Formation of Structure in the Universe
Large scale structure in the northern and southern hemispheres. Each
the 9,325 points in the image represents a galaxy. Note the "walls"
(Margaret J. Geller and Emilio
E. Falco, Harvard-Smithsonian Center for Astrophysics.)
image to enlarge it.)
The Big Bang is the best theory we have about the origin of the Universe, but this theory is far from complete. For example, in its simplest form, the theory does not explain the origin of the magnificent hierarchy of stars, galaxies, and clusters of galaxies formed from the fiery detritus of that initial spark. Yet clearly the Universe has patches of galaxies and voids of seemingly empty space. How can we account for this?
Cosmologists, who study the origin and evolution of the Universe, hope to augment Big Bang theory to account for the large-scale structure we clearly see today. There are several ideas on the drawing board that work within Big Bang theory to account for the formation of galaxies and other structures. The most popular and best developed idea is called inflation: it postulates that the early universe underwent a period of super-rapid expansion that "inflated" an atomic scale piece of the universe to scales larger than the visible universe in a fraction of a second.
Different ideas about how structure first formed -- inflation, topological defects, and others -- make very specific predictions about the size and location of temperature differences in the Cosmic Microwave Background (CMB). Temperature differences seen today from region to region across the sky point back to the density differences in the early Universe; slightly denser regions then appear as slightly warmer regions now in the CMB. Upon measuring the CMB to the utmost precision, WMAP can compare its survey results to these predictions to see which idea is correct. WMAP will study the slightest of density differences when the Universe was a thousandth of its current size, about 400,000 years after the Big Bang. Matter and radiation was relatively evenly distributed then, but certain regions had slightly higher or lower densities compared to others. The seeds of structure were thus planted in the earliest moments of the Universe.
The prevailing idea of how structure formed is that density differences appeared and gravity took over. In a classic case of "the rich get richer," regions with slightly higher densities had the gravitational potential to attract more matter, making them denser yet. The region of space that contains our Milky Way galaxy, for example, was only about 0.5% denser than neighboring regions 400,000 years after the Big Bang. About 15 million years later, the present-day Milky Way region was about 5% denser than surrounding regions. (Mind you, our Galaxy still wasn't formed at this point.) A billion years after this, though, our region was about twice as dense as neighboring regions. Cosmologists speculate that the inner part of the Milky Way galaxy began forming at this time.
Structure takes time, as you can imagine. The popular "cold dark matter" theory predicts that exotic dark matter (of a yet unknown and undetectable nature) and a little bit of normal matter (the stuff that the stars and ourselves are made of) clumped together to form long filaments, which were scattered across the Universe creating a web. This was the very first structure. The filaments were a result of millions of years of gravity carving out denser regions of space, leaving voids in its wake. Hydrogen was the predominant gas along these filaments. Hundreds of millions of years went by. Pockets of hydrogen condensed, and galaxies started forming along the filaments like beads on a string. Stars formed along with galaxies. Where filaments intersected, galaxy clusters arose.
Astronomers using ground- and space-based telescopes are now beginning to observe these primordial filaments and the first galaxies. Instruments include the Very Large Telescope in Chile, the Hubble Space Telescope and the Chandra X-ray Observatory. WMAP complements these telescopes by providing the "when" and "how" of structure formation. By the end of the WMAP survey we will have a well-defined understanding of the age and shape of the Universe, the age of the first galaxies and stars, and the ultimate fate of the Universe.