharp, “there may be more happening early in the universe than we previously thought. It will be interesting to see how this plays out in the more extensive surveys that are now being planned.”
Background: Cold Dark Matter and Galaxy Formation
Each of these studies, in various ways, addresses one of the most fundamental questions of cosmology: How did the Big Bang give rise to us? In the beginning, the matter that emerged from the primeval fireball was remarkably smooth and uniform. And yet now, some 13.7 billion years later, the matter in the universe is anything but uniform. Atoms have long since been swept up into planets, stars, and interstellar gas clouds. These objects, in turn, are organized into galaxies, which are grouped into clusters of galaxies, which are grouped into superclusters, and so on. How did that happen? What caused the universe to clump up in this way?
The short answer is “gravity”: the universal force of attraction. As astronomers have known for generations, gravity had the power to destabilize even the smoothest distribution of matter. Say that by chance, a given region of the primeval fireball just happened to have a few more particles than average. That would have made the mutual gravitational attraction among those particles a little bit stronger than average. But then the resulting imbalance of forces would have pulled the particles closer together and increased their mutual attraction still further. That would have accelerated their motion, decreased their separation, increased their attraction-on and on, faster and faster and faster. Conversely, a region that happened to have a few less particles than average would have tended to hollow out over time, as gravity pulled as more and more matter into the denser regions. Either way, the result would have been a distribution of matter that was very lumpy indeed-lumps that presumably gave rise to the stars, galaxies, and clusters of galaxies.
A longer and more complete answer is “gravity”-but gravity acting on a universe that has, literally, much more than meets the eye. Over the past three decades or so, astronomers have come to realize that the stars, galaxies, and clusters they can see through their telescopes dont contain nearly enough mass to clump up on their own. Instead, its now apparent that these visible objects are more like bright flecks of foam on a dark, swelling ocean. The “ocean waves,” in this case, consist of Cold Dark Matter: an utterly invisible essence that is thought to be a haze of weakly interacting elementary particles left over from the Big Bang. (The dark matter is “cold” because the particles are presumed to be moving fairly slowly, at much less than the speed of light.) But whatever it is, the dark matter permeates the cosmos, is immensely massive, and controls the evolution of everything we can see. It is the dark matter that undergoes gravitational collapse and makes the universe lumpy; all the ordinary matter, the stuff that makes up stars, galaxies, and us, simply gets carried along.
The process of gravitational collapse in a Cold Dark Matter dominated universe has been studied through many, many computer simulations. Some vivid examples have been posted on the Universe in a Box page prepared by the University of Chicagos Center for Cosmological Physics, an NSF-funded Physics Frontier Center. Many more examples can be found on the Cosmos In a Computer page posted by the University of Illinois National Center for Supercomputer Applications, one of the NSF-supported supercomputer centers.
Both of these sites also offer introductory tutorials on modern cosmology in general. Two sites that offer more extensive (and technical) tutorials are: http://www.astr.ua.edu/keel/galaxies/index.html, and http://www.astro.ucla.edu/~wright/cosmolog.htm.
Meanwhile, there are a number of experiments underway around the world to detect the dark matter particles. One major effort is the Cryogenic Dark Matter Search, which is being funded jointly by NSF and the Department of Energy. The NSF award abstract is available here.
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