A cosmic stork that's what astronomers need. Like curious children, they've been asking for decades, "Where do galaxies come from?" And, as with the tale about "the stork" parents used to tell their kids about where babies come from, the reality of galactic origins is a lot more complicated.
In a general sense, the origin of galaxies is understood. But like so many questions in science, this one fits the saying, "God is in the details." Discovering those details is extremely difficult and extremely important to astronomers.
"Galaxies are the building blocks of the universe, and therefore one of the greatest questions of modern astronomy is to understand how they form," says astronomer Sandy Faber of Californias Lick Observatory.
Thanks to some major theoretical advances in the last two decades, Faber and her colleagues have been able to answer that question at least, in its broad outlines.
able --> , helped amplify these ripples into more pronounced lumps and clumps. Those density variations are still visible today as fluctuations in the Cosmic Microwave Background (CMB), the "echo" of the Big Bang that is the earliest radiation astronomers can see.Dark matter, a mysterious form of matter unlike familiar atoms and subatomic particles, came into being almost immediately after the Big Bang, long before ordinary matter formed. Dark matter acted as gravitational "seeds" for the density variations to grow.
Because these areas of higher density had stronger gravity than their surroundings, they attracted more matter, and eventually, grew to become became the seeds of galaxies. (Today, galaxies are surrounded by dark matter halos, which are thought to be ten times larger and more massive than the galaxies' visible portions.)
This basic theory, which Faber helped create, has held up remarkably well, she says. "I dont think theres anything we said in that paper that has proven not to be true."
Though not proven, the theory has been further tested in recent years as astronomers have surveyed great numbers of galaxies in increasingly remote reaches of space. Caltech astronomer Richard Ellis says, "there is spectacular agreement" between theory and observations. Make a computer simulation that starts with the observed density fluctuations in the CMB and "cooks up" galaxies, says Ellis, and you end up with a picture that looks remarkably like the maps generated in the new galaxy surveys.
"I don't think it's a fluke that this agreement is there," Ellis says. "It shows that we have a very good understanding of how structure grows."
At the same time, Ellis says, many basic questions remain unanswered, not the least of which is why there are so many different kinds of galaxies. "Why do some have spiral arms and others not?" Ellis asks. "Why do they have different colors? Why is there a range of sizes?"
The answers are out there, but finding them means peering into the depths of space and back in time to make more detailed observations of galaxies throughout cosmic history. As Ellis says, "If you ask, When did galaxies form? It's a very continuous process, basically over the whole lifetime of the universe."
Last fall, Ellis and his colleagues used the Hubble Space Telescope, together with the giant Keck telescope in Hawaii, and were able to see back almost to the beginning of that process. They discovered a cluster of stars more than 13 billion light-years from Earth that formed less than 1 billion years after the Big Bang. (Even with the enormous power of these telescopes, the cluster would not have been detected without help from an intervening group of galaxies; the galaxies' dark matter halos, acting as a gravitational lens, magnified the distant cluster's brightness some 30 times.)
Possessing only a million stars, the cluster, which is just one-twentieth the size of our Milky Way, may represent one of the original galactic building blocks.
Nearer to Earth in space and in time, about 2 billion years after the Big Bang, astronomers see protogalaxies. These are compact objects (though larger than the suspected building blocks) that are forming new stars at a high rate and appear to represent an intermediate stage in galaxy formation.
About 4 billion to 5 billion years after the Big Bang that is, 8 billion or 9 billion years before now the first mature galaxies appear. These come in the same varieties astronomers see today in the nearby universe: spiral galaxies, with their classic pinwheel shapes; elliptical galaxies, which look like football- or basketball-shaped swarms of stars; and irregular galaxies, which are disorganized and ragged.
But as Ellis explains, "there are many more irregular-looking galaxies than we see today. So although we can find familiar-looking objects, the mixture of them is not familiar."
And even more important, the total number of galaxies at this time appears to be much higher than today between 3 and 10 times higher.
Next page: More pressing questions
advertisement
